ListView缓存策略
ListView和GridView一样，都继承自AbsListView。而缓存策略的实现是在AbsListView类中，所以两者的缓存策略也是一致的。
我们体验ListView的缓存最多的地方就是BaseAdapter.getView(int, View, ViewGroup)方法中了，一个典型的BaseAdapter的实现如下：
public class DemoAdapter extends BaseAdapter {
private Context mContext;
private List<Data> mDataList;
public DemoAdapter(Context context, List<Data> dataList) {
mContext = context;
mDataList = dataList;
}
@Override
public int getCount() {
return mDataList.size();
}
@Override
public Object getItem(int position) {
return null;
}
@Override
public long getItemId(int position) {
return 0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder viewHolder;
if (convertView == null) {
convertView = LayoutInflater.from(mContext).inflate(R.layout.item_location_hot_city, null);
viewHolder = new ViewHolder(convertView);
convertView.setTag(viewHolder);
} else {
viewHolder = (ViewHolder) convertView.getTag();
}
final Data data = mDataList.get(position);
viewHolder.tvTitle.setText(data.getName());
return convertView;
}
private static class ViewHolder {
View itemView;
TextView tvTitle;
public ViewHolder(View view) {
itemView = view;
tvTitle = (TextView) view.findViewById(R.id.tv_title);
}
}
}
在上面的方法中，一个非常重要的优化点就是getView方法中对convertView的判断。如果为空就需要我们创建一下；如果不为空，就说明是 缓存 的View，可以直接拿来填充数据。
那么，ListView如何管理缓存的View，什么时候调用BaseAdapter.getView方法并传入缓存的View或者null呢。这就是本节的重点，本节的讨论都是体现在convertView上。
1.1 RecycleBin
在讲AbsListView和ListView代码之前，先说一下AbsListView.RecycleBin，该类负责管理view的复用。RecycleBin 有两个等级的缓存：ActiveViews和ScrapViews。
ActiveViews是指显示在屏幕上的view ScrapViews是可能被adapter重新使用的老view，这样可以避免不必要的view创建。 RecycleBin里面的字段如下：
/**
* 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;
解释如下：
-
mFirstActivePosition、mActiveViews
mFirstActivePosition是指mActiveViews中第一个View在ListView中的position mActiveViews是指正在屏幕上显示的View；在layout发生前保存，供layout中进行复用，在layout后会将剩余没有复用的View降级到scrap中
-
mScrapViews、mViewTypeCount与mCurrentScrap
可以被Adapter作为convert view使用的View mScrapViews根据mViewTypeCount的值来确定有数组都多大，无论数组具体多大，mCurrentScrap = mScrapViews[0]都成立。但一般来说
- 当mViewTypeCount为1时，ScrapViews就是指mCurrentScrap
- 当mViewTypeCount大于1时，ScrapViews指mScrapViews
-
mTransientStateViewsById、mTransientStateViews、mSkippedScrap
当View.hasTransientState()为true时，会使用这上面的数据结构存储ScrapViews
当Adapter.hasStableIds()为true时，使用mTransientStateViewsById存储
当mDataChanged为false时，使用mTransientStateViews存储
否则，使用mSkippedScrap存储，该List里面的View稍后会被清除，不会被复用
RecycleBin的方法本质上就是对上面数据的一些操作。主要的方法有：
-
setViewTypeCount(int)
根据传入值为每个类型的数据元都申请一个List来存放缓存
-
fillActiveViews(int childCount, int firstActivePosition)
保存firstActivePosition的值，并将[0, childCount)范围的View保存到mActiveViews数组中
-
getActiveView(int)
与上面过程相反，首先将传入参数减去firstActivePosition得到View在mActiveViews数组中的下标，然后用下标取View
-
addScrapView(View, int)
将旧View保存到对应的ScrapViews、mTransientStateViewsById、mTransientStateViews、mSkippedScrap集合中
-
getScrapView(int)
从ScrapViews集合中获取旧View
-
getTransientStateView(int)
从mTransientStateViewsById、mTransientStateViews集合中获取旧View
在了解了RecycleBin的重要字段和方法之后，下面可以开始分析ListView的缓存机制了。我们以ListView初次layout、再次layout以及用户滑动三个过程来分析。
1.2 ListView初次layout
ListView初次layout，显然是没有任何子view以及缓存的view的，我们看看这种情况下ListView的流程。首先，onMeasure方法显然是不需要分析的，因为不涉及到缓存的设计。所以，我们直接看基类AbsListView的onLayout方法。
/**
* Subclasses should NOT override this method but
* {@link #layoutChildren()} instead.
*/
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
mInLayout = true;
final int childCount = getChildCount();
if (changed) {
for (int i = 0; i < childCount; i++) {
getChildAt(i).forceLayout();
}
mRecycler.markChildrenDirty();
}
layoutChildren();
mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;
// TODO: Move somewhere sane. This doesn't belong in onLayout().
if (mFastScroll != null) {
mFastScroll.onItemCountChanged(getChildCount(), mItemCount);
}
mInLayout = false;
}
这里第12行的判断中，changed是否为true都不重要，因为没有任何子view。然后19行layoutChildren()方法是一个要点，AbsListView并没有实现该方法，而是交给了子类来实现。所以，我们看看ListView.layoutChildren()方法：
@Override
protected void layoutChildren() {
final boolean blockLayoutRequests = mBlockLayoutRequests;
if (blockLayoutRequests) {
return;
}
mBlockLayoutRequests = true;
try {
super.layoutChildren();
invalidate();
if (mAdapter == null) {
resetList();
invokeOnItemScrollListener();
return;
}
final int childrenTop = mListPadding.top;
final int childrenBottom = mBottom - mTop - mListPadding.bottom;
final int childCount = getChildCount();
int index = 0;
int delta = 0;
View sel;
View oldSel = null;
View oldFirst = null;
View newSel = null;
...
