Android_ListView (基本使用 / RecycleBin机制 / 源码解析 / 异步图片错位解决方案)

最新推荐文章于 2022-04-26 08:15:00 发布

原创最新推荐文章于 2022-04-26 08:15:00 发布 · 981 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#android #listview

android 专栏收录该内容

38 篇文章

订阅专栏

本文详细探讨了Android ListView的基本使用，包括Adapter适配器模式，重点解析了ListView的RecycleBin机制和源码，特别是ListView的初始化与滑动加载逻辑。此外，还提出了针对异步图片加载错位问题的解决方案，通过设置Tag避免图片错位。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

Android_ListView (基本使用 / RecycleBin机制 / 源码解析 / 异步图片错位解决方案)

本文由 Luzhuo 编写,转发请保留该信息.
原文: http://blog.youkuaiyun.com/Rozol/article/details/78161840

把数据用列表的形式,动态滚动的方式,展示给用户.

ListView 作为界面展示的容器控件必然会直接或者间接的继承ViewGroup, 现在看看源代码的继承结构

public class ListView extends AbsListView { }

public abstract class AbsListView extends AdapterView<ListAdapter> implements TextWatcher, ViewTreeObserver.OnGlobalLayoutListener, Filter.FilterListener, ViewTreeObserver.OnTouchModeChangeListener, RemoteViewsAdapter.RemoteAdapterConnectionCallback { }

public abstract class AdapterView<T extends Adapter> extends ViewGroup { }

现在知道ListView确实是继承ViewGroup的,那么就会重写 onMeasure() onLayout() onDraw() 这三个基本的方法, 大家是否注意到继承ViewGroup的后,onMeasure() onLayout()会多次执行的问题(执行了4次onMeasure(), 2次onLayout()),以下是log.

基本使用

public class MainActivity extends AppCompatActivity {
    private ListView listView;
    private String[] listImage = Resource.grilImage;
    private BitmapUtils bitmapUtils;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        initView();
        initData();
    }

    private void initView() {
        bitmapUtils = new BitmapUtils(this);
        listView = (ListView) findViewById(R.id.listview);
    }

    private void initData() {
        ImageAdapter IAdapter = new ImageAdapter(MainActivity.this, listImage, bitmapUtils);
        listView.setAdapter(IAdapter);

        listView.setOnScrollListener(new PauseOnScrollListener(bitmapUtils, false, true));
    }
}

public class ImageAdapter extends BaseAdapter {
    private Context context;
    private String[] listImage;
    private LayoutInflater inflater;
    private BitmapUtils bitmapUtils;

    public ImageAdapter(Context context, String[] listImage, BitmapUtils bitmapUtils) {
        this.context = context;
        this.listImage = listImage;
        inflater = LayoutInflater.from(context);
        this.bitmapUtils = bitmapUtils;
    }

    @Override
    public int getCount() {
        return listImage == null ? 0 : listImage.length;
    }

    @Override
    public Object getItem(int position) {
        return listImage[position];
    }

    @Override
    public long getItemId(int position) {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder viewHolder;
        if(convertView == null){
            convertView = inflater.inflate(R.layout.item_list, null);
            viewHolder = new ViewHolder();

            viewHolder.imageview = (ImageView)convertView.findViewById(R.id.imageview);
            viewHolder.textview = (TextView)convertView.findViewById(R.id.textview);

            convertView.setTag(viewHolder);
        }else{
            viewHolder = (ViewHolder) convertView.getTag();
        }

        if(listImage.length != 0){
            bitmapUtils.display(viewHolder.imageview, listImage[position]);
            viewHolder.textview.setText("第"+position+"张图片");
        }
        return  convertView;
    }

    class ViewHolder{
        ImageView imageview;
        TextView textview;
    }
}

Adapter适配器模式

从上面的使用代码可以看出,要让ListView正常工作,就要设置Adapter,Adapter就是适配器

public class MyAdapter extends BaseAdapter {
    @Override
    public int getCount() {
        return 0;
    }

    @Override
    public Object getItem(int position) {
        return null;
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        return null;
    }
}

继承Adapter会被要求必须重写上述4个方法. 数据是不尽相同的, ListView只关心交互和展示的工作,不关心你数据是什么样的,从哪来的. 而Adapter的统一接口就解决的数据适配的问题.
示范图:

recycleBin机制

为了简洁说明ListView是怎么工作的,先讲下 AbsListView 类里的内部类 RecycleBin

public abstract class AbsListView extends AdapterView<ListAdapter> implements TextWatcher, ViewTreeObserver.OnGlobalLayoutListener, Filter.FilterListener, ViewTreeObserver.OnTouchModeChangeListener, RemoteViewsAdapter.RemoteAdapterConnectionCallback {

    final RecycleBin mRecycler = new RecycleBin();

    class RecycleBin {
        private RecyclerListener mRecyclerListener;

        /**
         * The position of the first view stored in mActiveViews.
         */
        private int mFirstActivePosition;

        /**
         * Views that were on screen at the start of layout. This array is populated at the start of
         * layout, and at the end of layout all view in mActiveViews are moved to mScrapViews.
         * Views in mActiveViews represent a contiguous range of Views, with position of the first
         * view store in mFirstActivePosition.
         */
        // ↓↓↓
        private View[] mActiveViews = new View[0];

        /**
         * Unsorted views that can be used by the adapter as a convert view.
         */
        // ↓↓↓
        private ArrayList<View>[] mScrapViews;

        private int mViewTypeCount;
        // ↓↓↓
        private ArrayList<View> mCurrentScrap;

        private ArrayList<View> mSkippedScrap;

        private SparseArray<View> mTransientStateViews;
        private LongSparseArray<View> mTransientStateViewsById;

        // ↓↓↓
        public void setViewTypeCount(int viewTypeCount) {
            if (viewTypeCount < 1) {
                throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
            }
            //noinspection unchecked
            ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
            for (int i = 0; i < viewTypeCount; i++) {
                scrapViews[i] = new ArrayList<View>();
            }
            mViewTypeCount = viewTypeCount;
            mCurrentScrap = scrapViews[0];
            mScrapViews = scrapViews;
        }

        /**
         * Fill ActiveViews with all of the children of the AbsListView.
         *
         * @param childCount The minimum number of views mActiveViews should hold
         * @param firstActivePosition The position of the first view that will be stored in
         *        mActiveViews
         */
        // ↓↓↓
        void fillActiveViews(int childCount, int firstActivePosition) {
            if (mActiveViews.length < childCount) {
                mActiveViews = new View[childCount];
            }
            mFirstActivePosition = firstActivePosition;

            //noinspection MismatchedReadAndWriteOfArray
            final View[] activeViews = mActiveViews;
            for (int i = 0; i < childCount; i++) {
                View child = getChildAt(i);
                AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
                // Don't put header or footer views into the scrap heap
                if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                    // Note:  We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in active views.
                    //        However, we will NOT place them into scrap views.
                    activeViews[i] = child;
                    // Remember the position so that setupChild() doesn't reset state.
                    lp.scrappedFromPosition = firstActivePosition + i;
                }
            }
        }

        /**
         * Get the view corresponding to the specified position. The view will be removed from
         * mActiveViews if it is found.
         *
         * @param position The position to look up in mActiveViews
         * @return The view if it is found, null otherwise
         */
        // ↓↓↓
        View getActiveView(int position) {
            int index = position - mFirstActivePosition;
            final View[] activeViews = mActiveViews;
            if (index >=0 && index < activeViews.length) {
                final View match = activeViews[index];
                activeViews[index] = null;
                return match;
            }
            return null;
        }

        /**
         * @return A view from the ScrapViews collection. These are unordered.
         */
        // ↓↓↓
        View getScrapView(int position) {
            final int whichScrap = mAdapter.getItemViewType(position);
            if (whichScrap < 0) {
                return null;
            }
            if (mViewTypeCount == 1) {
                return retrieveFromScrap(mCurrentScrap, position);
            } else if (whichScrap < mScrapViews.length) {
                return retrieveFromScrap(mScrapViews[whichScrap], position);
            }
            return null;
        }

        /**
         * Puts a view into the list of scrap views.
         * <p>
         * If the list data hasn't changed or the adapter has stable IDs, views
         * with transient state will be preserved for later retrieval.
         *
         * @param scrap The view to add
         * @param position The view's position within its parent
         */
        // ↓↓↓
        void addScrapView(View scrap, int position) {
            final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
            if (lp == null) {
                // Can't recycle, but we don't know anything about the view.
                // Ignore it completely.
                return;
            }

            lp.scrappedFromPosition = position;

            // Remove but don't scrap header or footer views, or views that
            // should otherwise not be recycled.
            final int viewType = lp.viewType;
            if (!shouldRecycleViewType(viewType)) {
                // Can't recycle. If it's not a header or footer, which have
                // special handling and should be ignored, then skip the scrap
                // heap and we'll fully detach the view later.
                if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
                    getSkippedScrap().add(scrap);
                }
                return;
            }

            scrap.dispatchStartTemporaryDetach();

            // The the accessibility state of the view may change while temporary
            // detached and we do not allow detached views to fire accessibility
            // events. So we are announcing that the subtree changed giving a chance
            // to clients holding on to a view in this subtree to refresh it.
            notifyViewAccessibilityStateChangedIfNeeded(
                    AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);

