編輯：關於Android編程
最近用到RecyclerView，想研究RecyclerView和ListView復用機制的區別，這篇文章以解析源碼的方式解析ListView復用機制。
/**
* 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.
*/
RecycleBin是2級的存儲結構，
ActiveViews: 當前屏幕上的活動View
ScrapViews: 廢棄View，可復用的舊View
//回收Listener，當View變為可回收，即ScrapView時，會通過mRecyclerListener通知注冊者，listener可通過setRecyclerListener注冊 private RecyclerListener mRecyclerListener; /** * The position of the first view stored in mActiveViews. */ // 第一個活動view的position，即第一個可視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. */ // 活動view的集合 private View[] mActiveViews = new View[0]; /** * Unsorted views that can be used by the adapter as a convert view. */ /**廢棄的可修復view集合，復用時傳遞到Adapter#getView方法的convertView參數。 * 因為item type可能大於1，只有view type相同的view之間才能復用，所以是個二維數組 */ private ArrayList[] mScrapViews; // ListView item type數量 private int mViewTypeCount; // 當前的廢棄view數組，定義這個成員是為了在mViewTypeCount為1時使用方便，不需要去取mScrapViews的第一個元素 private ArrayList mCurrentScrap; // 被跳過的，不能復用的view集合。view type小於0或者處理transient狀態的view不能被復用。 private ArrayList mSkippedScrap; // 處於transient狀態的view集合，處於transient狀態的view不能被復用，如view的動畫正在播放， // transient是瞬時、過渡的意思，關於transient狀態詳見android.view.View#PFLAG2_HAS_TRANSIENT_STATE private SparseArray mTransientStateViews; // 如果adapter的hasStableIds方法返回true，處於過度狀態的view保存到這裡。因為需要保存view的position，而且處於過度狀態的view一般很少， // 這2個成員用了稀疏數組。具體不需要case，知道是保存轉換狀態view的集合就行。 private LongSparseArray mTransientStateViewsById;
從RecycleBin成員變量的定義基本可以看出復用的原理：
1. 廢棄的view保存在一個數組中，復用時從中取出
2. 擁有相同view type的view之間才能復用，所以mScrapViews是個二維數組
3. 處於transient狀態的view不能被復用
public void setViewTypeCount(int viewTypeCount) {
if (viewTypeCount < 1) {
throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
}
//noinspection [] scrapViews = new ArrayList[viewTypeCount];
for (int i = 0; i < viewTypeCount; i++) {
scrapViews[i] = new ArrayList();
}
mViewTypeCount = viewTypeCount;
mCurrentScrap = scrapViews[0];
mScrapViews = scrapViews;
}
markChildrenDirty: 當ListView size或position變化時，設置mScrapViews和transient views的forceLayout flag，在下一次被復用時會重新布置。
public void markChildrenDirty() {
if (mViewTypeCount == 1) {
final ArrayList 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 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;
}
/**
* 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;
}
}
}
getActiveView: 查找mActiveView中指定position的view，找到後將從mActiveViews中移除
/**
* 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;
}
getScrapView: 查找復用的view，先通過position從adapter中找到view type，然後再從相應的scrap中查找
/**
* @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;
}
private View retrieveFromScrap(ArrayListscrapViews, 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; } }
/**
* Puts a view into the list of scrap views.
*
* 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)) { //不能被復用的view type，並且不為header或footer時加入skipped scrap // 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) { //view處於transient狀態 if (mAdapter != null && mAdapterHasStableIds) { //adpater has stable ids，加入mTransientStateViewsById中 // 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) { //數據未變化，加入mTransientStateViews中 // 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 { //adapter為null或adpater不為null，無stable ids，且數據變化，則丟棄scrap view // Otherwise, we'll have to remove the view and start over. getSkippedScrap().add(scrap); } } else { //不處於transient狀態，將scrap view加入mScarpViews中 if (mViewTypeCount == 1) { //view type count為1時，為方便，直接操作mCurrentScrap mCurrentScrap.add(scrap); } else { mScrapViews[viewType].add(scrap); } if (mRecyclerListener != null) { mRecyclerListener.onMovedToScrapHeap(scrap); } } }
/**
* 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 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();
}
long id = mAdapter.getItemId(mFirstActivePosition + i);
mTransientStateViewsById.put(id, victim);
} else if (!mDataChanged) {
if (mTransientStateViews == null) {
mTransientStateViews = new SparseArray();
}
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];
}
victim.dispatchStartTemporaryDetach();
lp.scrappedFromPosition = mFirstActivePosition + i;
scrapViews.add(victim);
if (hasListener) {
mRecyclerListener.onMovedToScrapHeap(victim);
}
}
}
}
pruneScrapViews();
}
//清除所有scrap view和transient view void clear(); //獲取當前position下的transient view，如無則返回null View getTransientStateView(int position); //獲取mSkippedScrap private ArrayListgetSkippedScrap(); //清除mSkippedScrap void removeSkippedScrap(); //裁剪mScarpViews，確保size不大於mActiveViews的size；移除已不是transient state的view private void pruneScrapViews(); //將所有scrap views放入指定的views中，這個方法沒弄明白，不知何時調用 void reclaimScrapViews(List views); //為每一個scrap和active view設置緩存背影色 void setCacheColorHint(int color); //從ListView層次中移除view private void removeDetachedView(View child, boolean animate);
android中GridView實現點擊查看更多功能示例
首先來看一下效果圖； 先說一下我的需求：查看群成員，如果超過15人則全部顯示，如果大於15人則先加載15人，其余的不顯示，點擊查看更多則加載全部。再來說一下我
[Android] TextSwitcher -- 做什麼的
TextSwitcher的Java Doc是這樣描述自己的： Specialized ViewSwitcher that contains only children o
Android開發之Android studio使用git（一）
使用AS從代碼托管中心下載項目（我使用的是Coding托管代碼）1、打開Android Studio，如果之前沒有打開過任何項目，那麼將會看到下面的啟動頁面，並選擇Che
Android 5.0 Usb調試攔截分析及修改
當我們調試安卓機器時,第一次插上usb線,會彈出一個授權的對話框,(前提是打開了usb調試功能)點擊確認,才會允許調試.如果我們想機器默認就可以調試該怎麼做呢?如果我們想
