通过此篇文章：http://blog.csdn.net/guolin_blog/article/details/44996879
在Android所有常用的原生控件当中，用法最复杂的应该就是ListView了，
ListView 父控件AbsListView
同时也会引伸出GridView， 父控件也是AbsListView
RecycleBin机制
那么在开始分析ListView的源码之前，还有一个东西是我们提前需要了解的，就是RecycleBin机制，它是在AbsListView中的一个内部类，所以所有继承自AbsListView的子类，也就是ListView和GridView，都可以使用这个机制。那我们来看一下RecycleBin中的主要代码，如下所示：
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.
*/
//布局开始时屏幕显示的view，这个数组会在布局开始时填充，布局结束后所有view被移至mScrapViews。（也包括是当前屏幕所展示的活跃view）
private View[] mActiveViews = new View[0];
/**
* Unsorted views that can be used by the adapter as a convert view.
*/
//ListView中所有的废弃缓存。这是一个数组，每一种布局类型的view都有一个自己的arraylist缓存
private ArrayList[] mScrapViews;
//listView中不同item布局的布局总数
private int mViewTypeCount;
//跟mScrapViews的区别是，mScrapViews是个队列数组，ArrayList[]类型，数组长度为mViewTypeCount，而默认ViewTypeCount = 1的情况下mCurrentScrap=mScrapViews[0]。
private ArrayList mCurrentScrap;
// 被跳过的，不能复用的view集合。view type小于0或者处理transient状态的view不能被复用
private ArrayList mSkippedScrap;
//处于transient状态的view集合，处于transient状态的view不能被复用，如view的动画正在播放，
// transient是瞬时、短暂的意思
private SparseArray mTransientStateViews;
//如果adapter的hasStableIds方法返回true，处于过度状态的view保存到这里。
//因为需要保存view的position，而且处于过度状态的view一般很少，
//知道是保存转换状态view的集合就行，itemId作为key（其实就是view的position）
private LongSparseArray mTransientStateViewsById
public void setViewTypeCount(int viewTypeCount) {
........
}
public void markChildrenDirty() {
if (mViewTypeCount == 1) {
.......
} else {
......
}
if (mTransientStateViews != null) {
......
}
if (mTransientStateViewsById != null) {
.....
}
}
public boolean shouldRecycleViewType(int viewType) {
return viewType >= 0;
}
/**
* Clears the scrap heap.
*/
void clear() {
if (mViewTypeCount == 1) {
final ArrayList scrap = mCurrentScrap;
clearScrap(scrap);
} else {
final int typeCount = mViewTypeCount;
for (int i = 0; i < typeCount; i++) {
final ArrayList 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 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 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.
*
* 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 getSkippedScrap() {
if (mSkippedScrap == null) {
mSkippedScrap = new ArrayList<>();
}
return mSkippedScrap;
}
/**
* Finish the removal of any views that skipped the scrap heap.
*/
void removeSkippedScrap() {
.......
}
/**
* 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];
}
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[] scrapViews = mScrapViews;
for (int i = 0; i < viewTypeCount; ++i) {
final ArrayList 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() {
.......
}
/**
* 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 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 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 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 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);
}
}
关于变量的解释都在代码注释中，下一篇文章会着重讲解缓存！
这面会着重说明layout（）方法！
因为layout（）方法会走两次！为什么layout（）方法会执行两次呢？
我猜测应该是顶层的
performTranversal函数在View的测量流程中会执行2次（View的绘制流程都始于ViewRootImpl的performTraversals方法）。
第一次Layout（）方法
代码比较多，我们挑重点解析：
首先可以确定的是，ListView当中目前还没有任何子View，数据都还是由Adapter管理的，并没有展示到界面上，
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
这面说明下mDataChanged，mDataChanged是AdapterView中的变量，因为listView本身的适配器Adapter是用到观察者模式的，调用
public void notifyDataSetChanged() {
mDataSetObservable.notifyChanged();
}
public void notifyChanged() {
synchronized(mObservers) {
// since onChanged() is implemented by the app, it could do anything, including
// removing itself from {@link mObservers} - and that could cause problems if
// an iterator is used on the ArrayList {@link mObservers}.