            // Don't scrap views that have transient state.
            final boolean scrapHasTransientState = scrap.hasTransientState();
            if (scrapHasTransientState) {
                if (mAdapter != null && mAdapterHasStableIds) {
                    // If the adapter has stable IDs, we can reuse the view for
                    // the same data.
                    if (mTransientStateViewsById == null) {
                        mTransientStateViewsById = new LongSparseArray<>();
                    }
                    mTransientStateViewsById.put(lp.itemId, scrap);
                } else if (!mDataChanged) {
                    // If the data hasn't changed, we can reuse the views at
                    // their old positions.
                    if (mTransientStateViews == null) {
                        mTransientStateViews = new SparseArray<>();
                    }
                    mTransientStateViews.put(position, scrap);
                } else {
                    // Otherwise, we'll have to remove the view and start over.
                    getSkippedScrap().add(scrap);
                }
            } else {
                if (mViewTypeCount == 1) {
                    mCurrentScrap.add(scrap);
                } else {
                    mScrapViews[viewType].add(scrap);
                }

                if (mRecyclerListener != null) {
                    mRecyclerListener.onMovedToScrapHeap(scrap);
                }
            }
        }
    }
}

以上代码是从源码中拷贝出来,并删掉了一些不重要的方法

成员变量:

View[] mActiveViews: 用于存放活动View(也就是在屏幕上展示的View)
int mFirstActivePosition: 存放mActiveViews中第一个View的Position(也就是第几个Item)
ArrayList<View>[] mScrapViews: 废弃的View,可通过Adapter转为convertView继续使用(看看基本使用Adapter代码,我们就是将这些废弃的view重复使用的)
ArrayList<View> mCurrentScrap: ViewTypeCount == 1 的废弃View会被存在这个集合里
方法:
public void setViewTypeCount(int viewTypeCount) {}: 该方法会根据传入的类型数量初始化 mScrapViews 和 mCurrentScrap; mScrapViews 存了不同类型View的集合, mCurrentScrap是mScrapViews的第一个集合; 看来ListView是可以传入多个类型的View的
void fillActiveViews(int childCount, int firstActivePosition) {}: 主要是将mActiveViews填满子View (保存屏幕上展示的View)
View getActiveView(int position) {}: 根据position获取mActiveViews里的View

View getScrapView(int position) {}: 源码主要调用了retrieveFromScrap(mCurrentScrap, position)方法,现在看看这个方法是干吗的?

private View retrieveFromScrap(ArrayList<View> scrapViews, int position) {
    final int size = scrapViews.size();
    if (size > 0) {
        // See if we still have a view for this position or ID.
        for (int i = 0; i < size; i++) {
            final View view = scrapViews.get(i);
            final AbsListView.LayoutParams params =
                    (AbsListView.LayoutParams) view.getLayoutParams();

            if (mAdapterHasStableIds) {
                final long id = mAdapter.getItemId(position);
                if (id == params.itemId) {
                    return scrapViews.remove(i);
                }
            } else if (params.scrappedFromPosition == position) {
                final View scrap = scrapViews.remove(i);
                clearAccessibilityFromScrap(scrap);
                return scrap;
            }
        }
        final View scrap = scrapViews.remove(size - 1);
        clearAccessibilityFromScrap(scrap);
        return scrap;
    } else {
        return null;
    }
}

看来getScrapView(int position)是获取废弃的View, 如果能获取到就返回View,获取不到就返回null

void addScrapView(View scrap, int position) {}: 把废弃的View添加到mCurrentScrap里, 把具有过渡效果的废弃View添加到mTransientStateViews里(带有过渡效果的View这里不做讲解)
可见recycleBin主要工作就是填满和获取展示View,添加和获取缓存View.

ListView的执行逻辑源码

ListView的初始化逻辑

ListView作为View容器控件,那么我们就从 onMeasure() onLayout() 这2个基本的被重写方法开始研究

onMeasure() 主要是测量ListView的大小; onLayout()用于确定子View的布局, 这才是核心, 该方法并没有在ListView中实现, 而是在抽象父类AbsListView中实现.

protected void onLayout(boolean changed, int l, int t, int r, int b) {
    super.onLayout(changed, l, t, r, b);

    mInLayout = true;

    final int childCount = getChildCount();
    // ↓↓↓ 1. 如果change == true, ListView的大小和位置发生变化 
    if (changed) {
        for (int i = 0; i < childCount; i++) {
            // ↓↓↓ 2. 那么就把所有子布局强制重绘
            getChildAt(i).forceLayout();
        }
        mRecycler.markChildrenDirty();
    }

    // ↓↓↓ 3. 调用子类ListView的layoutChildren()方法
    layoutChildren();
    mInLayout = false;

    mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;

    // TODO: Move somewhere sane. This doesn't belong in onLayout().
    if (mFastScroll != null) {
        mFastScroll.onItemCountChanged(getChildCount(), mItemCount);
    }
}

接着看看layoutChildren()做了什么

protected void layoutChildren() {
    final boolean blockLayoutRequests = mBlockLayoutRequests;
    if (blockLayoutRequests) {
        return;
    }

    mBlockLayoutRequests = true;

    try {
        super.layoutChildren();

        invalidate();

        if (mAdapter == null) {
            resetList();
            invokeOnItemScrollListener();
            return;
        }

        final int childrenTop = mListPadding.top;
        final int childrenBottom = mBottom - mTop - mListPadding.bottom;
        // ↓↓↓ 1. ListView中还未填充任务子View, 得到结果为0
        final int childCount = getChildCount();

        int index = 0;
        int delta = 0;

        View sel;
        View oldSel = null;
        View oldFirst = null;
        View newSel = null;

        // Remember stuff we will need down below
        switch (mLayoutMode) {
        case LAYOUT_SET_SELECTION:
            index = mNextSelectedPosition - mFirstPosition;
            if (index >= 0 && index < childCount) {
                newSel = getChildAt(index);
            }
            break;
        case LAYOUT_FORCE_TOP:
        case LAYOUT_FORCE_BOTTOM:
        case LAYOUT_SPECIFIC:
        case LAYOUT_SYNC:
            break;
        case LAYOUT_MOVE_SELECTION:
        default:
            // Remember the previously selected view
            index = mSelectedPosition - mFirstPosition;
            if (index >= 0 && index < childCount) {
                oldSel = getChildAt(index);
            }

            // Remember the previous first child
            oldFirst = getChildAt(0);

            if (mNextSelectedPosition >= 0) {
                delta = mNextSelectedPosition - mSelectedPosition;
            }

            // Caution: newSel might be null
            newSel = getChildAt(index + delta);
        }

        boolean dataChanged = mDataChanged;
        if (dataChanged) {
            handleDataChanged();
        }

        // Handle the empty set by removing all views that are visible
        // and calling it a day
        if (mItemCount == 0) {
            resetList();
            invokeOnItemScrollListener();
            return;
        } else if (mItemCount != mAdapter.getCount()) {
            throw new IllegalStateException("The content of the adapter has changed but "
                    + "ListView did not receive a notification. Make sure the content of "
                    + "your adapter is not modified from a background thread, but only from "
                    + "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
                    + "when its content changes. [in ListView(" + getId() + ", " + getClass()
                    + ") with Adapter(" + mAdapter.getClass() + ")]");
        }

        setSelectedPositionInt(mNextSelectedPosition);

        AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
        View accessibilityFocusLayoutRestoreView = null;
        int accessibilityFocusPosition = INVALID_POSITION;

        // Remember which child, if any, had accessibility focus. This must
        // occur before recycling any views, since that will clear
        // accessibility focus.
        final ViewRootImpl viewRootImpl = getViewRootImpl();
        if (viewRootImpl != null) {
            final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
            if (focusHost != null) {
                final View focusChild = getAccessibilityFocusedChild(focusHost);
                if (focusChild != null) {
                    if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
                            || focusChild.hasTransientState() || mAdapterHasStableIds) {
                        // The views won't be changing, so try to maintain
                        // focus on the current host and virtual view.
                        accessibilityFocusLayoutRestoreView = focusHost;
                        accessibilityFocusLayoutRestoreNode = viewRootImpl
                                .getAccessibilityFocusedVirtualView();
                    }

                    // If all else fails, maintain focus at the same
                    // position.
                    accessibilityFocusPosition = getPositionForView(focusChild);
                }
            }
        }

        View focusLayoutRestoreDirectChild = null;
        View focusLayoutRestoreView = null;

        // Take focus back to us temporarily to avoid the eventual call to
        // clear focus when removing the focused child below from messing
        // things up when ViewAncestor assigns focus back to someone else.
        final View focusedChild = getFocusedChild();
        if (focusedChild != null) {
            // TODO: in some cases focusedChild.getParent() == null

            // We can remember the focused view to restore after re-layout
            // if the data hasn't changed, or if the focused position is a
            // header or footer.
            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
                    || focusedChild.hasTransientState() || mAdapterHasStableIds) {
                focusLayoutRestoreDirectChild = focusedChild;
                // Remember the specific view that had focus.
                focusLayoutRestoreView = findFocus();
                if (focusLayoutRestoreView != null) {
                    // Tell it we are going to mess with it.
                    focusLayoutRestoreView.dispatchStartTemporaryDetach();
                }
            }
            requestFocus();
        }

