ListView复用机制的源码分析

在Android源码中适配器Adapter

下面是Adapter的源码

void registerDataSetObserver(DataSetObserver observer); void unregisterDataSetObserver(DataSetObserver observer); int getCount(); Object getItem(int position); long getItemId(int position); boolean hasStableIds(); View getView(int position, View convertView, ViewGroup parent); static final int IGNORE_ITEM_VIEW_TYPE = AdapterView.ITEM_VIEW_TYPE_IGNORE; int getItemViewType(int position); int getViewTypeCount(); static final int NO_SELECTION = Integer.MIN_VALUE; boolean isEmpty(); }

我仿造写一个适配器的

public class Reryclear { private Stack<View>[] views; //打造一个回收池 public Reryclear(int type) { views = new Stack[type]; for (int i = 0; i < type; i++) { views[i] = new Stack<View>(); } } public void addRecycledView(View view, int type) { views[type].push(view); } //添加 public View getRecycledView(View view, int type) { return views[type].pop(); } }

ListView的缓冲机制是在AbsListView中RecycleBin是缓存

*/ 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; } public void markChildrenDirty() { if (mViewTypeCount == 1) { final ArrayList<View> scrap = mCurrentScrap; final int scrapCount = scrap.size(); for (int i = 0; i < scrapCount; i++) { scrap.get(i).forceLayout(); } } else { final int typeCount = mViewTypeCount; for (int i = 0; i < typeCount; i++) { final ArrayList<View> scrap = mScrapViews[i]; final int scrapCount = scrap.size(); for (int j = 0; j < scrapCount; j++) { scrap.get(j).forceLayout(); } } } if (mTransientStateViews != null) { final int count = mTransientStateViews.size(); for (int i = 0; i < count; i++) { mTransientStateViews.valueAt(i).forceLayout(); } } if (mTransientStateViewsById != null) { final int count = mTransientStateViewsById.size(); for (int i = 0; i < count; i++) { mTransientStateViewsById.valueAt(i).forceLayout(); } } } public boolean shouldRecycleViewType(int viewType) { return viewType >= 0; } /** * Clears the scrap heap. */ void clear() { if (mViewTypeCount == 1) { final ArrayList<View> scrap = mCurrentScrap; clearScrap(scrap); } else { final int typeCount = mViewTypeCount; for (int i = 0; i < typeCount; i++) { final ArrayList<View> scrap = mScrapViews[i]; clearScrap(scrap); } } clearTransientStateViews(); } /** * 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; } View getTransientStateView(int position) { if (mAdapter != null && mAdapterHasStableIds && mTransientStateViewsById != null) { long id = mAdapter.getItemId(position); View result = mTransientStateViewsById.get(id); mTransientStateViewsById.remove(id); return result; } if (mTransientStateViews != null) { final int index = mTransientStateViews.indexOfKey(position); if (index >= 0) { View result = mTransientStateViews.valueAt(index); mTransientStateViews.removeAt(index); return result; } } return null; } /** * Dumps and fully detaches any currently saved views with transient * state. */ void clearTransientStateViews() { final SparseArray<View> viewsByPos = mTransientStateViews; if (viewsByPos != null) { final int N = viewsByPos.size(); for (int i = 0; i < N; i++) { removeDetachedView(viewsByPos.valueAt(i), false); } viewsByPos.clear(); } final LongSparseArray<View> viewsById = mTransientStateViewsById; if (viewsById != null) { final int N = viewsById.size(); for (int i = 0; i < N; i++) { removeDetachedView(viewsById.valueAt(i), false); } viewsById.clear(); } } /** * @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); } } } private ArrayList<View> getSkippedScrap() { if (mSkippedScrap == null) { mSkippedScrap = new ArrayList<>(); } return mSkippedScrap; } /** * Finish the removal of any views that skipped the scrap heap. */ void removeSkippedScrap() { if (mSkippedScrap == null) { return; } final int count = mSkippedScrap.size(); for (int i = 0; i < count; i++) { removeDetachedView(mSkippedScrap.get(i), false); } mSkippedScrap.clear(); } /** * Move all views remaining in mActiveViews to mScrapViews. */ void scrapActiveViews() { final View[] activeViews = mActiveViews; final boolean hasListener = mRecyclerListener != null; final boolean multipleScraps = mViewTypeCount > 1; ArrayList<View> scrapViews = mCurrentScrap; final int count = activeViews.length; for (int i = count - 1; i >= 0; i--) { final View victim = activeViews[i]; if (victim != null) { final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) victim.getLayoutParams(); final int whichScrap = lp.viewType; activeViews[i] = null; if (victim.hasTransientState()) { // Store views with transient state for later use. victim.dispatchStartTemporaryDetach(); if (mAdapter != null && mAdapterHasStableIds) { if (mTransientStateViewsById == null) { mTransientStateViewsById = new LongSparseArray<View>(); } long id = mAdapter.getItemId(mFirstActivePosition + i); mTransientStateViewsById.put(id, victim); } else if (!mDataChanged) { if (mTransientStateViews == null) { mTransientStateViews = new SparseArray<View>(); } mTransientStateViews.put(mFirstActivePosition + i, victim); } else if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { // The data has changed, we can't keep this view. removeDetachedView(victim, false); } } else if (!shouldRecycleViewType(whichScrap)) { // Discard non-recyclable views except headers/footers. if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { removeDetachedView(victim, false); } } else { // Store everything else on the appropriate scrap heap. if (multipleScraps) { scrapViews = mScrapViews[whichScrap]; } lp.scrappedFromPosition = mFirstActivePosition + i; removeDetachedView(victim, false); scrapViews.add(victim); if (hasListener) { mRecyclerListener.onMovedToScrapHeap(victim); } } } } pruneScrapViews(); } /** * At the end of a layout pass, all temp detached views should either be re-attached or * completely detached. This method ensures that any remaining view in the scrap list is * fully detached. */ void fullyDetachScrapViews() { final int viewTypeCount = mViewTypeCount; final ArrayList<View>[] scrapViews = mScrapViews; for (int i = 0; i < viewTypeCount; ++i) { final ArrayList<View> scrapPile = scrapViews[i]; for (int j = scrapPile.size() - 1; j >= 0; j--) { final View view = scrapPile.get(j); if (view.isTemporarilyDetached()) { removeDetachedView(view, false); } } } } /** * Makes sure that the size of mScrapViews does not exceed the size of * mActiveViews, which can happen if an adapter does not recycle its * views. Removes cached transient state views that no longer have * transient state. */ private void pruneScrapViews() { final int maxViews = mActiveViews.length; final int viewTypeCount = mViewTypeCount; final ArrayList<View>[] scrapViews = mScrapViews; for (int i = 0; i < viewTypeCount; ++i) { final ArrayList<View> scrapPile = scrapViews[i]; int size = scrapPile.size(); while (size > maxViews) { scrapPile.remove(--size); } } final SparseArray<View> transViewsByPos = mTransientStateViews; if (transViewsByPos != null) { for (int i = 0; i < transViewsByPos.size(); i++) { final View v = transViewsByPos.valueAt(i); if (!v.hasTransientState()) { removeDetachedView(v, false); transViewsByPos.removeAt(i); i--; } } } final LongSparseArray<View> transViewsById = mTransientStateViewsById; if (transViewsById != null) { for (int i = 0; i < transViewsById.size(); i++) { final View v = transViewsById.valueAt(i); if (!v.hasTransientState()) { removeDetachedView(v, false); transViewsById.removeAt(i); i--; } } } } /** * Puts all views in the scrap heap into the supplied list. */ void reclaimScrapViews(List<View> views) { if (mViewTypeCount == 1) { views.addAll(mCurrentScrap); } else { final int viewTypeCount = mViewTypeCount; final ArrayList<View>[] scrapViews = mScrapViews; for (int i = 0; i < viewTypeCount; ++i) { final ArrayList<View> scrapPile = scrapViews[i]; views.addAll(scrapPile); } } } /** * Updates the cache color hint of all known views. * * @param color The new cache color hint. */ void setCacheColorHint(int color) { if (mViewTypeCount == 1) { final ArrayList<View> scrap = mCurrentScrap; final int scrapCount = scrap.size(); for (int i = 0; i < scrapCount; i++) { scrap.get(i).setDrawingCacheBackgroundColor(color); } } else { final int typeCount = mViewTypeCount; for (int i = 0; i < typeCount; i++) { final ArrayList<View> scrap = mScrapViews[i]; final int scrapCount = scrap.size(); for (int j = 0; j < scrapCount; j++) { scrap.get(j).setDrawingCacheBackgroundColor(color); } } } // Just in case this is called during a layout pass final View[] activeViews = mActiveViews; final int count = activeViews.length; for (int i = 0; i < count; ++i) { final View victim = activeViews[i]; if (victim != null) { victim.setDrawingCacheBackgroundColor(color); } } } 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; } } private void clearScrap(final ArrayList<View> scrap) { final int scrapCount = scrap.size(); for (int j = 0; j < scrapCount; j++) { removeDetachedView(scrap.remove(scrapCount - 1 - j), false); } } private void clearAccessibilityFromScrap(View view) { view.clearAccessibilityFocus(); view.setAccessibilityDelegate(null); } private void removeDetachedView(View child, boolean animate) { child.setAccessibilityDelegate(null); AbsListView.this.removeDetachedView(child, animate); } }