// to avoid such problems, just march thru the list in the reverse order.
for (int i = mObservers.size() - 1; i >= 0; i--) {
mObservers.get(i).onChanged();
}
}
}
@Override
public void onChanged() {
mDataChanged = true;
}
这样数据量发生改变的时候mDataChanged就会变成true，当然默认值是false！
然后会走到下面的switch case 因为第一次肯定是默认，会走 switch中的default:
也就是1781行，childCount肯定会为0，然后会走到 fillFromTop(childrenTop)这个方法！
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);
}
从这个方法的注释中可以看出，它所负责的主要任务就是从mFirstPosition开始，自顶至底去填充ListView。而这个方法本身并没有什么逻辑，就是判断了一下mFirstPosition值的合法性，然后调用fillDown()方法，那么我们就有理由可以猜测，填充ListView的操作是在fillDown()方法中完成的。进入fillDown()方法，代码如下所示：
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;
}
可以看到，这里使用了一个while循环来执行重复逻辑，一开始nextTop的值是第一个子元素顶部距离整个ListView顶部的像素值，pos则是刚刚传入的mFirstPosition的值，而end是ListView底部减去顶部所得的像素值，mItemCount则是Adapter中的元素数量。因此一开始的情况下nextTop必定是小于end值的，并且pos也是小于mItemCount值的。那么每执行一次while循环，pos的值都会加1，并且nextTop也会增加，当nextTop大于等于end时，也就是子元素已经超出当前屏幕了，或者pos大于等于mItemCount时，也就是所有Adapter中的元素都被遍历结束了，就会跳出while循环。
那么while循环当中又做了什么事情呢？值得让人留意的就是第18行调用的makeAndAddView()方法，进入到这个方法当中，代码如下所示：
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;
}
这里在第5行尝试从RecycleBin当中快速获取一个active view，不过很遗憾的是目前RecycleBin当中还没有缓存任何的View，所以这里得到的值肯定是null。那么取得了null之后就会继续向下运行，到第16行会调用obtainView()方法来再次尝试获取一个View，这次的obtainView()方法是可以保证一定返回一个View的，于是下面立刻将获取到的View传入到了setupChild()方法当中。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：
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);
}
if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);
}
setItemViewLayoutParams(child, position);
if (AccessibilityManager.getInstance(mContext).isEnabled()) {
if (mAccessibilityDelegate == null) {
mAccessibilityDelegate = new ListItemAccessibilityDelegate();
}
if (child.getAccessibilityDelegate() == null) {
child.setAccessibilityDelegate(mAccessibilityDelegate);
}
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
return child;
}
final View transientView = mRecycler.getTransientStateView(position);
第一次 transientView 肯定会为null
这样接下来会往后走，
final View scrapView = mRecycler.getScrapView(position);
final View child = mAdapter.getView(position, scrapView, this);
可想而知
第一次layout的scrapView也肯定会为null，接下会从mAdapter.getView(position, scrapView, this)，适配器中拿到利用布局加载器生成的view（这面就不对适配器的getView（）方法做说明了！）
然后就会拿到view了
接下来：
private void setItemViewLayoutParams(View child, int position) {
final ViewGroup.LayoutParams vlp = child.getLayoutParams();
LayoutParams lp;
if (vlp == null) {
lp = (LayoutParams) generateDefaultLayoutParams();
} else if (!checkLayoutParams(vlp)) {
lp = (LayoutParams) generateLayoutParams(vlp);
} else {
lp = (LayoutParams) vlp;
}
if (mAdapterHasStableIds) {
lp.itemId = mAdapter.getItemId(position);
}
lp.viewType = mAdapter.getItemViewType(position);
lp.isEnabled = mAdapter.isEnabled(position);
if (lp != vlp) {
child.setLayoutParams(lp);
}
}
处理view的LayoutParams，、
在回到makeAndAddView（）这一方法！拿到view会 setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
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);
}
那么这个View也会作为obtainView()的结果进行返回，并最终传入到setupChild()方法当中。其实也就是说，第一次layout过程当中，所有的子View都是调用LayoutInflater的inflate()方法加载出来的，这样就会相对比较耗时，但是不用担心，后面就不会再有这种情况了！
setupChild()方法当中的代码虽然比较多，但是我们只看核心代码的话就非常简单了，刚才调用obtainView()方法获取到的子元素View，这里在第61行调用了addViewInLayout()方法将它添加到了ListView当中。那么根据fillDown()方法中的while循环，会让子元素View将整个ListView控件填满然后就跳出，也就是说即使我们的Adapter中有一千条数据，ListView也只会加载第一屏的数据，剩下的数据反正目前在屏幕上也看不到，所以不会去做多余的加载工作，这样就可以保证ListView中的内容能够迅速展示到屏幕上。
如果需要needToMeasure，会设置view的自身measue和位于父view的位置（layout（）方法），这样第一次layout()方法结束，并在父view展示了满屏的子view。