        // Pull all children into the RecycleBin.
        // These views will be reused if possible
        final int firstPosition = mFirstPosition;
        final RecycleBin recycleBin = mRecycler;
        // ↓↓↓ 2. 一旦数据源有变化, dataChanged == true
        if (dataChanged) {
            for (int i = 0; i < childCount; i++) {
                recycleBin.addScrapView(getChildAt(i), firstPosition+i);
            }
        } else {
            // ↓↓↓ 2.2 如果数据源, 执行该方法, 调用RecycleBin的fillActiveViews()缓存展示的所有view, 但是现在并没有展示的View
            recycleBin.fillActiveViews(childCount, firstPosition);
        }

        // Clear out old views
        // ↓↓↓ 2.3 从父容器中清除所有view, 现在没有可清理的view
        detachAllViewsFromParent();
        recycleBin.removeSkippedScrap();

        // ↓↓↓ 3. 判断布局模式, 默认布局模式为LAYOUT_NORMAL
        switch (mLayoutMode) {
        case LAYOUT_SET_SELECTION:
            if (newSel != null) {
                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
            } else {
                sel = fillFromMiddle(childrenTop, childrenBottom);
            }
            break;
        case LAYOUT_SYNC:
            sel = fillSpecific(mSyncPosition, mSpecificTop);
            break;
        case LAYOUT_FORCE_BOTTOM:
            sel = fillUp(mItemCount - 1, childrenBottom);
            adjustViewsUpOrDown();
            break;
        case LAYOUT_FORCE_TOP:
            mFirstPosition = 0;
            sel = fillFromTop(childrenTop);
            adjustViewsUpOrDown();
            break;
        case LAYOUT_SPECIFIC:
            final int selectedPosition = reconcileSelectedPosition();
            sel = fillSpecific(selectedPosition, mSpecificTop);
            /**
             * When ListView is resized, FocusSelector requests an async selection for the
             * previously focused item to make sure it is still visible. If the item is not
             * selectable, it won't regain focus so instead we call FocusSelector
             * to directly request focus on the view after it is visible.
             */
            if (sel == null && mFocusSelector != null) {
                final Runnable focusRunnable = mFocusSelector
                        .setupFocusIfValid(selectedPosition);
                if (focusRunnable != null) {
                    post(focusRunnable);
                }
            }
            break;
        case LAYOUT_MOVE_SELECTION:
            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
            break;
        default:
            // ↓↓↓ 3.1 于是会执行该部分代码
            if (childCount == 0) {
                if (!mStackFromBottom) {
                    final int position = lookForSelectablePosition(0, true);
                    setSelectedPositionInt(position);
                    // ↓↓↓ 3.2 执行方法
                    sel = fillFromTop(childrenTop);
                } else {
                    final int position = lookForSelectablePosition(mItemCount - 1, false);
                    setSelectedPositionInt(position);
                    sel = fillUp(mItemCount - 1, childrenBottom);
                }
            } else {
                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
                    sel = fillSpecific(mSelectedPosition,
                            oldSel == null ? childrenTop : oldSel.getTop());
                } else if (mFirstPosition < mItemCount) {
                    sel = fillSpecific(mFirstPosition,
                            oldFirst == null ? childrenTop : oldFirst.getTop());
                } else {
                    sel = fillSpecific(0, childrenTop);
                }
            }
            break;
        }

        // Flush any cached views that did not get reused above
        recycleBin.scrapActiveViews();

        // remove any header/footer that has been temp detached and not re-attached
        removeUnusedFixedViews(mHeaderViewInfos);
        removeUnusedFixedViews(mFooterViewInfos);

        if (sel != null) {
            // The current selected item should get focus if items are
            // focusable.
            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
                        focusLayoutRestoreView != null &&
                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
                if (!focusWasTaken) {
                    // Selected item didn't take focus, but we still want to
                    // make sure something else outside of the selected view
                    // has focus.
                    final View focused = getFocusedChild();
                    if (focused != null) {
                        focused.clearFocus();
                    }
                    positionSelector(INVALID_POSITION, sel);
                } else {
                    sel.setSelected(false);
                    mSelectorRect.setEmpty();
                }
            } else {
                positionSelector(INVALID_POSITION, sel);
            }
            mSelectedTop = sel.getTop();
        } else {
            final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
                    || mTouchMode == TOUCH_MODE_DONE_WAITING;
            if (inTouchMode) {
                // If the user's finger is down, select the motion position.
                final View child = getChildAt(mMotionPosition - mFirstPosition);
                if (child != null) {
                    positionSelector(mMotionPosition, child);
                }
            } else if (mSelectorPosition != INVALID_POSITION) {
                // If we had previously positioned the selector somewhere,
                // put it back there. It might not match up with the data,
                // but it's transitioning out so it's not a big deal.
                final View child = getChildAt(mSelectorPosition - mFirstPosition);
                if (child != null) {
                    positionSelector(mSelectorPosition, child);
                }
            } else {
                // Otherwise, clear selection.
                mSelectedTop = 0;
                mSelectorRect.setEmpty();
            }

            // Even if there is not selected position, we may need to
            // restore focus (i.e. something focusable in touch mode).
            if (hasFocus() && focusLayoutRestoreView != null) {
                focusLayoutRestoreView.requestFocus();
            }
        }

        // Attempt to restore accessibility focus, if necessary.
        if (viewRootImpl != null) {
            final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost();
            if (newAccessibilityFocusedView == null) {
                if (accessibilityFocusLayoutRestoreView != null
                        && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) {
                    final AccessibilityNodeProvider provider =
                            accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider();
                    if (accessibilityFocusLayoutRestoreNode != null && provider != null) {
                        final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId(
                                accessibilityFocusLayoutRestoreNode.getSourceNodeId());
                        provider.performAction(virtualViewId,
                                AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null);
                    } else {
                        accessibilityFocusLayoutRestoreView.requestAccessibilityFocus();
                    }
                } else if (accessibilityFocusPosition != INVALID_POSITION) {
                    // Bound the position within the visible children.
                    final int position = MathUtils.constrain(
                            accessibilityFocusPosition - mFirstPosition, 0,
                            getChildCount() - 1);
                    final View restoreView = getChildAt(position);
                    if (restoreView != null) {
                        restoreView.requestAccessibilityFocus();
                    }
                }
            }
        }

        // Tell focus view we are done mucking with it, if it is still in
        // our view hierarchy.
        if (focusLayoutRestoreView != null
                && focusLayoutRestoreView.getWindowToken() != null) {
            focusLayoutRestoreView.dispatchFinishTemporaryDetach();
        }

        mLayoutMode = LAYOUT_NORMAL;
        mDataChanged = false;
        if (mPositionScrollAfterLayout != null) {
            post(mPositionScrollAfterLayout);
            mPositionScrollAfterLayout = null;
        }
        mNeedSync = false;
        setNextSelectedPositionInt(mSelectedPosition);

        updateScrollIndicators();

        if (mItemCount > 0) {
            checkSelectionChanged();
        }

        invokeOnItemScrollListener();
    } finally {
        if (mFocusSelector != null) {
            mFocusSelector.onLayoutComplete();
        }
        if (!blockLayoutRequests) {
            mBlockLayoutRequests = false;
        }
    }
}

private View fillFromTop(int nextTop) {
    mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
    mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
    if (mFirstPosition < 0) {
        mFirstPosition = 0;
    }
    return fillDown(mFirstPosition, nextTop);
}

private View fillDown(int pos, int nextTop) {
    View selectedView = null;

    int end = (mBottom - mTop);
    if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
        end -= mListPadding.bottom;
    }

    // ↓↓↓ 3.2 循环计算子布局位置, 然后调用makeAndAddView()
    while (nextTop < end && pos < mItemCount) {
        // is this the selected item?
        boolean selected = pos == mSelectedPosition;
        View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);

        nextTop = child.getBottom() + mDividerHeight;
        if (selected) {
            selectedView = child;
        }
        pos++;
    }

    setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
    return selectedView;
}

private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
        boolean selected) {
    View child;


    if (!mDataChanged) {
        // Try to use an existing view for this position
        // ↓↓↓ 3.3 从RecycleBin里获取一个缓存的View, 但是缓存为空, 返回null
        child = mRecycler.getActiveView(position);
        if (child != null) {
            // Found it -- we're using an existing child
            // This just needs to be positioned
            setupChild(child, position, y, flow, childrenLeft, selected, true);

            return child;
        }
    }

    // Make a new view for this position, or convert an unused view if possible
    // ↓↓↓ 3.4 然而从RecycleBin里获取缓存的View失败, 则试图通过obtainView()方法获取一个View
    child = obtainView(position, mIsScrap);

    // This needs to be positioned and measured
    // ↓↓↓ 3.7 将适配器获得的View, 交给该方法
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

    return child;
}

View obtainView(int position, boolean[] isScrap) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

    isScrap[0] = false;

    // ...