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
// Handle the empty set by removing all views that are visible
// and calling it a day
if (mItemCount == 0) {
resetList();
invokeOnItemScrollListener();
return;
} else if (mItemCount != mAdapter.getCount()) {
throw new IllegalStateException("The content of the adapter has changed but "
+ "ListView did not receive a notification. Make sure the content of "
+ "your adapter is not modified from a background thread, but only from "
+ "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
+ "when its content changes. [in ListView(" + getId() + ", " + getClass()
+ ") with Adapter(" + mAdapter.getClass() + ")]");
}
setSelectedPositionInt(mNextSelectedPosition);
...
// 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_MOVE_SELECTION:
...
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();
// remove any header/footer that has been temp detached and not re-attached
removeUnusedFixedViews(mHeaderViewInfos);
removeUnusedFixedViews(mFooterViewInfos);
...
// Tell focus view we are done mucking with it, if it is still in
// our view hierarchy.
if (focusLayoutRestoreView != null
&& focusLayoutRestoreView.getWindowToken() != null) {
focusLayoutRestoreView.dispatchFinishTemporaryDetach();
}
mLayoutMode = LAYOUT_NORMAL;
mDataChanged = false;
if (mPositionScrollAfterLayout != null) {
post(mPositionScrollAfterLayout);
mPositionScrollAfterLayout = null;
}
mNeedSync = false;
setNextSelectedPositionInt(mSelectedPosition);
updateScrollIndicators();
if (mItemCount > 0) {
checkSelectionChanged();
}
invokeOnItemScrollListener();
} finally {
if (mFocusSelector != null) {
mFocusSelector.onLayoutComplete();
}
if (!blockLayoutRequests) {
mBlockLayoutRequests = false;
}
}
}
在上面的代码中，可以直奔重点，直接来到第63的if语句。dataChanged只有在数据源发生改变的情况下才会变成true，其它情况都是false，显然此时为false。所以会执行第68行的recycleBin.fillActiveViews(childCount, firstPosition)将ListView中的View进行缓存，可是目前ListView中还没有任何的子View，因此这一行暂时还起不了任何作用。
接下来，会来到第75行的switch语句，一般情况下ListView的 layout mode 为LAYOUT_NORMAL，所以会走default分支。由于childCount目前为0，且mStackFromBottom默认为false，表示默认从上往下进行布局，所以会执行第84行的fillFromTop()方法。
fillFromTop(int)会调用fillDown(int, int)方法从上到下填充ListView，直到加载完了一屏数据或者数据加载完毕：
/**
* Fills the list from top to bottom, starting with mFirstPosition
*
* @param nextTop The location where the top of the first item should be
* drawn
*
* @return The view that is currently selected
*/
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);
}
/**
* Fills the list from pos down to the end of the list view.
*
* @param pos The first position to put in the list
*
* @param nextTop The location where the top of the item associated with pos
* should be drawn
*
* @return The view that is currently selected, if it happens to be in the
* range that we draw.
*/
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;
}
在上面第37行的while循环条件中，前面的nextTop < end判断的是是否超过了屏幕，每次添加新view之后nextTop值都会累加，后面的pos < mItemCount判断的是是否所有数据已经显示完毕。 第40行的makeAndAddView应该是fill过程的重点，我们看看如何make、如何add view的。
/**
* Obtains the view and adds it to our list of children. The view can be
* made fresh, converted from an unused view, or used as is if it was in
* the recycle bin.
*
* @param position logical position in the list
* @param y top or bottom edge of the view to add
* @param flow {@code true} to align top edge to y, {@code false} to align
* bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @return the view that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
if (!mDataChanged) {
// Try to use an existing view for this position.
final View activeView = mRecycler.getActiveView(position);
if (activeView != null) {
// Found it. We're reusing an existing child, so it just needs
// to be positioned like a scrap view.
setupChild(activeView, position, y, flow, childrenLeft, selected, true);
return activeView;
}
}
// Make a new view for this position, or convert an unused view if
// possible.
final View child = obtainView(position, mIsScrap);
// This needs to be positioned and measured.
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}
mDataChanged显然还是false的，因此会尝试调用RecycleBin.getActiveView获取一个layout开始时fill的view（上面ListView.layoutChildren()方法的第68行recycleBin.fillActiveViews(childCount, firstPosition)会fill view到ActiveViews中），显然此时获取不到view。所以，接下来会执行第30行的obtainView方法创建或复用View，然后接着调用行第32行的setupChild方法放置并测量View。 那么obtainView()内部到底是怎么工作的呢？我们先进入到这个方法里面看一下：
/**
* Gets a view and have it show the data associated with the specified
* position. This is called when we have already discovered that the view
* is not available for reuse in the recycle bin. The only choices left are
* converting an old view or making a new one.
*
* @param position the position to display
* @param outMetadata an array of at least 1 boolean where the first entry
* will be set {@code true} if the view is currently
* attached to the window, {@code false} otherwise (e.g.