中的有点几个重要的方法

一，RecycleBin分析 /** * 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; } } } 1,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。 /** * 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; } 2,getActiveView() 这个方法和fillActiveViews()是对应的，用于从mActiveViews数组当中获取数据。该方法接收一个position参数，表示元素在ListView当中的位置，方法内部会自动将position值转换成mActiveViews数组对应的下标值。需要注意的是，mActiveViews当中所存储的View，一旦被获取了之后就会从mActiveViews当中移除，下次获取同样位置的View将会返回null，也就是说mActiveViews不能被重复利用。 /** * 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); } } } 3,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和mCurrentScrap这两个List来存储废弃View。 /** * @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; 4,getScrapView 用于从废弃缓存中取出一个View，这些废弃缓存中的View是没有顺序可言的，因此getScrapView()方法中的算法也非常简单，就是直接从mCurrentScrap当中获取尾部的一个scrap view进行返回。 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; } 5,setViewTypeCount() 我们都知道Adapter当中可以重写一个getViewTypeCount()来表示ListView中有几种类型的数据项，而setViewTypeCount()方法的作用就是为每种类型的数据项都单独启用一个RecycleBin缓存机制。实际上，getViewTypeCount()方法通常情况下使用的并不是很多，所以我们只要知道RecycleBin当中有这样一个功能就行了。