ListView 快取機制原始碼淺析
ListView的快取機制是通過RecycleBin實現對View的快取實現的。快取機制大大的提高了View的複用率，這也是為什麼ListView可以載入大量的Item也不會OOM的原因。當然了，為了進一步提高效能我們都會自定義一個ViewHolder來避免不必要的findViewById操作，這也是RecycleView與ListView的區別之一<RecycleView快取的是ViewHolder>。RecycleBin是ListView/GridView的父類AbsListView的內部類，這說明ListView/GridView的快取機制大致是相同的。
一、概覽
當item view 在螢幕之外，RecycleBin將其快取起來，當需要新的itemView，除非RecycleBin中沒有快取，否則就直接從快取中取，還記得在getView中有一個引數convertView就是從RecycleBin中取出來的。
二、RecycleBin
由上圖我們可以看大，RecycleBin主要通過兩個集合：View[] mActiveViews和ArrayList<View>[] mScrapViews來實現的。
下面看一下RecycleBin中需要重點關注的部分：
private View[] mActiveViews：快取當前螢幕可以看到的item view;這個陣列是為了防止多次layout帶來的效能即資料重複問題而存在的。因為多次layout會導致ListView重複多次載入當前的螢幕顯示的item views，為了避免資料重複以及多次例項化item views帶來的資源浪費，有必要將其快取起來。
private int mViewTypeCount：item view的種類；
private ArrayList<View>[] mScrapViews:注意到這是一個ArrayList<View>陣列，之所以是一個數組，是因為item view的種類可能大於一，而mScrapViews:就是快取每種item View的。還記得BaseAdapter裡面有一個getViewTypeCount（）方法嗎？這個方法的返回值就是mViewTypeCount。
private ArrayList<View> mCurrentScrap：當前種類的item view的快取。
public void setViewTypeCount(int viewTypeCount)：設定列表item的種類；
void fillActiveViews(int childCount, int firstActivePosition)：將當前螢幕可以看到的item view新增到mActiveViews中；
View getActiveView(int position)：根據position獲取mActiveViews中的item view;
void addScrapView(View scrap, int position):將廢棄的（通常是滾出螢幕外的）item view快取起來；簡單的看一下addSrapView的原始碼：
void addScrapView(View scrap, int position){ ....... if (mViewTypeCount == 1) { mCurrentScrap.add(scrap); } else { mScrapViews[viewType].add(scrap); } if (mRecyclerListener != null) { mRecyclerListener.onMovedToScrapHeap(scrap); } }
這驗證了我們對mScrapViews的分析；
View getScrapView(int position):根據position的值獲取一個廢棄的item view用於重用；
三、詳細快取流程
ListView是ViewGroup的子類，遵循自定義View的基本步驟，measure、layout、draw。快取部分我們重點分析layout過程。下面看一下ListView的layoutChildren部分原始碼：
// Pull all children into the RecycleBin.
// These views will be reused if possible
final int firstPosition = mFirstPosition;
final RecycleBin recycleBin = mRecycler;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}
// Clear out old views
detachAllViewsFromParent();
recycleBin.removeSkippedScrap();
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
if (newSel != null) {
sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
} else {
sel = fillFromMiddle(childrenTop, childrenBottom);
}
break;
case LAYOUT_SYNC:
sel = fillSpecific(mSyncPosition, mSpecificTop);
break;
case LAYOUT_FORCE_BOTTOM:
sel = fillUp(mItemCount - 1, childrenBottom);
adjustViewsUpOrDown();
break;
case LAYOUT_FORCE_TOP:
mFirstPosition = 0;
sel = fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
case LAYOUT_SPECIFIC:
sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
break;
case LAYOUT_MOVE_SELECTION:
sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
break;
default:
if (childCount == 0) {
if (!mStackFromBottom) {
final int position = lookForSelectablePosition(0, true);
setSelectedPositionInt(position);
sel = fillFromTop(childrenTop);
} else {
final int position = lookForSelectablePosition(mItemCount - 1, false);
setSelectedPositionInt(position);
sel = fillUp(mItemCount - 1, childrenBottom);
}
} else {
if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
sel = fillSpecific(mSelectedPosition,
oldSel == null ? childrenTop : oldSel.getTop());
} else if (mFirstPosition < mItemCount) {
sel = fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
} else {
sel = fillSpecific(0, childrenTop);
}
}
break;
}
// Flush any cached views that did not get reused above
recycleBin.scrapActiveViews();
繼續向下看，mLayoutMode代表了佈局模式，預設情況下是LAYOUT_NORMAL，因此會走default語句中。因為childCount是0，並且預設的順序從上到下的，因此會執行fillFromTop -- 執行該函式就可以將Adapter中的資料填充到ListView中了，填充完成之後，childCount就大於0了。下面看一下fillFromTop的原始碼：
private View fillFromTop(int nextTop) {
mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
if (mFirstPosition < 0) {
mFirstPosition = 0;
}
return fillDown(mFirstPosition, nextTop);
} private View fillDown(int pos, int nextTop) {
View selectedView = null;
int end = (mBottom - mTop);
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
end -= mListPadding.bottom;
}
while (nextTop < end && pos < mItemCount) {
// is this the selected item?
boolean selected = pos == mSelectedPosition;
View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
nextTop = child.getBottom() + mDividerHeight;
if (selected) {
selectedView = child;
}
pos++;
}
setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
return selectedView;
} private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
View child;
if (!mDataChanged) {
// Try to use an existing view for this position
child = mRecycler.getActiveView(position);
if (child != null) {
// Found it -- we're using an existing child
// This just needs to be positioned
setupChild(child, position, y, flow, childrenLeft, selected, true);
return child;
}
}
// Make a new view for this position, or convert an unused view if possible
child = obtainView(position, mIsScrap);
// This needs to be positioned and measured
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}......
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();
}
}
......
return child;下面看一下第二次layout的時候，會發生什麼
// Pull all children into the RecycleBin.
// These views will be reused if possible
final int firstPosition = mFirstPosition;
final RecycleBin recycleBin = mRecycler;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}
// Clear out old views
detachAllViewsFromParent();
recycleBin.removeSkippedScrap();
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
if (newSel != null) {
sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
} else {
sel = fillFromMiddle(childrenTop, childrenBottom);
}
break;
case LAYOUT_SYNC:
sel = fillSpecific(mSyncPosition, mSpecificTop);
break;
case LAYOUT_FORCE_BOTTOM:
sel = fillUp(mItemCount - 1, childrenBottom);
adjustViewsUpOrDown();
break;
case LAYOUT_FORCE_TOP:
mFirstPosition = 0;
sel = fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
case LAYOUT_SPECIFIC:
sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
break;
case LAYOUT_MOVE_SELECTION:
sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
break;
default:
if (childCount == 0) {
if (!mStackFromBottom) {
final int position = lookForSelectablePosition(0, true);
setSelectedPositionInt(position);
sel = fillFromTop(childrenTop);
} else {
final int position = lookForSelectablePosition(mItemCount - 1, false);
setSelectedPositionInt(position);
sel = fillUp(mItemCount - 1, childrenBottom);
}
} else {
if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
sel = fillSpecific(mSelectedPosition,
oldSel == null ? childrenTop : oldSel.getTop());
} else if (mFirstPosition < mItemCount) {
sel = fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
} else {
sel = fillSpecific(0, childrenTop);
}
}
break;
}
// Flush any cached views that did not get reused above
recycleBin.scrapActiveViews();滑動過程中，會產生大量的Item View，RecycleBin也發揮了重要的作用。這也是ListView不會OOM的原因。