* newly-inflated or remained scrap for multiple layout
* passes)
*
* @return A view displaying the data associated with the specified position
*/
View obtainView(int position, boolean[] outMetadata) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");
outMetadata[0] = false;
// Check whether we have a transient state view. Attempt to re-bind the
// data and discard the view if we fail.
final View transientView = mRecycler.getTransientStateView(position);
if (transientView != null) {
final LayoutParams params = (LayoutParams) transientView.getLayoutParams();
// If the view type hasn't changed, attempt to re-bind the data.
if (params.viewType == mAdapter.getItemViewType(position)) {
final View updatedView = mAdapter.getView(position, transientView, this);
// If we failed to re-bind the data, scrap the obtained view.
if (updatedView != transientView) {
setItemViewLayoutParams(updatedView, position);
mRecycler.addScrapView(updatedView, position);
}
}
outMetadata[0] = true;
// Finish the temporary detach started in addScrapView().
transientView.dispatchFinishTemporaryDetach();
return transientView;
}
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 if (child.isTemporarilyDetached()) {
outMetadata[0] = true;
// Finish the temporary detach started in addScrapView().
child.dispatchFinishTemporaryDetach();
}
}
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
...
setItemViewLayoutParams(child, position);
...
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
return child;
}
obtainView()方法中代码包含了非常重要的逻辑，整个ListView最重要的内容就在这里。首先，会设置outMetadata[0]为false，这里的outMetadata实际上就是mIsScrap变量，该变量后面会用到。然后调用RecycleBin.getTransientStateView方法获取transient状态的scrap view。显然，目前没有这样的view。接着会到第45行执行RecycleBin.getScapView方法获取一个scrap view，显然，目前还是没有这样的view，所以scrapView为null。最后，会调用Adapter.getView方法来获取一个view。
回想一下文章开头在ListView的缓存策略写的一个典型的BaseAdapter的实现，如果传入的convertView为null，我们就会inflate一个view并返回。返回的view也会作为obtainView的结果进行返回，最终传入setupChild中：
/**
* Adds a view as a child and make sure it is measured (if necessary) and
* positioned properly.
*
* @param child the view to add
* @param position the position of this child
* @param y the y position relative to which this view will be positioned
* @param flowDown {@code true} to align top edge to y, {@code false} to
* align bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @param isAttachedToWindow {@code true} if the view is already attached
* to the window, e.g. whether it was reused, or
* {@code false} otherwise
*/
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
boolean selected, boolean isAttachedToWindow) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");
final boolean isSelected = selected && shouldShowSelector();
final boolean updateChildSelected = isSelected != child.isSelected();
final int mode = mTouchMode;
final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL
&& mMotionPosition == position;
final boolean updateChildPressed = isPressed != child.isPressed();
final boolean needToMeasure = !isAttachedToWindow || updateChildSelected
|| child.isLayoutRequested();
// Respect layout params that are already in the view. Otherwise make
// some up...
AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
}
p.viewType = mAdapter.getItemViewType(position);
p.isEnabled = mAdapter.isEnabled(position);
// Set up view state before attaching the view, since we may need to
// rely on the jumpDrawablesToCurrentState() call that occurs as part
// of view attachment.
if (updateChildSelected) {
child.setSelected(isSelected);
}
if (updateChildPressed) {
child.setPressed(isPressed);
}
if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
if (child instanceof Checkable) {
((Checkable) child).setChecked(mCheckStates.get(position));
} else if (getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB) {
child.setActivated(mCheckStates.get(position));
}
}
if ((isAttachedToWindow && !p.forceAdd) || (p.recycledHeaderFooter
&& p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
attachViewToParent(child, flowDown ? -1 : 0, p);
// If the view was previously attached for a different position,
// then manually jump the drawables.
if (isAttachedToWindow
&& (((AbsListView.LayoutParams) child.getLayoutParams()).scrappedFromPosition)
!= position) {
child.jumpDrawablesToCurrentState();
}
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
p.recycledHeaderFooter = true;
}
addViewInLayout(child, flowDown ? -1 : 0, p, true);
// add view in layout will reset the RTL properties. We have to re-resolve them
child.resolveRtlPropertiesIfNeeded();
}
if (needToMeasure) {
final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
mListPadding.left + mListPadding.right, p.width);
final int lpHeight = p.height;
final int childHeightSpec;
if (lpHeight > 0) {
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
} else {
cleanupLayoutState(child);
}
final int w = child.getMeasuredWidth();
final int h = child.getMeasuredHeight();
final int childTop = flowDown ? y : y - h;
if (needToMeasure) {
final int childRight = childrenLeft + w;
final int childBottom = childTop + h;
child.layout(childrenLeft, childTop, childRight, childBottom);
} else {
child.offsetLeftAndRight(childrenLeft - child.getLeft());
child.offsetTopAndBottom(childTop - child.getTop());
}
if (mCachingStarted && !child.isDrawingCacheEnabled()) {
child.setDrawingCacheEnabled(true);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
setupChild方法的传入参数中，isAttachedToWindow传入的是mIsScrap[0]，该值在obtainView方法中设置为了false。所以，我们略过一些状态的判定，可以直接来到59行的if语句，显然条件都不满足，因此会走70行的else分支。在该分支中会调用ViewGroup.addViewInLayout方法将child添加到ListView中。 同时，由于isAttachedToWindow为false，所以needToMeasure初始化为了true。因此，child会在第91行完成measure操作，在103行完成layout操作。
值得一提的是，在层层返回到layoutChildren方法之后，方法会继续执行到第105行的recycleBin.scrapActiveViews();语句，该方法会将ActionViews所有剩下的View移动到ScrapView集合中。也就是说，ActionViews的生命周期仅仅只存在于layout过程中。
至此，ListView初次layout的缓存流程已经探究完毕。下面我们看再次layout时的缓存流程。
1.3 ListView再次layout
再次layout主要是探究有缓存的情况下，ListView如何处理缓存的。
我们直接从layoutChildren方法开始：
@Override
protected void layoutChildren() {
final boolean blockLayoutRequests = mBlockLayoutRequests;
if (blockLayoutRequests) {
return;
}
mBlockLayoutRequests = true;
try {
super.layoutChildren();
invalidate();
if (mAdapter == null) {
resetList();
invokeOnItemScrollListener();
return;
}
final int childrenTop = mListPadding.top;
final int childrenBottom = mBottom - mTop - mListPadding.bottom;
final int childCount = getChildCount();
int index = 0;
int delta = 0;
View sel;
View oldSel = null;
View oldFirst = null;
View newSel = null;
...