第二次Layout
layoutChildren()过程会执行两次，而这个过程当中涉及到向ListView中添加子元素，如果相同的逻辑执行两遍的话，那么ListView中就会存在一份重复的数据了。因此ListView在layoutChildren()过程当中做了第二次Layout的逻辑处理，非常巧妙地解决了这个问题，
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;
// Remember stuff we will need down below
switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
index = mNextSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
newSel = getChildAt(index);
}
break;
case LAYOUT_FORCE_TOP:
case LAYOUT_FORCE_BOTTOM:
case LAYOUT_SPECIFIC:
case LAYOUT_SYNC:
break;
case LAYOUT_MOVE_SELECTION:
default:
// Remember the previously selected view
index = mSelectedPosition - mFirstPosition;
if (index >= 0 && index < childCount) {
oldSel = getChildAt(index);
}
// Remember the previous first child
oldFirst = getChildAt(0);
if (mNextSelectedPosition >= 0) {
delta = mNextSelectedPosition - mSelectedPosition;
}
// Caution: newSel might be null
newSel = getChildAt(index + delta);
}
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);
AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
View accessibilityFocusLayoutRestoreView = null;
int accessibilityFocusPosition = INVALID_POSITION;
// Remember which child, if any, had accessibility focus. This must
// occur before recycling any views, since that will clear
// accessibility focus.
final ViewRootImpl viewRootImpl = getViewRootImpl();
if (viewRootImpl != null) {
final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
if (focusHost != null) {
final View focusChild = getAccessibilityFocusedChild(focusHost);
if (focusChild != null) {
if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
|| (focusChild.hasTransientState() && mAdapterHasStableIds)) {
// The views won't be changing, so try to maintain
// focus on the current host and virtual view.
accessibilityFocusLayoutRestoreView = focusHost;
accessibilityFocusLayoutRestoreNode = viewRootImpl
.getAccessibilityFocusedVirtualView();
}
// If all else fails, maintain focus at the same
// position.
accessibilityFocusPosition = getPositionForView(focusChild);
}
}
}
View focusLayoutRestoreDirectChild = null;
View focusLayoutRestoreView = null;
// Take focus back to us temporarily to avoid the eventual call to
// clear focus when removing the focused child below from messing
// things up when ViewAncestor assigns focus back to someone else.
final View focusedChild = getFocusedChild();
if (focusedChild != null) {
// TODO: in some cases focusedChild.getParent() == null
// We can remember the focused view to restore after re-layout
// if the data hasn't changed, or if the focused position is a
// header or footer.
if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
|| focusedChild.hasTransientState() || mAdapterHasStableIds) {
focusLayoutRestoreDirectChild = focusedChild;
// Remember the specific view that had focus.
focusLayoutRestoreView = findFocus();
if (focusLayoutRestoreView != null) {
// Tell it we are going to mess with it.
focusLayoutRestoreView.dispatchStartTemporaryDetach();
}
}
requestFocus();
}
// 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:
final int selectedPosition = reconcileSelectedPosition();
sel = fillSpecific(selectedPosition, mSpecificTop);
/**
* When ListView is resized, FocusSelector requests an async selection for the
* previously focused item to make sure it is still visible. If the item is not
* selectable, it won't regain focus so instead we call FocusSelector
* to directly request focus on the view after it is visible.