    // ↓↓↓ 3.5 从RecycleBin里获取一个缓存的废弃的View, 现在RecycleBin里没有缓存任何View, 所以返回null
    final View scrapView = mRecycler.getScrapView(position);
    // ↓↓↓ 3.6 并且从适配器里获取一个View, 这个只要我们给了数据, 一定能获取到
    final View child = mAdapter.getView(position, scrapView, this);
    if (scrapView != null) {
        if (child != scrapView) {
            // Failed to re-bind the data, return scrap to the heap.
            mRecycler.addScrapView(scrapView, position);
        } else {
            if (child.isTemporarilyDetached()) {
                isScrap[0] = true;

                // Finish the temporary detach started in addScrapView().
                child.dispatchFinishTemporaryDetach();
            } else {
                // we set isScrap to "true" only if the view is temporarily detached.
                // if the view is fully detached, it is as good as a view created by the
                // adapter
                isScrap[0] = false;
            }

        }
    }

    if (mCacheColorHint != 0) {
        child.setDrawingCacheBackgroundColor(mCacheColorHint);
    }

    if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
        child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
    }

    setItemViewLayoutParams(child, position);

    if (AccessibilityManager.getInstance(mContext).isEnabled()) {
        if (mAccessibilityDelegate == null) {
            mAccessibilityDelegate = new ListItemAccessibilityDelegate();
        }
        if (child.getAccessibilityDelegate() == null) {
            child.setAccessibilityDelegate(mAccessibilityDelegate);
        }
    }

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);

    return child;
}

private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");

    final boolean isSelected = selected && shouldShowSelector();
    final boolean updateChildSelected = isSelected != child.isSelected();
    final int mode = mTouchMode;
    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position;
    final boolean updateChildPressed = isPressed != child.isPressed();
    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();

    // Respect layout params that are already in the view. Otherwise make some up...
    // noinspection unchecked
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
    if (p == null) {
        p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
    }
    p.viewType = mAdapter.getItemViewType(position);
    p.isEnabled = mAdapter.isEnabled(position);

    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
        attachViewToParent(child, flowDown ? -1 : 0, p);
    } else {
        p.forceAdd = false;
        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
            p.recycledHeaderFooter = true;
        }
        // ↓↓↓ 3.8 把获得的View添加到ListView中, 这样就完成了一次onLayout的加载
        addViewInLayout(child, flowDown ? -1 : 0, p, true);
    }

    if (updateChildSelected) {
        child.setSelected(isSelected);
    }

    if (updateChildPressed) {
        child.setPressed(isPressed);
    }

    if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
        if (child instanceof Checkable) {
            ((Checkable) child).setChecked(mCheckStates.get(position));
        } else if (getContext().getApplicationInfo().targetSdkVersion
                >= android.os.Build.VERSION_CODES.HONEYCOMB) {
            child.setActivated(mCheckStates.get(position));
        }
    }

    if (needToMeasure) {
        final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
                mListPadding.left + mListPadding.right, p.width);
        final int lpHeight = p.height;
        final int childHeightSpec;
        if (lpHeight > 0) {
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
        } else {
            childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
                    MeasureSpec.UNSPECIFIED);
        }
        child.measure(childWidthSpec, childHeightSpec);
    } else {
        cleanupLayoutState(child);
    }

    final int w = child.getMeasuredWidth();
    final int h = child.getMeasuredHeight();
    final int childTop = flowDown ? y : y - h;

    if (needToMeasure) {
        final int childRight = childrenLeft + w;
        final int childBottom = childTop + h;
        child.layout(childrenLeft, childTop, childRight, childBottom);
    } else {
        child.offsetLeftAndRight(childrenLeft - child.getLeft());
        child.offsetTopAndBottom(childTop - child.getTop());
    }

    if (mCachingStarted && !child.isDrawingCacheEnabled()) {
        child.setDrawingCacheEnabled(true);
    }

    if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition)
            != position) {
        child.jumpDrawablesToCurrentState();
    }

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}

第一次onLayout, 由于第一次, 所以屏幕上和RecycleBin的缓存里并没有View, 只有通过适配器获取View, 直至将屏幕填满.

接下来我们来看看第二次onLayout, 与第一次的差别是, 这一次屏幕上已经有View了

protected void onLayout(boolean changed, int l, int t, int r, int b) {
    super.onLayout(changed, l, t, r, b);

    mInLayout = true;

    final int childCount = getChildCount();
    if (changed) {
        for (int i = 0; i < childCount; i++) {
            getChildAt(i).forceLayout();
        }
        mRecycler.markChildrenDirty();
    }

    // ↓↓↓ 1.第二次的onLayout执行还是从这里开始
    layoutChildren();
    mInLayout = false;

    mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;

    // TODO: Move somewhere sane. This doesn't belong in onLayout().
    if (mFastScroll != null) {
        mFastScroll.onItemCountChanged(getChildCount(), mItemCount);
    }
}

protected void layoutChildren() {
    final boolean blockLayoutRequests = mBlockLayoutRequests;
    if (blockLayoutRequests) {
        return;
    }

    mBlockLayoutRequests = true;

    try {
        super.layoutChildren();

        invalidate();

        if (mAdapter == null) {
            resetList();
            invokeOnItemScrollListener();
            return;
        }

        final int childrenTop = mListPadding.top;
        final int childrenBottom = mBottom - mTop - mListPadding.bottom;
        // 2. 获取子View的数量, 由于屏幕已经填充了View, 所以这里返回的数字 > 0
        final int childCount = getChildCount();

        int index = 0;
        int delta = 0;

        View sel;
        View oldSel = null;
        View oldFirst = null;
        View newSel = null;

        // Remember stuff we will need down below
        switch (mLayoutMode) {
        case LAYOUT_SET_SELECTION:
            index = mNextSelectedPosition - mFirstPosition;
            if (index >= 0 && index < childCount) {
                newSel = getChildAt(index);
            }
            break;
        case LAYOUT_FORCE_TOP:
        case LAYOUT_FORCE_BOTTOM:
        case LAYOUT_SPECIFIC:
        case LAYOUT_SYNC:
            break;
        case LAYOUT_MOVE_SELECTION:
        default:
            // Remember the previously selected view
            index = mSelectedPosition - mFirstPosition;
            if (index >= 0 && index < childCount) {
                oldSel = getChildAt(index);
            }

            // Remember the previous first child
            oldFirst = getChildAt(0);

            if (mNextSelectedPosition >= 0) {
                delta = mNextSelectedPosition - mSelectedPosition;
            }

            // Caution: newSel might be null
            newSel = getChildAt(index + delta);
        }


        boolean dataChanged = mDataChanged;
        if (dataChanged) {
            handleDataChanged();
        }

        // Handle the empty set by removing all views that are visible
        // and calling it a day
        if (mItemCount == 0) {
            resetList();
            invokeOnItemScrollListener();
            return;
        } else if (mItemCount != mAdapter.getCount()) {
            throw new IllegalStateException("The content of the adapter has changed but "
                    + "ListView did not receive a notification. Make sure the content of "
                    + "your adapter is not modified from a background thread, but only from "
                    + "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
                    + "when its content changes. [in ListView(" + getId() + ", " + getClass()
                    + ") with Adapter(" + mAdapter.getClass() + ")]");
        }

        setSelectedPositionInt(mNextSelectedPosition);

        AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
        View accessibilityFocusLayoutRestoreView = null;
        int accessibilityFocusPosition = INVALID_POSITION;

        // Remember which child, if any, had accessibility focus. This must
        // occur before recycling any views, since that will clear
        // accessibility focus.
        final ViewRootImpl viewRootImpl = getViewRootImpl();
        if (viewRootImpl != null) {
            final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
            if (focusHost != null) {
                final View focusChild = getAccessibilityFocusedChild(focusHost);
                if (focusChild != null) {
                    if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
                            || focusChild.hasTransientState() || mAdapterHasStableIds) {
                        // The views won't be changing, so try to maintain
                        // focus on the current host and virtual view.
                        accessibilityFocusLayoutRestoreView = focusHost;
                        accessibilityFocusLayoutRestoreNode = viewRootImpl
                                .getAccessibilityFocusedVirtualView();
                    }

                    // If all else fails, maintain focus at the same
                    // position.
                    accessibilityFocusPosition = getPositionForView(focusChild);
                }
            }
        }

        View focusLayoutRestoreDirectChild = null;
        View focusLayoutRestoreView = null;

        // Take focus back to us temporarily to avoid the eventual call to
        // clear focus when removing the focused child below from messing
        // things up when ViewAncestor assigns focus back to someone else.
        final View focusedChild = getFocusedChild();
        if (focusedChild != null) {
            // TODO: in some cases focusedChild.getParent() == null

            // We can remember the focused view to restore after re-layout
            // if the data hasn't changed, or if the focused position is a
            // header or footer.
            if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
                    || focusedChild.hasTransientState() || mAdapterHasStableIds) {
                focusLayoutRestoreDirectChild = focusedChild;
                // Remember the specific view that had focus.
                focusLayoutRestoreView = findFocus();
                if (focusLayoutRestoreView != null) {
                    // Tell it we are going to mess with it.
                    focusLayoutRestoreView.dispatchStartTemporaryDetach();
                }
            }
            requestFocus();
        }

        // Pull all children into the RecycleBin.
        // These views will be reused if possible
        final int firstPosition = mFirstPosition;
        final RecycleBin recycleBin = mRecycler;
        if (dataChanged) {
            for (int i = 0; i < childCount; i++) {
                recycleBin.addScrapView(getChildAt(i), firstPosition+i);
            }
        } else {
            // ↓↓↓ 3. 将屏幕上的View全部添加到RecycleBin里
            recycleBin.fillActiveViews(childCount, firstPosition);
        }