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
// Handle the empty set by removing all views that are visible
// and calling it a day
if (mItemCount == 0) {
resetList();
invokeOnItemScrollListener();
return;
} else if (mItemCount != mAdapter.getCount()) {
throw new IllegalStateException("The content of the adapter has changed but "
+ "ListView did not receive a notification. Make sure the content of "
+ "your adapter is not modified from a background thread, but only from "
+ "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
+ "when its content changes. [in ListView(" + getId() + ", " + getClass()
+ ") with Adapter(" + mAdapter.getClass() + ")]");
}
setSelectedPositionInt(mNextSelectedPosition);
...
// 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_MOVE_SELECTION:
...
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();
// remove any header/footer that has been temp detached and not re-attached
removeUnusedFixedViews(mHeaderViewInfos);
removeUnusedFixedViews(mFooterViewInfos);
...
// Tell focus view we are done mucking with it, if it is still in
// our view hierarchy.
if (focusLayoutRestoreView != null
&& focusLayoutRestoreView.getWindowToken() != null) {
focusLayoutRestoreView.dispatchFinishTemporaryDetach();
}
mLayoutMode = LAYOUT_NORMAL;
mDataChanged = false;
if (mPositionScrollAfterLayout != null) {
post(mPositionScrollAfterLayout);
mPositionScrollAfterLayout = null;
}
mNeedSync = false;
setNextSelectedPositionInt(mSelectedPosition);
updateScrollIndicators();
if (mItemCount > 0) {
checkSelectionChanged();
}
invokeOnItemScrollListener();
} finally {
if (mFocusSelector != null) {
mFocusSelector.onLayoutComplete();
}
if (!blockLayoutRequests) {
mBlockLayoutRequests = false;
}
}
}
还是和初次layout一样，会执行第68行的recycleBin.fillActiveViews方法，不过由于此时ListView中有View了，所以这些子View都会被缓存到RecycleBin中。接着执行第72行的detachAllViewsFromParent方法将children的mParent以及自己置为null，这样会暂时从ListView中detach，待稍后在复用过程中调用attachViewToParent方法重新attach。 接着来到了第75行的switch，我们还是进入了default分支。由于此时childCount不为0，所以会走else分支。又由于mSelectedPosition默认为INVALID_POSITION即-1，所以会走94行的分支，执行fillSpecific操作。
/**
* Put a specific item at a specific location on the screen and then build
* up and down from there.
*
* @param position The reference view to use as the starting point
* @param top Pixel offset from the top of this view to the top of the
* reference view.
*
* @return The selected view, or null if the selected view is outside the
* visible area.
*/
private View fillSpecific(int position, int top) {
boolean tempIsSelected = position == mSelectedPosition;
View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
// Possibly changed again in fillUp if we add rows above this one.
mFirstPosition = position;
View above;
View below;
final int dividerHeight = mDividerHeight;
if (!mStackFromBottom) {
above = fillUp(position - 1, temp.getTop() - dividerHeight);
// This will correct for the top of the first view not touching the top of the list
adjustViewsUpOrDown();
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooHigh(childCount);
}
} else {
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
// This will correct for the bottom of the last view not touching the bottom of the list
adjustViewsUpOrDown();
above = fillUp(position - 1, temp.getTop() - dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
correctTooLow(childCount);
}
}
if (tempIsSelected) {
return temp;
} else if (above != null) {
return above;
} else {
return below;
}
}
fillSpecific()方法和fillUp()、fillDown()方法功能差不多，都是fill操作，不同的是fillSpecific()方法会先将指定位置的子View先加载到屏幕上，然后再从该View往上以及往下fill其它子View。
这里我们直接关注重点方法——makeAndAddView：
/**
* Obtains the view and adds it to our list of children. The view can be
* made fresh, converted from an unused view, or used as is if it was in
* the recycle bin.
*
* @param position logical position in the list
* @param y top or bottom edge of the view to add
* @param flow {@code true} to align top edge to y, {@code false} to align
* bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @return the view that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
if (!mDataChanged) {
// Try to use an existing view for this position.
final View activeView = mRecycler.getActiveView(position);
if (activeView != null) {
// Found it. We're reusing an existing child, so it just needs
// to be positioned like a scrap view.
setupChild(activeView, position, y, flow, childrenLeft, selected, true);
return activeView;
}
}
// Make a new view for this position, or convert an unused view if
// possible.
final View child = obtainView(position, mIsScrap);
// This needs to be positioned and measured.
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}
还是先尝试从ActiveViews中获取View，显然这次可以了。接着调用第23行的setupChild方法并返回了actionView。既然如何，ListView就不会执行下面第30行的obtainView方法了，因为ActiveViews中获取的View肯定是上一刻还显示在屏幕上的，无需让Adapter再次inflate布局或者重新更新的UI值。
我们注意到上面第23行的setupChild方法中，最后一个参数为true，该参数是isAttachedToWindow。因此needToMeasure为false，表示child不需要重新measure、layout；另外，也会导致下面第59行的if为true，进而导致调用attachViewToParent方法，让child重新attach到ListView上：
/**
* Adds a view as a child and make sure it is measured (if necessary) and
* positioned properly.
