ListView、GridView、RecyclerView分析

ListView

列表数据显示需要4个元素，分别是：

• [1] 用来展示列表的控件ListView
• [2] 给列表装配数据的适配器adapter
• [3] 需要展示的数据集
• [4] 数据集展示需要的item布局View

ListView 复用模板

 @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        //有视图缓存，复用视图
        if (convertView == null) {
            convertView = View.inflate(context, R.layout.item, null);//视图获取方法，可以用别的方法
        } else {
            //复用convertView
        }

        //进行数据绑定，返回item的View
        return convertView;
    }

ListView的优化模式
ListView通过Adapter模式、观察者模式、Item View复用机制实现了高效的列表显示。

观察者模式源码分析：

 if (mAdapter != null) {
            mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
            mOldItemCount = mItemCount;
            mItemCount = mAdapter.getCount();
            checkFocus();

            mDataSetObserver = new AdapterDataSetObserver();
            mAdapter.registerDataSetObserver(mDataSetObserver);

            mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

            int position;
            if (mStackFromBottom) {
                position = lookForSelectablePosition(mItemCount - 1, false);
            } else {
                position = lookForSelectablePosition(0, true);
            }
            setSelectedPositionInt(position);
            setNextSelectedPositionInt(position);

            if (mItemCount == 0) {
                // Nothing selected
                checkSelectionChanged();
            }
        } else {
            mAreAllItemsSelectable = true;
            checkFocus();
            // Nothing selected
            checkSelectionChanged();
        }

ListView设置adapter调用方法setAdapter(adapter)，在源码中 mDataSetObserver = new AdapterDataSetObserver();
mAdapter.registerDataSetObserver(mDataSetObserver); 看出观察者应该是AdapterDataSetObserver，但是通过进入更底层的源码分析发现，被观察的对象发生变化应该是AdapterView中onChanged()方法中的requestLayout()。

    class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            //这里是数据源发生改变时，进行更新，通知所有观察者进行相应操作
            requestLayout(); 
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;

            checkFocus();
            requestLayout();
        }

        public void clearSavedState() {
            mInstanceState = null;
        }
    }

所以真正的观察者是AdapterView或者其中ViewGroup的方法requestLayout()，ListView是AdapterView的子类，也可以说ListView是观察者，应用了观察者模式。

GridView

类似ListView。只有布局方式不同，其他机制基本一致。

RecyclerView

导入v7包：implementation ‘com.android.support:appcompat-v7:28.0.0’
代码模板：

public class ARecyclerAdapter<T> extends RecyclerView.Adapter<RecyclerView.ViewHolder> {
    protected List<T> tList;//集合数据源
    protected Context context;

    public ARecyclerAdapter(List<T> tList, Context context) {
        this.tList = tList;
        this.context = context;
    }

    @NonNull
    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
        //创建ViewHolder
        MyViewHolder holder;
        //添加item布局
        View view = LayoutInflater.from(context).inflate(R.layout.item, viewGroup, false);
        holder = new MyViewHolder(view);
        return holder;
    }

    @Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder viewHolder, int i) {
        //绑定数据
        MyViewHolder holder = (MyViewHolder) viewHolder;
        holder.setData();
    }

    @Override
    public int getItemCount() {
        return tList.size();
    }
    
    class MyViewHolder extends RecyclerView.ViewHolder{

        public MyViewHolder(@NonNull View itemView) {
            super(itemView);
        }

        public void setData() {
            
        }
    }
}

RecyclerView的优点：

1. ListView 以及GridView 的复用优化方法在RecyclerView中有封装好的方法，代码量大大减少。
2. 将布局方式抽象为LayoutManager，默认提供了LinearLayoutManager、GridLayoutManager、StaggeredGridLayoutManager 3种布局，对应线性布局、网络布局、交错布局，使用桥接方式把布局职责抽离出去，使得RecyclerView变得更为灵活。
3. RecyclerView对于Item View的控制更为精细，可以通过ItemDecotation为Item View 添加装饰，也可以通过ItemAnimator为Item View 添加动画。具有很强大的扩展性。

ItemDecoration

/**
 * RecyclerView 的分割线设置
 */
class SimpleItemDecoration : RecyclerView.ItemDecoration {

    var width: Int //屏幕宽
    var height: Int//屏幕高
    var item_height: Int //分割线高度
    var paint: Paint//画分割线画笔
    var itemPadding: Int = 0// 分割线距离左右的间距，默认是0

    constructor(context: Context) : super() {
        width = context.resources.displayMetrics.widthPixels
        height = context.resources.displayMetrics.heightPixels

        //初始化画笔
        paint = Paint(Paint.ANTI_ALIAS_FLAG or Paint.DITHER_FLAG) //抗锯齿 或者 使位图进行有利的抖动的位掩码标志
        paint.color = Color.BLACK
        item_height = 1
    }

    /**
     * item 上画分割线的方法
     *
     * Item图层和画的分割线的图层在同一图层，所以会出现重合情况，
     * 这时候就需要使用getItemOffsets方法，设置分割线偏移量，避免重合
     *
     */
    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDraw(c, parent, state)
        var itemCount = parent.childCount
        val child: Int
        for (i in 0 until itemCount) {
            var view = parent.getChildAt(i)
            var top = view.top
            var bottom = top + item_height
            c.drawRect(itemPadding.toFloat(), top.toFloat(), (width - itemPadding).toFloat(), bottom.toFloat(), paint)
        }
    }


    /**
     * 位于onDraw之上
     */
    override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(c, parent, state)
    }

    /**
     * item的偏移量
     */
    override fun getItemOffsets(outRect: Rect, view: View, parent: RecyclerView, state: RecyclerView.State) {
        super.getItemOffsets(outRect, view, parent, state)
        outRect.bottom = item_height // 分割线相对于item的偏移量
    }
}