RecycleBin机制
RecycleBin 为 AbsListView 中的一个内部类，因而所有继承自AbsListView的子类，即ListView和GridView，都可以使用这个机制，这个机制保障了 ListView实现上千条数据都不好OOM的最重要的一个原因
RecylceBin源码解析
参数解析
- RecyclerListener mRecyclerListener，RecyclerListener 中只有一个函数void onMovedToScrapHeap(View view)，作用是指明某个 view 被回收到了 recycler's scrap heap，那么该视图不再显示在屏幕上，与该视图相关联的任何昂贵资源都应丢弃
- int mFirstActivePosition ，第一个可见元素的 position 值
- View[] mActiveViews ，直接复用的view（处于可见状态的view）
- ArrayList
[] mScrapViews，间接复用的view（处于不可见状态的view）。可以被适配器用作convert view 的无序 view 数组。特别指明：这里是一个数组，因为如果adapter中数据有多种类型，那么就会有多个ScrapViews。 - int mViewTypeCount，View 类型总数
- ArrayList
mCurrentScrap，与 mScrapViews 类似，当 mViewTypeCount = 1情况下，mCurrentScrap = mScrapViews [0]
下面三个参数分别对应addScrapView()方法中 scrapHasTransientState 的三个情况
- SparseArray
mTransientStateViews，If the data hasn't changed, we can reuse the views at their old positions. - LongSparseArray
mTransientStateViewsById， If the adapter has stable IDs,we can reuse the view forthe same data. - ArrayList
mSkippedScrap，Otherwise, we'll have to remove the view and start over.
方法解析
-
void markChildrenDirty()，为每个子类调用 forceLayout()。将
mScrapView中回收回来的View设置一样标志，在下次被复用到 ListView 中时，告诉 viewroot 重新 layout 该 view。forceLayout()方法只是设置标志，并不会通知其 parent 来重新 layout -
boolean shouldRecycleViewType(int viewType)，判断给定的 view 的 viewType 指明是否可以回收回。viewType < 0可以回收。指定忽略的（ ITEM_VIEW_TYPE_IGNORE = -1），或者是 HeaderView / FootView（ITEM_VIEW_TYPE_HEADER_OR_FOOTER = -2）是不被回收的。如有特殊需要可以将自己定义的viewType设置为-1，否则，将会浪费内存，导致OOM。
-
void clear()，清空废弃 View 堆，并将这些View从窗口中Detach。
-
void fillActiveViews(int childCount, int firstActivePosition)，第一个参数为
mActiveViews要存储的最少的view的数量，第二个参数表示ListView中第一个可见元素的 position 值，调用这个方法后，会根据传入的参数将ListView中的指定元素存入mActiveView数组中 -
View getActiveView(int position)，与 fillActiveViews() 相对应，用于从
mActiveView数组中 获取对应 View，该方法接受一个 position 参数，表示元素在ListView中的位置，方法内部会将 position 的值自动转化为mActiveView数组对应的下标值。特别之处在于，一旦找当指定位置相对应的 View，该 View 将被移除，下一次获取相同位置的 View 将会返回 null，即mActiveView不能被复用 -
void addScrapView(View scrap, int position) ，scrap 为要添加的 View，position 为 View 在父类中的位置。放入时位置赋给 scrappedFromPosition 。有 transient 状态的 view 不会被 scrap（废弃），会被加入
mSkippedScrap。就是将移出可视区域的 view，设置它的scrappedFromPosition，然后从窗口中 detach 该 view，并根据 viewType 加入到mScrapView中。该方法会调用 mRecyclerListener 接口的函数 onMovedToScrapHeap
mRecyclerListener 的设置可通过 AbsListView 的 setRecyclerListener方法。
当 view 被回收准备再利用的时候设置要通知的监听器， 可以用来释放跟 view 有关的资源。
-
View getScrapView(int position)，A view from the ScrapViews collection. These are unordered。内部实际上是调用 retrieveFromScrap() 方法。
-
View retrieveFromScrap(ArrayList
scrapViews, int position) ，1.如果有 mAdapterHasStableIds ，并且适配器 position 的 itemId = (AbsListView.LayoutParams) view.getLayoutParams().itemId，则返回该 view
2.如果有 view.scrappedFromPosition = position的，直接返回该view
3.否则返回
mScrapView中的最后一个4.如果缓存中没有 view，则返回 null
第四种情况，ListView 稳定后，显示N个 item，此时
mScrapView是没有缓存 view 的，当我们向上滑动一小块（第一个 item 并未移除屏幕），新的 view 将显示，此时ListView 会调用 Adapter.getView，但是缓存中没有，因此 convertView 是 null，因而需要分配一块内润来创建新的 convertView。若第一个 item 完全移除屏幕，第一个 view就会被detach，并加入到mScrapView，当我们继续滑动，需要显示新的 view，此时系统会从mScrapView中找 position 对应的 view，由于尚未将第一个 view 加入到mScrapView中，所以找不到，则从mScrapView中取得最后一个缓存的 view 传递给 convertView。若第一个 item 完全移除屏幕且加入到mScrapView中，向下滑动加载新的 item，则直接从缓存中查找 position 对应的 view 返回 -
void removeSkippedScrap()，清空
mSkippedScrap -
void scrapActiveViews() ，将
mActiveViews中剩余的 view 放入mScrapViews。实际上就是将mActiveView中未使用的 view 回收（因为，此时已经移出可视区域了）。会调用mRecyclerListener.onMovedToScrapHeap(scrap) -
void fullyDetachScrapViews()，在布局阶段结束时，应重新附加所有临时分离视图或完全分离它们。 此方法可确保将
mScrapViews中的所有剩余视图完全分离。 -
void pruneScrapViews()，确保
mScrapViews的大小不超过mActiveViews的大小，如果适配器不回收其视图，则可能导致超出大小。如果超过，系统认为程序并没有复用convertView，而是每次都是创建一个新的view，为了避免产生大量的闲置内存且增加OOM的风险，系统会在每次回收后，去检查一下，将超过的部分释放掉，节约内存降低OOM风险。 -
void reclaimScrapViews(List
views) ，将mScrapViews中的所有 View 放入提供的列表中。只看到有AbsListView.reclaimViews有调用到，但没有其它方法使用这个函数，可能在特殊情况下会使用到，但目前从framework中，看不出来。 -
void setCacheColorHint(int color)，Updates the cache color hint of all known views.更新view的缓存颜色提示setDrawingCacheBackgroundColor。为所有的view绘置它们的背景色。
附源代码：
/**
* The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
* storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
* start of a layout. By construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews are old views that
* could potentially be used by the adapter to avoid allocating views unnecessarily.
*
* @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener)
* @see android.widget.AbsListView.RecyclerListener
*/
class RecycleBin {
@UnsupportedAppUsage
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.
*/
@UnsupportedAppUsage
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.
clearScrapForRebind(scrap);
getSkippedScrap().add(scrap);
}
} else {
clearScrapForRebind(scrap);
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.
// Traverse backwards to find the most recently used scrap view
for (int i = size - 1; i >= 0; 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);
clearScrapForRebind(scrap);
return scrap;
}
}
final View scrap = scrapViews.remove(size - 1);
clearScrapForRebind(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 clearScrapForRebind(View view) {
view.clearAccessibilityFocus();
view.setAccessibilityDelegate(null);
}
private void removeDetachedView(View child, boolean animate) {
child.setAccessibilityDelegate(null);
AbsListView.this.removeDetachedView(child, animate);
}
}