        // Clear out old views
        // ↓↓↓ 4. 从父容器中清除所有的view
        detachAllViewsFromParent();
        recycleBin.removeSkippedScrap();

        switch (mLayoutMode) {
        case LAYOUT_SET_SELECTION:
            if (newSel != null) {
                sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
            } else {
                sel = fillFromMiddle(childrenTop, childrenBottom);
            }
            break;
        case LAYOUT_SYNC:
            sel = fillSpecific(mSyncPosition, mSpecificTop);
            break;
        case LAYOUT_FORCE_BOTTOM:
            sel = fillUp(mItemCount - 1, childrenBottom);
            adjustViewsUpOrDown();
            break;
        case LAYOUT_FORCE_TOP:
            mFirstPosition = 0;
            sel = fillFromTop(childrenTop);
            adjustViewsUpOrDown();
            break;
        case LAYOUT_SPECIFIC:
            final int selectedPosition = reconcileSelectedPosition();
            sel = fillSpecific(selectedPosition, mSpecificTop);
            /**
             * When ListView is resized, FocusSelector requests an async selection for the
             * previously focused item to make sure it is still visible. If the item is not
             * selectable, it won't regain focus so instead we call FocusSelector
             * to directly request focus on the view after it is visible.
             */
            if (sel == null && mFocusSelector != null) {
                final Runnable focusRunnable = mFocusSelector
                        .setupFocusIfValid(selectedPosition);
                if (focusRunnable != null) {
                    post(focusRunnable);
                }
            }
            break;
        case LAYOUT_MOVE_SELECTION:
            sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
            break;
        default:
            // ↓↓↓ 5. 这里的childCount > 0
            if (childCount == 0) {
                if (!mStackFromBottom) {
                    final int position = lookForSelectablePosition(0, true);
                    setSelectedPositionInt(position);
                    sel = fillFromTop(childrenTop);
                } else {
                    final int position = lookForSelectablePosition(mItemCount - 1, false);
                    setSelectedPositionInt(position);
                    sel = fillUp(mItemCount - 1, childrenBottom);
                }
            } else {
                // ↓↓↓ 6. 所以执行以下判断代码
                // ↓↓↓ 6.1 由于是新加载的, 所以不会有被选择的条目, 所以 mSelectedPosition = -1, 改判断不成立
                if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
                    sel = fillSpecific(mSelectedPosition,
                            oldSel == null ? childrenTop : oldSel.getTop());
                // ↓↓↓ 6.2 所以这 mFirstPosition = 0, mItemCount >= 0, 判断成立
                } else if (mFirstPosition < mItemCount) {
                    // ↓↓↓ 7. 可见该方法会去获取缓存View,然后优先填充指定位置的View, 然后填满其他位置的View, 第二次onLayout结束
                    sel = fillSpecific(mFirstPosition,
                            oldFirst == null ? childrenTop : oldFirst.getTop());
                } else {
                    sel = fillSpecific(0, childrenTop);
                }
            }
            break;
        }

        // Flush any cached views that did not get reused above
        recycleBin.scrapActiveViews();

        // remove any header/footer that has been temp detached and not re-attached
        removeUnusedFixedViews(mHeaderViewInfos);
        removeUnusedFixedViews(mFooterViewInfos);

        if (sel != null) {
            // The current selected item should get focus if items are
            // focusable.
            if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
                final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
                        focusLayoutRestoreView != null &&
                        focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
                if (!focusWasTaken) {
                    // Selected item didn't take focus, but we still want to
                    // make sure something else outside of the selected view
                    // has focus.
                    final View focused = getFocusedChild();
                    if (focused != null) {
                        focused.clearFocus();
                    }
                    positionSelector(INVALID_POSITION, sel);
                } else {
                    sel.setSelected(false);
                    mSelectorRect.setEmpty();
                }
            } else {
                positionSelector(INVALID_POSITION, sel);
            }
            mSelectedTop = sel.getTop();
        } else {
            final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
                    || mTouchMode == TOUCH_MODE_DONE_WAITING;
            if (inTouchMode) {
                // If the user's finger is down, select the motion position.
                final View child = getChildAt(mMotionPosition - mFirstPosition);
                if (child != null) {
                    positionSelector(mMotionPosition, child);
                }
            } else if (mSelectorPosition != INVALID_POSITION) {
                // If we had previously positioned the selector somewhere,
                // put it back there. It might not match up with the data,
                // but it's transitioning out so it's not a big deal.
                final View child = getChildAt(mSelectorPosition - mFirstPosition);
                if (child != null) {
                    positionSelector(mSelectorPosition, child);
                }
            } else {
                // Otherwise, clear selection.
                mSelectedTop = 0;
                mSelectorRect.setEmpty();
            }

            // Even if there is not selected position, we may need to
            // restore focus (i.e. something focusable in touch mode).
            if (hasFocus() && focusLayoutRestoreView != null) {
                focusLayoutRestoreView.requestFocus();
            }
        }

        // Attempt to restore accessibility focus, if necessary.
        if (viewRootImpl != null) {
            final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost();
            if (newAccessibilityFocusedView == null) {
                if (accessibilityFocusLayoutRestoreView != null
                        && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) {
                    final AccessibilityNodeProvider provider =
                            accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider();
                    if (accessibilityFocusLayoutRestoreNode != null && provider != null) {
                        final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId(
                                accessibilityFocusLayoutRestoreNode.getSourceNodeId());
                        provider.performAction(virtualViewId,
                                AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null);
                    } else {
                        accessibilityFocusLayoutRestoreView.requestAccessibilityFocus();
                    }
                } else if (accessibilityFocusPosition != INVALID_POSITION) {
                    // Bound the position within the visible children.
                    final int position = MathUtils.constrain(
                            accessibilityFocusPosition - mFirstPosition, 0,
                            getChildCount() - 1);
                    final View restoreView = getChildAt(position);
                    if (restoreView != null) {
                        restoreView.requestAccessibilityFocus();
                    }
                }
            }
        }

        // Tell focus view we are done mucking with it, if it is still in
        // our view hierarchy.
        if (focusLayoutRestoreView != null
                && focusLayoutRestoreView.getWindowToken() != null) {
            focusLayoutRestoreView.dispatchFinishTemporaryDetach();
        }

        mLayoutMode = LAYOUT_NORMAL;
        mDataChanged = false;
        if (mPositionScrollAfterLayout != null) {
            post(mPositionScrollAfterLayout);
            mPositionScrollAfterLayout = null;
        }
        mNeedSync = false;
        setNextSelectedPositionInt(mSelectedPosition);

        updateScrollIndicators();

        if (mItemCount > 0) {
            checkSelectionChanged();
        }

        invokeOnItemScrollListener();
    } finally {
        if (mFocusSelector != null) {
            mFocusSelector.onLayoutComplete();
        }
        if (!blockLayoutRequests) {
            mBlockLayoutRequests = false;
        }
    }
}

private View fillSpecific(int position, int top) {
    boolean tempIsSelected = position == mSelectedPosition;
    // ↓↓↓ 6.3 获取缓存View并显示
    View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
    // Possibly changed again in fillUp if we add rows above this one.
    mFirstPosition = position;

    View above;
    View below;

    final int dividerHeight = mDividerHeight;
    if (!mStackFromBottom) {
        // ↓↓↓ 6.8 向上填满view
        above = fillUp(position - 1, temp.getTop() - dividerHeight);
        // This will correct for the top of the first view not touching the top of the list
        adjustViewsUpOrDown();
        // ↓↓↓ 6.9 向下填满view
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);
        int childCount = getChildCount();
        if (childCount > 0) {
            correctTooHigh(childCount);
        }
    } else {
        below = fillDown(position + 1, temp.getBottom() + dividerHeight);
        // This will correct for the bottom of the last view not touching the bottom of the list
        adjustViewsUpOrDown();
        above = fillUp(position - 1, temp.getTop() - dividerHeight);
        int childCount = getChildCount();
        if (childCount > 0) {
             correctTooLow(childCount);
        }
    }

    if (tempIsSelected) {
        return temp;
    } else if (above != null) {
        return above;
    } else {
        return below;
    }
}

private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) {
    View child;

    if (!mDataChanged) {
        // Try to use an existing view for this position
        // ↓↓↓ 6.4 从RecycleBin里获取缓存的View
        child = mRecycler.getActiveView(position);
        if (child != null) {
            // Found it -- we're using an existing child
            // This just needs to be positioned
            // ↓↓↓ 6.5 获取到view后执行该方法
            setupChild(child, position, y, flow, childrenLeft, selected, true);

            return child;
        }
    }

    // Make a new view for this position, or convert an unused view if possible
    child = obtainView(position, mIsScrap);

    // This needs to be positioned and measured
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

    return child;
}

private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");

    final boolean isSelected = selected && shouldShowSelector();
    final boolean updateChildSelected = isSelected != child.isSelected();
    final int mode = mTouchMode;
    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
            mMotionPosition == position;
    final boolean updateChildPressed = isPressed != child.isPressed();
    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();

    // Respect layout params that are already in the view. Otherwise make some up...
    // noinspection unchecked
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
    if (p == null) {
        p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
    }
    p.viewType = mAdapter.getItemViewType(position);
    p.isEnabled = mAdapter.isEnabled(position);