*
* @param child the view to add
* @param position the position of this child
* @param y the y position relative to which this view will be positioned
* @param flowDown {@code true} to align top edge to y, {@code false} to
* align bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @param isAttachedToWindow {@code true} if the view is already attached
* to the window, e.g. whether it was reused, or
* {@code false} otherwise
*/
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
boolean selected, boolean isAttachedToWindow) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");
final boolean isSelected = selected && shouldShowSelector();
final boolean updateChildSelected = isSelected != child.isSelected();
final int mode = mTouchMode;
final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL
&& mMotionPosition == position;
final boolean updateChildPressed = isPressed != child.isPressed();
final boolean needToMeasure = !isAttachedToWindow || updateChildSelected
|| child.isLayoutRequested();
// Respect layout params that are already in the view. Otherwise make
// some up...
AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
}
p.viewType = mAdapter.getItemViewType(position);
p.isEnabled = mAdapter.isEnabled(position);
// Set up view state before attaching the view, since we may need to
// rely on the jumpDrawablesToCurrentState() call that occurs as part
// of view attachment.
if (updateChildSelected) {
child.setSelected(isSelected);
}
if (updateChildPressed) {
child.setPressed(isPressed);
}
if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
if (child instanceof Checkable) {
((Checkable) child).setChecked(mCheckStates.get(position));
} else if (getContext().getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.HONEYCOMB) {
child.setActivated(mCheckStates.get(position));
}
}
if ((isAttachedToWindow && !p.forceAdd) || (p.recycledHeaderFooter
&& p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
attachViewToParent(child, flowDown ? -1 : 0, p);
// If the view was previously attached for a different position,
// then manually jump the drawables.
if (isAttachedToWindow
&& (((AbsListView.LayoutParams) child.getLayoutParams()).scrappedFromPosition)
!= position) {
child.jumpDrawablesToCurrentState();
}
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
p.recycledHeaderFooter = true;
}
addViewInLayout(child, flowDown ? -1 : 0, p, true);
// add view in layout will reset the RTL properties. We have to re-resolve them
child.resolveRtlPropertiesIfNeeded();
}
if (needToMeasure) {
final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
mListPadding.left + mListPadding.right, p.width);
final int lpHeight = p.height;
final int childHeightSpec;
if (lpHeight > 0) {
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),
MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
} else {
cleanupLayoutState(child);
}
final int w = child.getMeasuredWidth();
final int h = child.getMeasuredHeight();
final int childTop = flowDown ? y : y - h;
if (needToMeasure) {
final int childRight = childrenLeft + w;
final int childBottom = childTop + h;
child.layout(childrenLeft, childTop, childRight, childBottom);
} else {
child.offsetLeftAndRight(childrenLeft - child.getLeft());
child.offsetTopAndBottom(childTop - child.getTop());
}
if (mCachingStarted && !child.isDrawingCacheEnabled()) {
child.setDrawingCacheEnabled(true);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
回想一下第59行if的意义
- 第一次layout时由于没有可用的缓存，所以创建了新的View并使isAttachedToWindow为false，这样会调用addViewInLayout方法向ListView添加一个新View
- 第二次layout由于有可用的缓存，且缓存View由于detachAllViewsFromParent()方法的调用从而暂时处于detach状态；接着在成功复用到了View后，调用setupChild方法时传入参数isAttachedToWindow为true，这样就会执行attachViewToParent方法了，使child恢复了attach状态
至此第二次layout过程结束了，下面研究一下用户滑动ListView时发生了什么。
1.4 用户滑动
ListView的onTouchEvent实现是在AbsListView中，也就是说GridView也有着同样的机制。
@Override
public boolean onTouchEvent(MotionEvent ev) {
if (!isEnabled()) {
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return isClickable() || isLongClickable();
}
if (mPositionScroller != null) {
mPositionScroller.stop();
}
if (mIsDetaching || !isAttachedToWindow()) {
// Something isn't right.
// Since we rely on being attached to get data set change notifications,
// don't risk doing anything where we might try to resync and find things
// in a bogus state.
return false;
}
startNestedScroll(SCROLL_AXIS_VERTICAL);
if (mFastScroll != null && mFastScroll.onTouchEvent(ev)) {
return true;
}
initVelocityTrackerIfNotExists();
final MotionEvent vtev = MotionEvent.obtain(ev);
final int actionMasked = ev.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
mNestedYOffset = 0;
}
vtev.offsetLocation(0, mNestedYOffset);
switch (actionMasked) {
case MotionEvent.ACTION_DOWN: {
onTouchDown(ev);
break;
}
case MotionEvent.ACTION_MOVE: {
onTouchMove(ev, vtev);
break;
}
case MotionEvent.ACTION_UP: {
onTouchUp(ev);
break;
}
case MotionEvent.ACTION_CANCEL: {
onTouchCancel();
break;
}
case MotionEvent.ACTION_POINTER_UP: {
onSecondaryPointerUp(ev);
final int x = mMotionX;
final int y = mMotionY;
final int motionPosition = pointToPosition(x, y);
if (motionPosition >= 0) {
// Remember where the motion event started
final View child = getChildAt(motionPosition - mFirstPosition);
mMotionViewOriginalTop = child.getTop();
mMotionPosition = motionPosition;
}
mLastY = y;
break;
}
case MotionEvent.ACTION_POINTER_DOWN: {
// New pointers take over dragging duties
final int index = ev.getActionIndex();
final int id = ev.getPointerId(index);
final int x = (int) ev.getX(index);
final int y = (int) ev.getY(index);
mMotionCorrection = 0;
mActivePointerId = id;
mMotionX = x;
mMotionY = y;
final int motionPosition = pointToPosition(x, y);
if (motionPosition >= 0) {
// Remember where the motion event started
final View child = getChildAt(motionPosition - mFirstPosition);
mMotionViewOriginalTop = child.getTop();
mMotionPosition = motionPosition;
}
mLastY = y;
break;
}
}
if (mVelocityTracker != null) {
mVelocityTracker.addMovement(vtev);
}
vtev.recycle();
return true;
}
MotionEvent的事件种类太多了，而我们只关心移动的事件，所以直接看onTouchMove方法即可：
private void onTouchMove(MotionEvent ev, MotionEvent vtev) {
if (mHasPerformedLongPress) {
// Consume all move events following a successful long press.
