从源码开始学习ListView
前言
这个是我人生中写的地一篇博客，如果有写的不好，思维混乱的地方请请请请轻喷。并谢谢指出不对的地方，我好修改。毕竟博客写出来更多的是为了记录学习。
依旧现行感谢一下几个博客对我的指导以及借鉴。如果有看的不明白的可以移步去了解，排名不分前后。
Yogurt_QaQ ：ListView 源码研究 带你从源码中赞叹谷歌大吊们的代码艺术
康志：Android ListView工作原理完全解析，带你从源码的角度彻底理解
孙群：源码解析ListView中的RecycleBin机制
foxleezh：Android系统源码分析--如何下载和阅读Android源码
一、ListView的组成
1、ListView的继承关系
ListView-->AbsListView-->AdapterView-->ViewGroup-->View
由上可知，ListView继承于AbsListView继承AdapterView，再往后就是View的继承，后面暂时不说。等我慢慢看完，学习完在做总结。AdapterView可以知道，google的工程师把ListView分成了两部分，ListView和Adapter。
2、浅谈Adapter
google为了让listview适应更多的需求，承载更多的数据类型。所以使用了Adpater去承载数据，起到一个桥梁的作用，当然adapter还有一个重要的方法getview()；这个后面会详细提到
二、ListView的源码研究
1、RcycleBin机制
这个机制是Listview处理成大数据量时不会OOM的重要部分，先看源码，如下。
/**
* The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
* storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
* start of a layout. By construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews are old views that
* could potentially be used by the adapter to avoid allocating views unnecessarily.
*
* @see android.widget.AbsListView#setRecyclerListener(android.widget.AbsListView.RecyclerListener)
* @see android.widget.AbsListView.RecyclerListener
*/
class RecycleBin {
private RecyclerListener mRecyclerListener;
/**
* The position of the first view stored in mActiveViews.
*/
private int mFirstActivePosition;
/**
* Views that were on screen at the start of layout. This array is populated at the start of
* layout, and at the end of layout all view in mActiveViews are moved to mScrapViews.
* Views in mActiveViews represent a contiguous range of Views, with position of the first
* view store in mFirstActivePosition.
*/
private View[] mActiveViews = new View[0];
/**
* Unsorted views that can be used by the adapter as a convert view.
*/
private ArrayList<View>[] mScrapViews;
private int mViewTypeCount;
private ArrayList<View> mCurrentScrap;
private ArrayList<View> mSkippedScrap;
private SparseArray<View> mTransientStateViews;
private LongSparseArray<View> mTransientStateViewsById;
public void setViewTypeCount(int viewTypeCount) {
if (viewTypeCount < 1) {
throw new IllegalArgumentException("Can't have a viewTypeCount < 1");
}
//noinspection unchecked
ArrayList<View>[] scrapViews = new ArrayList[viewTypeCount];
for (int i = 0; i < viewTypeCount; i++) {
scrapViews[i] = new ArrayList<View>();
}
mViewTypeCount = viewTypeCount;
mCurrentScrap = scrapViews[0];
mScrapViews = scrapViews;
}
public void markChildrenDirty() {
if (mViewTypeCount == 1) {
final ArrayList<View> scrap = mCurrentScrap;
final int scrapCount = scrap.size();
for (int i = 0; i < scrapCount; i++) {
scrap.get(i).forceLayout();
}
} else {
final int typeCount = mViewTypeCount;
for (int i = 0; i < typeCount; i++) {
final ArrayList<View> scrap = mScrapViews[i];
final int scrapCount = scrap.size();
for (int j = 0; j < scrapCount; j++) {
scrap.get(j).forceLayout();
}
}
}
if (mTransientStateViews != null) {
final int count = mTransientStateViews.size();
for (int i = 0; i < count; i++) {
mTransientStateViews.valueAt(i).forceLayout();
}
}
if (mTransientStateViewsById != null) {
final int count = mTransientStateViewsById.size();
for (int i = 0; i < count; i++) {
mTransientStateViewsById.valueAt(i).forceLayout();
}
}
}
public boolean shouldRecycleViewType(int viewType) {
return viewType >= 0;
}
/**
* Clears the scrap heap.
*/
void clear() {
if (mViewTypeCount == 1) {
final ArrayList<View> scrap = mCurrentScrap;
clearScrap(scrap);
} else {
final int typeCount = mViewTypeCount;
for (int i = 0; i < typeCount; i++) {
final ArrayList<View> scrap = mScrapViews[i];
clearScrap(scrap);
}
}
clearTransientStateViews();
}
/**
* Fill ActiveViews with all of the children of the AbsListView.