    // ↓↓↓ 6.6 recycled = true 这是参数传进来的, 第一次onLayout时p.forceAdd被标记为false. (p.forceAdd = false;)
    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter
            && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
        // ↓↓↓ 6.7 因此执行这句, 让所有View都处于attach状态, 即显示
        attachViewToParent(child, flowDown ? -1 : 0, p);
    } else {
        p.forceAdd = false;
        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
            p.recycledHeaderFooter = true;
        }
        addViewInLayout(child, flowDown ? -1 : 0, p, true);
    }

    if (updateChildSelected) {
        child.setSelected(isSelected);
    }

    if (updateChildPressed) {
        child.setPressed(isPressed);
    }

    if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
        if (child instanceof Checkable) {
            ((Checkable) child).setChecked(mCheckStates.get(position));
        } else if (getContext().getApplicationInfo().targetSdkVersion
                >= android.os.Build.VERSION_CODES.HONEYCOMB) {
            child.setActivated(mCheckStates.get(position));
        }
    }

    if (needToMeasure) {
        final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
                mListPadding.left + mListPadding.right, p.width);
        final int lpHeight = p.height;
        final int childHeightSpec;
        if (lpHeight > 0) {
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
        } else {
            childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
                    MeasureSpec.UNSPECIFIED);
        }
        child.measure(childWidthSpec, childHeightSpec);
    } else {
        cleanupLayoutState(child);
    }

    final int w = child.getMeasuredWidth();
    final int h = child.getMeasuredHeight();
    final int childTop = flowDown ? y : y - h;

    if (needToMeasure) {
        final int childRight = childrenLeft + w;
        final int childBottom = childTop + h;
        child.layout(childrenLeft, childTop, childRight, childBottom);
    } else {
        child.offsetLeftAndRight(childrenLeft - child.getLeft());
        child.offsetTopAndBottom(childTop - child.getTop());
    }

    if (mCachingStarted && !child.isDrawingCacheEnabled()) {
        child.setDrawingCacheEnabled(true);
    }

    if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition)
            != position) {
        child.jumpDrawablesToCurrentState();
    }

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}

第二次onLayout主要做的是: 将屏幕上的View全部缓存到RecycleBin里, 然后将他们从父容器中清除(detach);
然后从RecycleBin里获取缓存的View, 并将他们添加到父容器(attach), 然后依次向上, 向下填充满屏幕
另外 attachViewToParent 和 detachAllViewsFromParent 是组合使用的

ListView初始化过程图

ListView滑动加载逻辑

两次的onLayout只是初始化了第一屏的view, 当我们手指上下滑的时候, ListView会重复利用滑出屏幕的条目, 这才是ListView的神奇之处. 想要了解滑动事件是如何处理的, 那么就要从 onTouchEvent() 入手.

public boolean onTouchEvent(MotionEvent ev) {
    if (!isEnabled()) {
        // A disabled view that is clickable still consumes the touch
        // events, it just doesn't respond to them.
        return isClickable() || isLongClickable();
    }

    if (mPositionScroller != null) {
        mPositionScroller.stop();
    }

    if (mIsDetaching || !isAttachedToWindow()) {
        // Something isn't right.
        // Since we rely on being attached to get data set change notifications,
        // don't risk doing anything where we might try to resync and find things
        // in a bogus state.
        return false;
    }

    startNestedScroll(SCROLL_AXIS_VERTICAL);

    if (mFastScroll != null && mFastScroll.onTouchEvent(ev)) {
        return true;
    }

    initVelocityTrackerIfNotExists();
    final MotionEvent vtev = MotionEvent.obtain(ev);

    final int actionMasked = ev.getActionMasked();
    if (actionMasked == MotionEvent.ACTION_DOWN) {
        mNestedYOffset = 0;
    }
    vtev.offsetLocation(0, mNestedYOffset);
    switch (actionMasked) {
        case MotionEvent.ACTION_DOWN: {
            onTouchDown(ev);
            break;
        }

        // ↓↓↓ 1. 各种的触摸事件, 我们只需关心移动事件Move即可
        case MotionEvent.ACTION_MOVE: {
            onTouchMove(ev, vtev);
            break;
        }

        case MotionEvent.ACTION_UP: {
            onTouchUp(ev);
            break;
        }

        case MotionEvent.ACTION_CANCEL: {
            onTouchCancel();
            break;
        }

        case MotionEvent.ACTION_POINTER_UP: {
            onSecondaryPointerUp(ev);
            final int x = mMotionX;
            final int y = mMotionY;
            final int motionPosition = pointToPosition(x, y);
            if (motionPosition >= 0) {
                // Remember where the motion event started
                final View child = getChildAt(motionPosition - mFirstPosition);
                mMotionViewOriginalTop = child.getTop();
                mMotionPosition = motionPosition;
            }
            mLastY = y;
            break;
        }

        case MotionEvent.ACTION_POINTER_DOWN: {
            // New pointers take over dragging duties
            final int index = ev.getActionIndex();
            final int id = ev.getPointerId(index);
            final int x = (int) ev.getX(index);
            final int y = (int) ev.getY(index);
            mMotionCorrection = 0;
            mActivePointerId = id;
            mMotionX = x;
            mMotionY = y;
            final int motionPosition = pointToPosition(x, y);
            if (motionPosition >= 0) {
                // Remember where the motion event started
                final View child = getChildAt(motionPosition - mFirstPosition);
                mMotionViewOriginalTop = child.getTop();
                mMotionPosition = motionPosition;
            }
            mLastY = y;
            break;
        }
    }

    if (mVelocityTracker != null) {
        mVelocityTracker.addMovement(vtev);
    }
    vtev.recycle();
    return true;
}

private void onTouchMove(MotionEvent ev, MotionEvent vtev) {
    if (mHasPerformedLongPress) {
        // Consume all move events following a successful long press.
        return;
    }

    int pointerIndex = ev.findPointerIndex(mActivePointerId);
    if (pointerIndex == -1) {
        pointerIndex = 0;
        mActivePointerId = ev.getPointerId(pointerIndex);
    }

    if (mDataChanged) {
        // Re-sync everything if data has been changed
        // since the scroll operation can query the adapter.
        layoutChildren();
    }

    final int y = (int) ev.getY(pointerIndex);

    // ↓↓↓ 2. 这里的 mTouchMode = TOUCH_MODE_SCROLL
    switch (mTouchMode) {
        case TOUCH_MODE_DOWN:
        case TOUCH_MODE_TAP:
        case TOUCH_MODE_DONE_WAITING:
            // Check if we have moved far enough that it looks more like a
            // scroll than a tap. If so, we'll enter scrolling mode.
            if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) {
                break;
            }
            // Otherwise, check containment within list bounds. If we're
            // outside bounds, cancel any active presses.
            final View motionView = getChildAt(mMotionPosition - mFirstPosition);
            final float x = ev.getX(pointerIndex);
            if (!pointInView(x, y, mTouchSlop)) {
                setPressed(false);
                if (motionView != null) {
                    motionView.setPressed(false);
                }
                removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
                        mPendingCheckForTap : mPendingCheckForLongPress);
                mTouchMode = TOUCH_MODE_DONE_WAITING;
                updateSelectorState();
            } else if (motionView != null) {
                // Still within bounds, update the hotspot.
                final float[] point = mTmpPoint;
                point[0] = x;
                point[1] = y;
                transformPointToViewLocal(point, motionView);
                motionView.drawableHotspotChanged(point[0], point[1]);
            }
            break;
        case TOUCH_MODE_SCROLL:
        case TOUCH_MODE_OVERSCROLL:
            // ↓↓↓ 3. 于是也就执行该分支的代码
            scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev);
            break;
    }
}

private void scrollIfNeeded(int x, int y, MotionEvent vtev) {
    int rawDeltaY = y - mMotionY;
    int scrollOffsetCorrection = 0;
    int scrollConsumedCorrection = 0;
    if (mLastY == Integer.MIN_VALUE) {
        rawDeltaY -= mMotionCorrection;
    }
    if (dispatchNestedPreScroll(0, mLastY != Integer.MIN_VALUE ? mLastY - y : -rawDeltaY, mScrollConsumed, mScrollOffset)) {
        rawDeltaY += mScrollConsumed[1];
        scrollOffsetCorrection = -mScrollOffset[1];
        scrollConsumedCorrection = mScrollConsumed[1];
        if (vtev != null) {
            vtev.offsetLocation(0, mScrollOffset[1]);
            mNestedYOffset += mScrollOffset[1];
        }
    }
    final int deltaY = rawDeltaY;
    int incrementalDeltaY =
            mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY;
    int lastYCorrection = 0;

    if (mTouchMode == TOUCH_MODE_SCROLL) {
        if (PROFILE_SCROLLING) {
            if (!mScrollProfilingStarted) {
                Debug.startMethodTracing("AbsListViewScroll");
                mScrollProfilingStarted = true;
            }
        }

        if (mScrollStrictSpan == null) {
            // If it's non-null, we're already in a scroll.
            mScrollStrictSpan = StrictMode.enterCriticalSpan("AbsListView-scroll");
        }

        if (y != mLastY) {
            // We may be here after stopping a fling and continuing to scroll.
            // If so, we haven't disallowed intercepting touch events yet.
            // Make sure that we do so in case we're in a parent that can intercept.
            if ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0 && Math.abs(rawDeltaY) > mTouchSlop) {
                final ViewParent parent = getParent();
                if (parent != null) {
                    parent.requestDisallowInterceptTouchEvent(true);
                }
            }

            final int motionIndex;
            if (mMotionPosition >= 0) {
                motionIndex = mMotionPosition - mFirstPosition;
            } else {
                // If we don't have a motion position that we can reliably track,
                // pick something in the middle to make a best guess at things below.
                motionIndex = getChildCount() / 2;
            }

            int motionViewPrevTop = 0;
            View motionView = this.getChildAt(motionIndex);
            if (motionView != null) {
                motionViewPrevTop = motionView.getTop();
            }