return;
}
int pointerIndex = ev.findPointerIndex(mActivePointerId);
if (pointerIndex == -1) {
pointerIndex = 0;
mActivePointerId = ev.getPointerId(pointerIndex);
}
if (mDataChanged) {
// Re-sync everything if data has been changed
// since the scroll operation can query the adapter.
layoutChildren();
}
final int y = (int) ev.getY(pointerIndex);
switch (mTouchMode) {
case TOUCH_MODE_DOWN:
case TOUCH_MODE_TAP:
case TOUCH_MODE_DONE_WAITING:
// Check if we have moved far enough that it looks more like a
// scroll than a tap. If so, we'll enter scrolling mode.
if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) {
break;
}
// Otherwise, check containment within list bounds. If we're
// outside bounds, cancel any active presses.
final View motionView = getChildAt(mMotionPosition - mFirstPosition);
final float x = ev.getX(pointerIndex);
if (!pointInView(x, y, mTouchSlop)) {
setPressed(false);
if (motionView != null) {
motionView.setPressed(false);
}
removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
mPendingCheckForTap : mPendingCheckForLongPress);
mTouchMode = TOUCH_MODE_DONE_WAITING;
updateSelectorState();
} else if (motionView != null) {
// Still within bounds, update the hotspot.
final float[] point = mTmpPoint;
point[0] = x;
point[1] = y;
transformPointToViewLocal(point, motionView);
motionView.drawableHotspotChanged(point[0], point[1]);
}
break;
case TOUCH_MODE_SCROLL:
case TOUCH_MODE_OVERSCROLL:
scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev);
break;
}
}
onTouchMove方法会对mTouchMode做一个switch，手指在屏幕上滑动时，对应的mode为TOUCH_MODE_SCROLL，所以我们直接来到了scrollIfNeeded方法。由于方法实在有点长，所以省略了不相关的一些代码：
private void scrollIfNeeded(int x, int y, MotionEvent vtev) {
int rawDeltaY = y - mMotionY;
int scrollOffsetCorrection = 0;
int scrollConsumedCorrection = 0;
if (mLastY == Integer.MIN_VALUE) {
rawDeltaY -= mMotionCorrection;
}
...
final int deltaY = rawDeltaY;
int incrementalDeltaY =
mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY;
int lastYCorrection = 0;
if (mTouchMode == TOUCH_MODE_SCROLL) {
if (PROFILE_SCROLLING) {
if (!mScrollProfilingStarted) {
Debug.startMethodTracing("AbsListViewScroll");
mScrollProfilingStarted = true;
}
}
if (mScrollStrictSpan == null) {
// If it's non-null, we're already in a scroll.
mScrollStrictSpan = StrictMode.enterCriticalSpan("AbsListView-scroll");
}
if (y != mLastY) {
// We may be here after stopping a fling and continuing to scroll.
// If so, we haven't disallowed intercepting touch events yet.
// Make sure that we do so in case we're in a parent that can intercept.
if ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0 &&
Math.abs(rawDeltaY) > mTouchSlop) {
final ViewParent parent = getParent();
if (parent != null) {
parent.requestDisallowInterceptTouchEvent(true);
}
}
final int motionIndex;
if (mMotionPosition >= 0) {
motionIndex = mMotionPosition - mFirstPosition;
} else {
// If we don't have a motion position that we can reliably track,
// pick something in the middle to make a best guess at things below.
motionIndex = getChildCount() / 2;
}
int motionViewPrevTop = 0;
View motionView = this.getChildAt(motionIndex);
if (motionView != null) {
motionViewPrevTop = motionView.getTop();
}
// No need to do all this work if we're not going to move anyway
boolean atEdge = false;
if (incrementalDeltaY != 0) {
atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
}
// Check to see if we have bumped into the scroll limit
motionView = this.getChildAt(motionIndex);
if (motionView != null) {
// Check if the top of the motion view is where it is
// supposed to be
final int motionViewRealTop = motionView.getTop();
if (atEdge) {
// Apply overscroll
int overscroll = -incrementalDeltaY -
(motionViewRealTop - motionViewPrevTop);
if (dispatchNestedScroll(0, overscroll - incrementalDeltaY, 0, overscroll,
mScrollOffset)) {
lastYCorrection -= mScrollOffset[1];
if (vtev != null) {
vtev.offsetLocation(0, mScrollOffset[1]);
mNestedYOffset += mScrollOffset[1];
}
} else {
final boolean atOverscrollEdge = overScrollBy(0, overscroll,
0, mScrollY, 0, 0, 0, mOverscrollDistance, true);
if (atOverscrollEdge && mVelocityTracker != null) {
// Don't allow overfling if we're at the edge
mVelocityTracker.clear();
}
final int overscrollMode = getOverScrollMode();
if (overscrollMode == OVER_SCROLL_ALWAYS ||
(overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS &&
!contentFits())) {
if (!atOverscrollEdge) {
mDirection = 0; // Reset when entering overscroll.