*/
if (sel == null && mFocusSelector != null) {
final Runnable focusRunnable = mFocusSelector
.setupFocusIfValid(selectedPosition);
if (focusRunnable != null) {
post(focusRunnable);
}
}
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();
// remove any header/footer that has been temp detached and not re-attached
removeUnusedFixedViews(mHeaderViewInfos);
removeUnusedFixedViews(mFooterViewInfos);
if (sel != null) {
// The current selected item should get focus if items are
// focusable.
if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
focusLayoutRestoreView != null &&
focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
if (!focusWasTaken) {
// Selected item didn't take focus, but we still want to
// make sure something else outside of the selected view
// has focus.
final View focused = getFocusedChild();
if (focused != null) {
focused.clearFocus();
}
positionSelector(INVALID_POSITION, sel);
} else {
sel.setSelected(false);
mSelectorRect.setEmpty();
}
} else {
positionSelector(INVALID_POSITION, sel);
}
mSelectedTop = sel.getTop();
} else {
final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
|| mTouchMode == TOUCH_MODE_DONE_WAITING;
if (inTouchMode) {
// If the user's finger is down, select the motion position.
final View child = getChildAt(mMotionPosition - mFirstPosition);
if (child != null) {
positionSelector(mMotionPosition, child);
}
} else if (mSelectorPosition != INVALID_POSITION) {
// If we had previously positioned the selector somewhere,
// put it back there. It might not match up with the data,
// but it's transitioning out so it's not a big deal.
final View child = getChildAt(mSelectorPosition - mFirstPosition);
if (child != null) {
positionSelector(mSelectorPosition, child);
}
} else {
// Otherwise, clear selection.
mSelectedTop = 0;
mSelectorRect.setEmpty();
}
// Even if there is not selected position, we may need to
// restore focus (i.e. something focusable in touch mode).
if (hasFocus() && focusLayoutRestoreView != null) {
focusLayoutRestoreView.requestFocus();
}
}
// Attempt to restore accessibility focus, if necessary.
if (viewRootImpl != null) {
final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost();
if (newAccessibilityFocusedView == null) {
if (accessibilityFocusLayoutRestoreView != null
&& accessibilityFocusLayoutRestoreView.isAttachedToWindow()) {
final AccessibilityNodeProvider provider =
accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider();
if (accessibilityFocusLayoutRestoreNode != null && provider != null) {
final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId(
accessibilityFocusLayoutRestoreNode.getSourceNodeId());
provider.performAction(virtualViewId,
AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null);
} else {
accessibilityFocusLayoutRestoreView.requestAccessibilityFocus();
}
} else if (accessibilityFocusPosition != INVALID_POSITION) {
// Bound the position within the visible children.
