【android】Adapter 的 convertView 复用浅析

目录

• 操作环境
• 导读
• 参数说明
• 源码浅析

操作环境

• 操作系统： win7-64bit 旗舰版
• android 版本： android-23

导读

平时在使用 ListView 或 GridView 时，常需要自己复写 Adapter 的 View getView(int position, View convertView, ViewGroup parent) 方法，但对其中的 convertView 却不太了解，没法做到物尽其用；殊不知其用途可不小，可以在一定程度上避免 OOM

参数说明

• 在 Adapter.java 中，对参数 convertView 的说明如下：

  • 说明： convertView 是一个可重用的 View，当然，也有可能是 Null；
  • 良好的习惯是，在使用之前都检测一下；

    /**
     * Get a View that displays the data at the specified position in the data set. You can either
     * create a View manually or inflate it from an XML layout file. When the View is inflated, the
     * parent View (GridView, ListView...) will apply default layout parameters unless you use
     * {@link android.view.LayoutInflater#inflate(int, android.view.ViewGroup, boolean)}
     * to specify a root view and to prevent attachment to the root.
     * 
     * @param position The position of the item within the adapter's data set of the item whose view
     *        we want.
     * @param convertView The old view to reuse, if possible. Note: You should check that this view
     *        is non-null and of an appropriate type before using. If it is not possible to convert
     *        this view to display the correct data, this method can create a new view.
     *        Heterogeneous lists can specify their number of view types, so that this View is
     *        always of the right type (see {@link #getViewTypeCount()} and
     *        {@link #getItemViewType(int)}).
     * @param parent The parent that this view will eventually be attached to
     * @return
     */
    View getView(int position, View convertView, ViewGroup parent);

源码浅析

• 再来看 AbsListView.java 的 obtainView 函数：
  
  • 说明：该函数的主要作用之一就是对 convertView 的复用；
  • AbsListView 维持：View[] mActiveViews 和 ArrayList mScrapViews 来对 convertView 进行回收管理的；
  • mScrapViews 主要用于存放不在 ListView 屏幕内那些 convertView，其内部的 convertView 的排列是没有一定顺序的，使用的时候需留意；
  • mActiveViews 则专门用于保存在 ListView 中显示的 convertView；
  • 对于具有临时状态的 convertView，那么该 View 将不被纳入回收操作；
  • 对于类型为 ITEM_VIEW_TYPE_HEADER_OR_FOOTER 的视图，也是不进行回收操作的；
  • 对于其他的 convertView：首先通过 getScrapView( position ) 从 mScrapViews 中查询( 如果不存在，则需由之后的 getView 创建；如果存在，则将其返回，实现View的复用查询操作 )；在调用 getView 函数后，如果得到一个新的 convertView，那么之前的 converView 将被继续保留在 mScrapViews 中，等待下一次根据 position 的查询；

    /**
     * Get a view and have it show the data associated with the specified
     * position. This is called when we have already discovered that the view is
     * not available for reuse in the recycle bin. The only choices left are
     * converting an old view or making a new one.
     *
     * @param position The position to display
     * @param isScrap Array of at least 1 boolean, the first entry will become true if
     *                the returned view was taken from the scrap heap, false if otherwise.
     *
     * @return A view displaying the data associated with the specified position
     */
    View obtainView(int position, boolean[] isScrap) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");

        isScrap[0] = false;

        // Check whether we have a transient state view. Attempt to re-bind the
        // data and discard the view if we fail.
        final View transientView = mRecycler.getTransientStateView(position);
        if (transientView != null) {
            final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

            // If the view type hasn't changed, attempt to re-bind the data.
            if (params.viewType == mAdapter.getItemViewType(position)) {
                final View updatedView = mAdapter.getView(position, transientView, this);

                // If we failed to re-bind the data, scrap the obtained view.
                if (updatedView != transientView) {
                    setItemViewLayoutParams(updatedView, position);
                    mRecycler.addScrapView(updatedView, position);
                }
            }

            isScrap[0] = true;

            // Finish the temporary detach started in addScrapView().
            transientView.dispatchFinishTemporaryDetach();
            return transientView;
        }

        final View scrapView = mRecycler.getScrapView(position);
        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 {
                isScrap[0] = true;

                // Finish the temporary detach started in addScrapView().
                child.dispatchFinishTemporaryDetach();
            }
        }

        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;
    }

• 总结：
  
  • RecycleBin 提供了 mScrapViews 和 mActiveViews 来实现复用机制；
  • AbsListView 已经提供了 RecycleBin 机制，为我们尽可能的提供复用机制；但在平时开发使用的过程中，我们还是应当尽量避免无谓的 new 操作；
  • 对于常用的 ListView 或者 GridView，如果 convertView 的布局大体都相同，可以尽量重用，避免 item 数量过大或过于分配频繁导致 OOM；

参考资料

• http://www.android100.org/html/201507/26/168809.html