mTouchMode = TOUCH_MODE_OVERSCROLL;
}
if (incrementalDeltaY > 0) {
mEdgeGlowTop.onPull((float) -overscroll / getHeight(),
(float) x / getWidth());
if (!mEdgeGlowBottom.isFinished()) {
mEdgeGlowBottom.onRelease();
}
invalidateTopGlow();
} else if (incrementalDeltaY < 0) {
mEdgeGlowBottom.onPull((float) overscroll / getHeight(),
1.f - (float) x / getWidth());
if (!mEdgeGlowTop.isFinished()) {
mEdgeGlowTop.onRelease();
}
invalidateBottomGlow();
}
}
}
}
mMotionY = y + lastYCorrection + scrollOffsetCorrection;
}
mLastY = y + lastYCorrection + scrollOffsetCorrection;
}
} else if (mTouchMode == TOUCH_MODE_OVERSCROLL) {
...
}
}
上面的方法中我们需要看的就是第57行的trackMotionScroll方法，由于滑动的每个事件都会触发上面的方法，所以方法会被调用很多次。trackMotionScroll代码如下：
/**
* Track a motion scroll
*
* @param deltaY Amount to offset mMotionView. This is the accumulated delta since the motion
* began. Positive numbers mean the user's finger is moving down the screen.
* @param incrementalDeltaY Change in deltaY from the previous event.
* @return true if we're already at the beginning/end of the list and have nothing to do.
*/
boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
final int childCount = getChildCount();
if (childCount == 0) {
return true;
}
final int firstTop = getChildAt(0).getTop();
final int lastBottom = getChildAt(childCount - 1).getBottom();
final Rect listPadding = mListPadding;
// "effective padding" In this case is the amount of padding that affects
// how much space should not be filled by items. If we don't clip to padding
// there is no effective padding.
int effectivePaddingTop = 0;
int effectivePaddingBottom = 0;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
effectivePaddingTop = listPadding.top;
effectivePaddingBottom = listPadding.bottom;
}
// FIXME account for grid vertical spacing too?
final int spaceAbove = effectivePaddingTop - firstTop;
final int end = getHeight() - effectivePaddingBottom;
final int spaceBelow = lastBottom - end;
final int height = getHeight() - mPaddingBottom - mPaddingTop;
if (deltaY < 0) {
deltaY = Math.max(-(height - 1), deltaY);
} else {
deltaY = Math.min(height - 1, deltaY);
}
if (incrementalDeltaY < 0) {
incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
} else {
incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
}
final int firstPosition = mFirstPosition;
// Update our guesses for where the first and last views are
if (firstPosition == 0) {
mFirstPositionDistanceGuess = firstTop - listPadding.top;
} else {
mFirstPositionDistanceGuess += incrementalDeltaY;
}
if (firstPosition + childCount == mItemCount) {
mLastPositionDistanceGuess = lastBottom + listPadding.bottom;
} else {
mLastPositionDistanceGuess += incrementalDeltaY;
}
final boolean cannotScrollDown = (firstPosition == 0 &&
firstTop >= listPadding.top && incrementalDeltaY >= 0);
final boolean cannotScrollUp = (firstPosition + childCount == mItemCount &&
lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0);
if (cannotScrollDown || cannotScrollUp) {
return incrementalDeltaY != 0;
}
final boolean down = incrementalDeltaY < 0;
final boolean inTouchMode = isInTouchMode();
if (inTouchMode) {
hideSelector();
}
final int headerViewsCount = getHeaderViewsCount();
final int footerViewsStart = mItemCount - getFooterViewsCount();
int start = 0;
int count = 0;
if (down) {
int top = -incrementalDeltaY;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
top += listPadding.top;
}
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (child.getBottom() >= top) {
break;
} else {
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
mRecycler.addScrapView(child, position);
}
}
}
} else {
int bottom = getHeight() - incrementalDeltaY;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
bottom -= listPadding.bottom;
}
for (int i = childCount - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getTop() <= bottom) {
break;
} else {
start = i;
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
mRecycler.addScrapView(child, position);
}
}
}
}
mMotionViewNewTop = mMotionViewOriginalTop + deltaY;
mBlockLayoutRequests = true;
if (count > 0) {
detachViewsFromParent(start, count);
mRecycler.removeSkippedScrap();
}
// invalidate before moving the children to avoid unnecessary invalidate
// calls to bubble up from the children all the way to the top
if (!awakenScrollBars()) {
invalidate();
}
offsetChildrenTopAndBottom(incrementalDeltaY);
if (down) {
mFirstPosition += count;
}
final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
fillGap(down);
}
mRecycler.fullyDetachScrapViews();
if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
final int childIndex = mSelectedPosition - mFirstPosition;
if (childIndex >= 0 && childIndex < getChildCount()) {
positionSelector(mSelectedPosition, getChildAt(childIndex));
}
} else if (mSelectorPosition != INVALID_POSITION) {
final int childIndex = mSelectorPosition - mFirstPosition;
if (childIndex >= 0 && childIndex < getChildCount()) {
positionSelector(INVALID_POSITION, getChildAt(childIndex));
}
} else {
mSelectorRect.setEmpty();
}
mBlockLayoutRequests = false;
invokeOnItemScrollListener();
return false;
}
这个方法接收两个参数，deltaY表示从手指按下时的位置到当前手指位置的距离，incrementalDeltaY则表示据上次触发event事件手指在Y方向上位置的改变量，那么其实我们就可以通过incrementalDeltaY的正负值情况来判断用户是向上还是向下滑动的了。如第71行所示，如果incrementalDeltaY小于0，说明是向下滑动，否则就是向上滑动。