            // No need to do all this work if we're not going to move anyway
            boolean atEdge = false;
            // ↓↓↓ 4. 如果移动增量不为0, 说明用户进行了移动操作, 执行该方法
            if (incrementalDeltaY != 0) {
                // ↓↓↓ 4.1 执行该方法, 传递按下的Y坐标, 和Y轴移动增量
                atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
            }

            // Check to see if we have bumped into the scroll limit
            motionView = this.getChildAt(motionIndex);
            if (motionView != null) {
                // Check if the top of the motion view is where it is
                // supposed to be
                final int motionViewRealTop = motionView.getTop();
                if (atEdge) {
                    // Apply overscroll

                    int overscroll = -incrementalDeltaY - (motionViewRealTop - motionViewPrevTop);
                    if (dispatchNestedScroll(0, overscroll - incrementalDeltaY, 0, overscroll, mScrollOffset)) {
                        lastYCorrection -= mScrollOffset[1];
                        if (vtev != null) {
                            vtev.offsetLocation(0, mScrollOffset[1]);
                            mNestedYOffset += mScrollOffset[1];
                        }
                    } else {
                        final boolean atOverscrollEdge = overScrollBy(0, overscroll, 0, mScrollY, 0, 0, 0, mOverscrollDistance, true);

                        if (atOverscrollEdge && mVelocityTracker != null) {
                            // Don't allow overfling if we're at the edge
                            mVelocityTracker.clear();
                        }

                        final int overscrollMode = getOverScrollMode();
                        if (overscrollMode == OVER_SCROLL_ALWAYS ||
                                (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && !contentFits())) {
                            if (!atOverscrollEdge) {
                                mDirection = 0; // Reset when entering overscroll.
                                mTouchMode = TOUCH_MODE_OVERSCROLL;
                            }
                            if (incrementalDeltaY > 0) {
                                mEdgeGlowTop.onPull((float) -overscroll / getHeight(), (float) x / getWidth());
                                if (!mEdgeGlowBottom.isFinished()) {
                                    mEdgeGlowBottom.onRelease();
                                }
                                invalidateTopGlow();
                            } else if (incrementalDeltaY < 0) {
                                mEdgeGlowBottom.onPull((float) overscroll / getHeight(), 1.f - (float) x / getWidth());
                                if (!mEdgeGlowTop.isFinished()) {
                                    mEdgeGlowTop.onRelease();
                                }
                                invalidateBottomGlow();
                            }
                        }
                    }
                }
                mMotionY = y + lastYCorrection + scrollOffsetCorrection;
            }
            mLastY = y + lastYCorrection + scrollOffsetCorrection;
        }
    } else if (mTouchMode == TOUCH_MODE_OVERSCROLL) {
        // ...
    }
}

boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
    final int childCount = getChildCount();
    if (childCount == 0) {
        return true;
    }

    final int firstTop = getChildAt(0).getTop();
    final int lastBottom = getChildAt(childCount - 1).getBottom();

    final Rect listPadding = mListPadding;

    // "effective padding" In this case is the amount of padding that affects
    // how much space should not be filled by items. If we don't clip to padding
    // there is no effective padding.
    int effectivePaddingTop = 0;
    int effectivePaddingBottom = 0;
    if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
        effectivePaddingTop = listPadding.top;
        effectivePaddingBottom = listPadding.bottom;
    }

     // FIXME account for grid vertical spacing too?
    final int spaceAbove = effectivePaddingTop - firstTop;
    final int end = getHeight() - effectivePaddingBottom;
    final int spaceBelow = lastBottom - end;

    final int height = getHeight() - mPaddingBottom - mPaddingTop;
    if (deltaY < 0) {
        deltaY = Math.max(-(height - 1), deltaY);
    } else {
        deltaY = Math.min(height - 1, deltaY);
    }

    if (incrementalDeltaY < 0) {
        incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
    } else {
        incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
    }

    final int firstPosition = mFirstPosition;

    // Update our guesses for where the first and last views are
    if (firstPosition == 0) {
        mFirstPositionDistanceGuess = firstTop - listPadding.top;
    } else {
        mFirstPositionDistanceGuess += incrementalDeltaY;
    }
    if (firstPosition + childCount == mItemCount) {
        mLastPositionDistanceGuess = lastBottom + listPadding.bottom;
    } else {
        mLastPositionDistanceGuess += incrementalDeltaY;
    }

    final boolean cannotScrollDown = (firstPosition == 0 && firstTop >= listPadding.top && incrementalDeltaY >= 0);
    final boolean cannotScrollUp = (firstPosition + childCount == mItemCount && lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0);

    if (cannotScrollDown || cannotScrollUp) {
        return incrementalDeltaY != 0;
    }

    // ↓↓↓ 5. 当 incrementalDeltaY < 0 时, 表示向下滑动, 否则为向上滑动
    final boolean down = incrementalDeltaY < 0;

    final boolean inTouchMode = isInTouchMode();
    if (inTouchMode) {
        hideSelector();
    }

    final int headerViewsCount = getHeaderViewsCount();
    final int footerViewsStart = mItemCount - getFooterViewsCount();

    int start = 0;
    int count = 0;

    // ↓↓↓ 6. 当向下滑动时
    if (down) {
        int top = -incrementalDeltaY;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            top += listPadding.top;
        }
        for (int i = 0; i < childCount; i++) {
            // ↓↓↓ 6.1 获取子View
            final View child = getChildAt(i);
            if (child.getBottom() >= top) {
                break;
            } else {
                // ↓↓↓ 6.2 如果子View的bottom < top 时, 则说明子View已经滑出屏幕了
                count++;
                int position = firstPosition + i;
                if (position >= headerViewsCount && position < footerViewsStart) {
                    // The view will be rebound to new data, clear any
                    // system-managed transient state.
                    child.clearAccessibilityFocus();
                    // ↓↓↓ 6.3 将其添加到RecycleBin的废弃View里, 并count++
                    mRecycler.addScrapView(child, position);
                }
            }
        }
    } else {
        int bottom = getHeight() - incrementalDeltaY;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            bottom -= listPadding.bottom;
        }
        for (int i = childCount - 1; i >= 0; i--) {
            final View child = getChildAt(i);
            if (child.getTop() <= bottom) {
                break;
            } else {
                start = i;
                count++;
                int position = firstPosition + i;
                if (position >= headerViewsCount && position < footerViewsStart) {
                    // The view will be rebound to new data, clear any
                    // system-managed transient state.
                    child.clearAccessibilityFocus();
                    mRecycler.addScrapView(child, position);
                }
            }
        }
    }

    mMotionViewNewTop = mMotionViewOriginalTop + deltaY;

    mBlockLayoutRequests = true;

    if (count > 0) {
        // ↓↓↓ 7. 如果有需要被回收的View, 则将这些View从父容器中清除 (detach)
        detachViewsFromParent(start, count);
        mRecycler.removeSkippedScrap();
    }

    // invalidate before moving the children to avoid unnecessary invalidate
    // calls to bubble up from the children all the way to the top
    if (!awakenScrollBars()) {
       invalidate();
    }

    // ↓↓↓ 8. 然后根据手指滑动的增量incrementalDeltaY, 移动这些View, 也就产生了listview里的itemView的移动效果
    offsetChildrenTopAndBottom(incrementalDeltaY);

    if (down) {
        mFirstPosition += count;
    }

    final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
    // ↓↓↓ 9. 如果一个View的底部移入了屏幕, 或者 一个View的顶部移入了屏幕, 则执行该方法
    if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
        fillGap(down);
    }

    mRecycler.fullyDetachScrapViews();
    if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
        final int childIndex = mSelectedPosition - mFirstPosition;
        if (childIndex >= 0 && childIndex < getChildCount()) {
            positionSelector(mSelectedPosition, getChildAt(childIndex));
        }
    } else if (mSelectorPosition != INVALID_POSITION) {
        final int childIndex = mSelectorPosition - mFirstPosition;
        if (childIndex >= 0 && childIndex < getChildCount()) {
            positionSelector(INVALID_POSITION, getChildAt(childIndex));
        }
    } else {
        mSelectorRect.setEmpty();
    }

    mBlockLayoutRequests = false;

    invokeOnItemScrollListener();

    return false;
}

abstract void fillGap(boolean down);

void fillGap(boolean down) {
    final int count = getChildCount();
    // ↓↓↓ 9.1 如果向下滑
    if (down) {
        int paddingTop = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            paddingTop = getListPaddingTop();
        }
        final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight : paddingTop;
        // ↓↓↓ 9.2 则执行该方法, 来填充View
        fillDown(mFirstPosition + count, startOffset);
        correctTooHigh(getChildCount());
    } else {
        int paddingBottom = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            paddingBottom = getListPaddingBottom();
        }
        final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight : getHeight() - paddingBottom;
        fillUp(mFirstPosition - 1, startOffset);
        correctTooLow(getChildCount());
    }
}

private View fillDown(int pos, int nextTop) {
    // ...

