Listview缓存机制分析(四)

6 RecycleBin解析

RecycleBin是AbsListView的内部类, 是ListView能够实现成百上千条数据都不会OOM最重要的一个原因. 主要变量如下,

private View[] mActiveViews = new View[0]; // 保存listview中可见的元素(view)private ArrayList<View>[] mScrapViews; // ArrayList组private int mViewTypeCount;private ArrayList<View> mCurrentScrap; // // 当有某个View确定要废弃掉的时候(比如滚动出了屏幕), mScrapViews或者mCurrentScrap就会缓存   // 该view,当mViewTypeCount大于1时,缓存在mCurrentScrap中,否则缓存在mScrapViews中。     // BaseAdapter中的mViewTypeCount为1,其实就是一个分类private ArrayList<View> mSkippedScrap;private SparseArray<View> mTransientStateViews; // 缓存短暂的view

1, fillActiveViews

fillActiveViews() 这个方法接收两个参数，第一个参数表示要存储的view的数量，第二个参数表示ListView中第一个可见元素的position值。RecycleBin当中使用mActiveViews这个数组来存储View，调用这个方法后就会根据传入的参数来将ListView中的指定元素存储到mActiveViews数组当中。

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;                }            }        }

2, getActiveView

getActiveView() 这个方法和fillActiveViews()是对应的，用于从mActiveViews数组当中获取数据,返回之后activeViews对应位置就为null,所以mActiveViews不能被重复利用。

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;        }

3, addScrapView

addScrapView() 用于将一个废弃的View进行缓存，该方法接收一个View参数，当有某个View确定要废弃掉的时候(比如滚动出了屏幕)，就应该调用这个方法来对View进行缓存，RecycleBin当中使用mScrapViews或者mCurrentScrap这两个List来存储废弃View。

void addScrapView(View scrap, int position) {•••if (mViewTypeCount == 1) {                    mCurrentScrap.add(scrap);                } else {                    mScrapViews[viewType].add(scrap);                }•••}

4, getScrapView

和addScrapView的作用刚好相反

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;        }