final int position = MathUtils.constrain(
accessibilityFocusPosition - mFirstPosition, 0,
getChildCount() - 1);
final View restoreView = getChildAt(position);
if (restoreView != null) {
restoreView.requestAccessibilityFocus();
}
}
}
}
// 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;
}
}
}
final int childCount = getChildCount();
同样还是这段代码，调用getChildCount()方法来获取子View的数量，只不过现在得到的值不会再是0了，而是ListView中一屏可以显示的子View数量，因为我们刚刚在第一次Layout过程当中向ListView添加了这么多的子View。
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}
调用了RecycleBin的fillActiveViews()方法，
这次效果可就不一样了，因为目前ListView中已经有子View了，这样所有的子View都会被缓存到RecycleBin的mActiveViews数组当中，后面将会用到它们。
detachAllViewsFromParent()
接下来将会是非常非常重要的一个操作，调用了detachAllViewsFromParent()方法。这个方法会将所有ListView当中的子View全部清除掉，从而保证第二次Layout过程不会产生一份重复的数据。那有的朋友可能会问了，这样把已经加载好的View又清除掉，待会还要再重新加载一遍，这不是严重影响效率吗？不用担心，还记得我们刚刚调用了RecycleBin的fillActiveViews()方法来缓存子View吗，待会儿将会直接使用这些缓存好的View来进行加载，而并不会重新执行一遍inflate过程，因此效率方面并不会有什么明显的影响。
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;
这样与第一次的Layout大不相同，childCount不会为0，
这样就会走else这面的代码！
那么我们接着，由于不再等于0了，因此会进入到else语句当中。而else语句中又有三个逻辑判断，第一个逻辑判断不成立，因为默认情况下我们没有选中任何子元素，mSelectedPosition应该等于-1。第二个逻辑判断通常是成立的，因为mFirstPosition的值一开始是等于0的，只要adapter中的数据大于0条件就成立。那么进入到fillSpecific()方法当中，代码如下所示：
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()这算是一个新方法了，不过其实它和fillUp()、fillDown()方法功能也是差不多的，主要的区别在于，fillSpecific()方法会优先将指定位置的子View先加载到屏幕上，然后再加载该子View往上以及往下的其它子View。那么由于这里我们传入的position就是第一个子View的位置，于是fillSpecific()方法的作用就基本上和fillDown()方法是差不多的了，这里我们就不去关注太多它的细节，而是将精力放在makeAndAddView()方法上面。再次回到makeAndAddView()方法，代码如下所示：
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;
}
因为
final View activeView = mRecycler.getActiveView(position);
第二次Layout的时候activeView肯定不为空了，因为前面已经
recycleBin.fillActiveViews(childCount, firstPosition);
这样了，所以会再次走向
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);
}
因为这面的isAttachedToWindow是true，可以看到，setupChild()方法的最后一个参数是recycled，然后在
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();
}
会对这个变量进行判断，由于recycled现在是true，所以会执行attachViewToParent()方法，而第一次Layout过程则是执行的else语句中的addViewInLayout()方法。这两个方法最大的区别在于，如果我们需要向ViewGroup中添加一个新的子View，应该调用addViewInLayout()方法，而如果是想要将一个之前detach的View重新attach到ViewGroup上，就应该调用attachViewToParent()方法。那么由于前面在layoutChildren()方法当中调用了detachAllViewsFromParent()方法，这样ListView中所有的子View都是处于detach状态的，所以这里attachViewToParent()方法是正确的选择。再次绑定到ViewGroup中！
protected void attachViewToParent(View child, int index, LayoutParams params) {
child.mLayoutParams = params;
if (index < 0) {
index = mChildrenCount;
}
addInArray(child, index);
child.mParent = this;
child.mPrivateFlags = (child.mPrivateFlags & ~PFLAG_DIRTY_MASK
& ~PFLAG_DRAWING_CACHE_VALID)
| PFLAG_DRAWN | PFLAG_INVALIDATED;
this.mPrivateFlags |= PFLAG_INVALIDATED;
if (child.hasFocus()) {
requestChildFocus(child, child.findFocus());
}
dispatchVisibilityAggregated(isAttachedToWindow() && getWindowVisibility() == VISIBLE
&& isShown());
notifySubtreeAccessibilityStateChangedIfNeeded();
}
通过addInArray(child, index);这个方法把原来detach的view数组加进去
经历了这样一个detach又attach的过程，ListView中所有的子View又都可以正常显示出来了，那么第二次Layout过程结束。
这样ListView就会第一次把itemView全屏展示出来了！
下文将会讲解ListView缓存策略及滑动展示复用view！