android-listview-分析

分析

从Adapter.getView方法入手, 找到了 AbsListView.obtainView,

  /**
   * 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) {
      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();
          }
      }
...... // 省略部分代码
      
      return child;
  }

我们把目光放在mRecycler上，其实就是一个RecyclerBin对象，负责缓存View对象，这是ListView复用机制的精华所在，有必要好好刨析这个类. RecyclerBin中各个变量的定义如下：

//回收Listener，当View变为可回收，即ScrapView时，会通过mRecyclerListener通知注册者，listener可通过setRecyclerListener注册

private RecyclerListener mRecyclerListener;
/**
 * The position of the first view stored in mActiveViews.
 */
// 第一个活动view的position，即第一个可视view的position
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.
 */
// 活动view的集合
private View[] mActiveViews = new View[0];
/**
 * Unsorted views that can be used by the adapter as a convert view.
 */
/**废弃的可修复view集合，复用时传递到Adapter#getView方法的convertView参数。
 * 因为item type可能大于1，只有view type相同的view之间才能复用，所以是个二维数组
 */
private ArrayList<View>[] mScrapViews;
// ListView item type数量
private int mViewTypeCount;
// 当前的废弃view数组，定义这个成员是为了在mViewTypeCount为1时使用方便，不需要去取mScrapViews的第一个元素
private ArrayList<View> mCurrentScrap;
// 被跳过的，不能复用的view集合。view type小于0或者处理transient状态的view不能被复用。
private ArrayList<View> mSkippedScrap;
// 处于 transient状态的view集合，处于transient状态的view不能被复用，如view的动画正在播放，
// transient是瞬时、过渡的意思，关于transient状态详见android.view.View#PFLAG2_HAS_TRANSIENT_STATE
private SparseArray<View> mTransientStateViews;
// 如果adapter的hasStableIds方法返回true，处于过度状态的view保存到这里。因为需要保存view的position，而且处于过度状态的view一般很少，
// 这2个成员用了稀疏数组。具体不需要case，知道是保存转换状态view的集合就行。
private LongSparseArray<View> mTransientStateViewsById;

分析

参考