    while (nextTop < end && pos < mItemCount) {
        // is this the selected item?
        boolean selected = pos == mSelectedPosition;
        // ↓↓↓ 9.3 就需要去创建或者获取需要的View
        View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);

        nextTop = child.getBottom() + mDividerHeight;
        if (selected) {
            selectedView = child;
        }
        pos++;
    }

    setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
    return selectedView;
}

private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) {
    View child;

    if (!mDataChanged) {
        // Try to use an existing view for this position
        // ↓↓↓ 9.4 获取缓存的View, 既然View都已经展示在屏幕上了, 那么就没有可用的缓存View 了
        child = mRecycler.getActiveView(position);
        if (child != null) {
            // Found it -- we're using an existing child
            // This just needs to be positioned
            setupChild(child, position, y, flow, childrenLeft, selected, true);

            return child;
        }
    }

    // Make a new view for this position, or convert an unused view if possible
    // ↓↓↓ 9.5 于是就会调用该方法去获取
    child = obtainView(position, mIsScrap);

    // This needs to be positioned and measured
    // ↓↓↓ 9.8 获取到View后, 执行该方法
    setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

    return child;
}

View obtainView(int position, boolean[] isScrap) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

    isScrap[0] = false;

    // ...

    // ↓↓↓ 9.6 获取一个废弃的View, 可能获取到, 可能获取不到
    final View scrapView = mRecycler.getScrapView(position);
    // ↓↓↓ 9.7 从适配器里获取一个View, 这是必然能获取到的, 因为getView参数收了scrapView, 如果该scrapView的不为null, 我们将赋值, 为null, 我们将创建
    final View child = mAdapter.getView(position, scrapView, this);
    if (scrapView != null) {
        if (child != scrapView) {
            // Failed to re-bind the data, return scrap to the heap.
            mRecycler.addScrapView(scrapView, position);
        } else {
            if (child.isTemporarilyDetached()) {
                isScrap[0] = true;

                // Finish the temporary detach started in addScrapView().
                child.dispatchFinishTemporaryDetach();
            } else {
                // we set isScrap to "true" only if the view is temporarily detached.
                // if the view is fully detached, it is as good as a view created by the
                // adapter
                isScrap[0] = false;
            }

        }
    }

    if (mCacheColorHint != 0) {
        child.setDrawingCacheBackgroundColor(mCacheColorHint);
    }

    if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
        child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
    }

    setItemViewLayoutParams(child, position);

    if (AccessibilityManager.getInstance(mContext).isEnabled()) {
        if (mAccessibilityDelegate == null) {
            mAccessibilityDelegate = new ListItemAccessibilityDelegate();
        }
        if (child.getAccessibilityDelegate() == null) {
            child.setAccessibilityDelegate(mAccessibilityDelegate);
        }
    }

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);

    return child;
}

private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");

    final boolean isSelected = selected && shouldShowSelector();
    final boolean updateChildSelected = isSelected != child.isSelected();
    final int mode = mTouchMode;
    final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
            mMotionPosition == position;
    final boolean updateChildPressed = isPressed != child.isPressed();
    final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();

    // Respect layout params that are already in the view. Otherwise make some up...
    // noinspection unchecked
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
    if (p == null) {
        p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
    }
    p.viewType = mAdapter.getItemViewType(position);
    p.isEnabled = mAdapter.isEnabled(position);

    if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
        // 9.9 然后就将View添加到父容器中 (attach), 基本流程就到这里结束了
        attachViewToParent(child, flowDown ? -1 : 0, p);
    } else {
        p.forceAdd = false;
        if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
            p.recycledHeaderFooter = true;
        }
        addViewInLayout(child, flowDown ? -1 : 0, p, true);
    }

    if (updateChildSelected) {
        child.setSelected(isSelected);
    }

    if (updateChildPressed) {
        child.setPressed(isPressed);
    }

    if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
        if (child instanceof Checkable) {
            ((Checkable) child).setChecked(mCheckStates.get(position));
        } else if (getContext().getApplicationInfo().targetSdkVersion
                >= android.os.Build.VERSION_CODES.HONEYCOMB) {
            child.setActivated(mCheckStates.get(position));
        }
    }

    if (needToMeasure) {
        final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
                mListPadding.left + mListPadding.right, p.width);
        final int lpHeight = p.height;
        final int childHeightSpec;
        if (lpHeight > 0) {
            childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
        } else {
            childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
                    MeasureSpec.UNSPECIFIED);
        }
        child.measure(childWidthSpec, childHeightSpec);
    } else {
        cleanupLayoutState(child);
    }

    final int w = child.getMeasuredWidth();
    final int h = child.getMeasuredHeight();
    final int childTop = flowDown ? y : y - h;

    if (needToMeasure) {
        final int childRight = childrenLeft + w;
        final int childBottom = childTop + h;
        child.layout(childrenLeft, childTop, childRight, childBottom);
    } else {
        child.offsetLeftAndRight(childrenLeft - child.getLeft());
        child.offsetTopAndBottom(childTop - child.getTop());
    }

    if (mCachingStarted && !child.isDrawingCacheEnabled()) {
        child.setDrawingCacheEnabled(true);
    }

    if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition)
            != position) {
        child.jumpDrawablesToCurrentState();
    }

    Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}

可见ListView在向下滑动时, 会将移出屏幕的View添加到RecycleBin的废弃View里, 然后将其标记为detach状态, 然后移动屏幕上的View, 让他们产生对应的滑动
同时, 屏幕上方的View会进入屏幕, 首先从RecycleBin里获取缓存View, 获取不到就到Adapter里去获取

ListView滑动过程图

异步加载图片错位解决方案

通过设置Tag的方式解决:
- 方式一:
  - ImageView.setTag(imageUrl); // 对ImageView设置Tag
  - ImageView imageView = (ImageView) ListView.findViewWithTag(imageUrl); // 从ListVIew中寻找Tag,返回不为null说明该ImageView还在展示中,为其设置图片
  - 原理: 由于ImageView是被不断的回收利用的, 每次对同一个ImageView对象设置Tag都会被覆盖, 所以以前的ImageView的Tag是找不着的.

案例:

方式一实现代码:

Adapter代码, 跟往常一样

public class Case1Adapter extends BaseAdapter {
    private LayoutInflater inflater;
    private List<String> mDatas;

    public Case1Adapter(Context context, List<String> datas) {
        this.inflater = LayoutInflater.from(context);
        this.mDatas = datas;
    }

    @Override
    public int getCount() {
        return mDatas == null ? 0 : mDatas.size();
    }

    @Override
    public Object getItem(int i) {
        return mDatas.get(i);
    }

    @Override
    public long getItemId(int i) {
        return i;
    }

    @Override
    public View getView(int i, View convertView, ViewGroup viewGroup) {
        ViewHolder viewHolder;
        if(convertView == null){
            convertView = inflater.inflate(R.layout.item, null);
            viewHolder = new ViewHolder();

            viewHolder.sync = (TextView)convertView.findViewById(R.id.sync);
            viewHolder.async = (TextView)convertView.findViewById(R.id.async);

            convertView.setTag(viewHolder);
        }else{
            viewHolder = (ViewHolder) convertView.getTag();
        }

        viewHolder.sync.setText("当前位置为: " + i);

        if (mDatas.size() != 0) {
            AsyncUtil.ayncLoaderData((ListView) viewGroup, viewHolder.async, mDatas.get(i));
        }

        return  convertView;
    }

    class ViewHolder{
        TextView sync;
        TextView async;
    }
}

这里主要看DataTask线程池里的操作

public class AsyncUtil {

    public static void ayncLoaderData(ListView listView, TextView textView, String s) {
        if (textView == null || s == null) return;

        textView.setEnabled(false);

        new DataTask(listView, textView, s).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
    }

    /**
     * 线程池
     * @author Luzhuo
     */
    private static class DataTask extends AsyncTask<Object, Void, Void>{
        private ListView mListView;
        private TextView mTextView;
        private String mData;

        public DataTask(ListView listView, TextView textView, String s) {
            this.mListView = listView;
            this.mTextView = textView;
            this.mData = s;
        }

        @Override
        protected void onPreExecute() {
            // 设置Tag
            mTextView.setTag(mData);
        }

        /**
         * 设置延迟, 模拟图片的网络请求耗时
         */
        @Override
        protected Void doInBackground(Object... params) {
            SystemClock.sleep(500);
            return null;
        }

        /**
         * 设置文字, 模拟图片设置
         * @param result
         */
        @Override
        protected void onPostExecute(Void result) {
            // 获取有该Tag的View
            TextView tv = (TextView) mListView.findViewWithTag(mData);
            if (tv == null) return;

            mTextView.setText("异步数据: " + mData);
            mTextView.setEnabled(true);
        }
    }
}