下面将会进行一个边界值检测的过程，可以看到，从第89行开始，当ListView向下滑动的时候，就会进入一个for循环当中，从上往下依次获取子View，第91行当中，如果该子View的bottom值已经小于top值了，就说明这个子View已经移出屏幕了，所以会调用RecycleBin.addScrapView()方法将这个View加入到scrap view当中，并将count计数器加1，计数器用于记录有多少个子View被移出了屏幕。那么如果是ListView向上滑动的话，其实过程是基本相同的，只不过变成了从下往上依次获取子View，然后判断该子View的top值是不是大于bottom值了，如果大于的话说明子View已经移出了屏幕，同样把它加入到废弃缓存中，并将计数器加1。
接下来在第132行，会根据当前计数器的值来进行一个detach操作，它的作用就是把所有移出屏幕的子View全部detach掉。紧接着在第142行调用了offsetChildrenTopAndBottom()方法，并将incrementalDeltaY作为参数传入，这个方法的作用是让ListView中所有的子View都按照传入的参数值进行相应的偏移，这样就实现了随着手指的拖动，ListView的内容也会随着滚动的效果。
然后在第149行会进行判断，如果ListView中最后一个View的底部已经移入了屏幕，或者ListView中第一个View的顶部移入了屏幕，就会调用fillGap()方法，那么因此我们就可以猜出fillGap()方法是用来加载屏幕外数据的。该方法是个抽象方法，需要子类实现，所以我们看看ListView.fillGap方法：
/**
* {@inheritDoc}
*/
@Override
void fillGap(boolean down) {
final int count = getChildCount();
if (down) {
int paddingTop = 0;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
paddingTop = getListPaddingTop();
}
final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
paddingTop;
fillDown(mFirstPosition + count, startOffset);
correctTooHigh(getChildCount());
} else {
int paddingBottom = 0;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
paddingBottom = getListPaddingBottom();
}
final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
getHeight() - paddingBottom;
fillUp(mFirstPosition - 1, startOffset);
correctTooLow(getChildCount());
}
}
无论是fillDown还是fillUp，里面都是通过makeAndAddView方法来得到View，不过此时有些不同，因为ActiveViews已经在layout过程完成后被清空了，所以会执行obtainView方法来获取view：
/**
* Obtains the view and adds it to our list of children. The view can be
* made fresh, converted from an unused view, or used as is if it was in
* the recycle bin.
*
* @param position logical position in the list
* @param y top or bottom edge of the view to add
* @param flow {@code true} to align top edge to y, {@code false} to align
* bottom edge to y
* @param childrenLeft left edge where children should be positioned
* @param selected {@code true} if the position is selected, {@code false}
* otherwise
* @return the view that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,
boolean selected) {
if (!mDataChanged) {
// Try to use an existing view for this position.
final View activeView = mRecycler.getActiveView(position);
if (activeView != null) {
// Found it. We're reusing an existing child, so it just needs
// to be positioned like a scrap view.
setupChild(activeView, position, y, flow, childrenLeft, selected, true);
return activeView;
}
}
// Make a new view for this position, or convert an unused view if
// possible.
final View child = obtainView(position, mIsScrap);
// This needs to be positioned and measured.
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}
/**
* Gets a view and have it show the data associated with the specified
* position. This is called when we have already discovered that the view
* is not available for reuse in the recycle bin. The only choices left are
* converting an old view or making a new one.
*
* @param position the position to display
* @param outMetadata an array of at least 1 boolean where the first entry
* will be set {@code true} if the view is currently
* attached to the window, {@code false} otherwise (e.g.
* newly-inflated or remained scrap for multiple layout
* passes)
*
* @return A view displaying the data associated with the specified position
*/
View obtainView(int position, boolean[] outMetadata) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");
outMetadata[0] = false;
// Check whether we have a transient state view. Attempt to re-bind the
// data and discard the view if we fail.
final View transientView = mRecycler.getTransientStateView(position);
if (transientView != null) {
final LayoutParams params = (LayoutParams) transientView.getLayoutParams();
// If the view type hasn't changed, attempt to re-bind the data.
if (params.viewType == mAdapter.getItemViewType(position)) {
final View updatedView = mAdapter.getView(position, transientView, this);
// If we failed to re-bind the data, scrap the obtained view.
if (updatedView != transientView) {
setItemViewLayoutParams(updatedView, position);
mRecycler.addScrapView(updatedView, position);
}
}
outMetadata[0] = true;
// Finish the temporary detach started in addScrapView().
transientView.dispatchFinishTemporaryDetach();
return transientView;
}
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 if (child.isTemporarilyDetached()) {
outMetadata[0] = true;
// Finish the temporary detach started in addScrapView().
child.dispatchFinishTemporaryDetach();
}
}
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
...
setItemViewLayoutParams(child, position);
...
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
return child;
}
在上面第61行会调用RecycleBin.getTransientStateView方法获取transient状态的ScrapView，一般是没有的。所以会执行第83行的RecycleBin.getScrapView方法从ScrapViews里面获取一个View。在trackMotionScroll方法中我们会将任何移出屏幕的View添加到ScrapViews中，所以肯定是可以取到这个View的。因此，会在第84行中调用Adapter.getView方法让Adapter复用该View，并填充对应的数据，这样这个View看起来就像是全新的一样。
当然，如果复用View时，控件没有处理好，也是会出现复用引起的bug的，值得注意。一个典型的问题就是，如果某数据值为true就设置ImageView显示某个图片，但是值为false时又没有做任何处理，快速滑动时就会出现ImageView的显示与实际数据对不上的情况。
那么，以上就是ListView缓存机制的全部内容。