*
* @param childCount The minimum number of views mActiveViews should hold
* @param firstActivePosition The position of the first view that will be stored in
* mActiveViews
*/
void fillActiveViews(int childCount, int firstActivePosition) {
if (mActiveViews.length < childCount) {
mActiveViews = new View[childCount];
}
mFirstActivePosition = firstActivePosition;
//noinspection MismatchedReadAndWriteOfArray
final View[] activeViews = mActiveViews;
for (int i = 0; i < childCount; i++) {
View child = getChildAt(i);
AbsListView.LayoutParams lp = (AbsListView.LayoutParams) child.getLayoutParams();
// Don't put header or footer views into the scrap heap
if (lp != null && lp.viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
// Note: We do place AdapterView.ITEM_VIEW_TYPE_IGNORE in active views.
// However, we will NOT place them into scrap views.
activeViews[i] = child;
// Remember the position so that setupChild() doesn't reset state.
lp.scrappedFromPosition = firstActivePosition + i;
}
}
}
/**
* Get the view corresponding to the specified position. The view will be removed from
* mActiveViews if it is found.
*
* @param position The position to look up in mActiveViews
* @return The view if it is found, null otherwise
*/
View getActiveView(int position) {
int index = position - mFirstActivePosition;
final View[] activeViews = mActiveViews;
if (index >=0 && index < activeViews.length) {
final View match = activeViews[index];
activeViews[index] = null;
return match;
}
return null;
}
View getTransientStateView(int position) {
if (mAdapter != null && mAdapterHasStableIds && mTransientStateViewsById != null) {
long id = mAdapter.getItemId(position);
View result = mTransientStateViewsById.get(id);
mTransientStateViewsById.remove(id);
return result;
}
if (mTransientStateViews != null) {
final int index = mTransientStateViews.indexOfKey(position);
if (index >= 0) {
View result = mTransientStateViews.valueAt(index);
mTransientStateViews.removeAt(index);
return result;
}
}
return null;
}
/**
* Dumps and fully detaches any currently saved views with transient
* state.
*/
void clearTransientStateViews() {
final SparseArray<View> viewsByPos = mTransientStateViews;
if (viewsByPos != null) {
final int N = viewsByPos.size();
for (int i = 0; i < N; i++) {
removeDetachedView(viewsByPos.valueAt(i), false);
}
viewsByPos.clear();
}
final LongSparseArray<View> viewsById = mTransientStateViewsById;
if (viewsById != null) {
final int N = viewsById.size();
for (int i = 0; i < N; i++) {
removeDetachedView(viewsById.valueAt(i), false);
}
viewsById.clear();
}
}
/**
* @return A view from the ScrapViews collection. These are unordered.
*/
View getScrapView(int position) {
final int whichScrap = mAdapter.getItemViewType(position);
if (whichScrap < 0) {
return null;
}
if (mViewTypeCount == 1) {
return retrieveFromScrap(mCurrentScrap, position);
} else if (whichScrap < mScrapViews.length) {
return retrieveFromScrap(mScrapViews[whichScrap], position);
}
return null;
}
/**
* Puts a view into the list of scrap views.
* <p>
* If the list data hasn't changed or the adapter has stable IDs, views
* with transient state will be preserved for later retrieval.
*
* @param scrap The view to add
* @param position The view's position within its parent
*/
void addScrapView(View scrap, int position) {
final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
if (lp == null) {
// Can't recycle, but we don't know anything about the view.
// Ignore it completely.
return;
}
lp.scrappedFromPosition = position;
// Remove but don't scrap header or footer views, or views that
// should otherwise not be recycled.
final int viewType = lp.viewType;
if (!shouldRecycleViewType(viewType)) {
// Can't recycle. If it's not a header or footer, which have
// special handling and should be ignored, then skip the scrap
// heap and we'll fully detach the view later.
if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
getSkippedScrap().add(scrap);
}
return;
}
scrap.dispatchStartTemporaryDetach();
// The the accessibility state of the view may change while temporary
// detached and we do not allow detached views to fire accessibility
// events. So we are announcing that the subtree changed giving a chance
// to clients holding on to a view in this subtree to refresh it.
notifyViewAccessibilityStateChangedIfNeeded(
AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);
// Don't scrap views that have transient state.
final boolean scrapHasTransientState = scrap.hasTransientState();
if (scrapHasTransientState) {
if (mAdapter != null && mAdapterHasStableIds) {
// If the adapter has stable IDs, we can reuse the view for
// the same data.
if (mTransientStateViewsById == null) {
mTransientStateViewsById = new LongSparseArray<>();
}
mTransientStateViewsById.put(lp.itemId, scrap);
} else if (!mDataChanged) {
// If the data hasn't changed, we can reuse the views at
// their old positions.
if (mTransientStateViews == null) {
mTransientStateViews = new SparseArray<>();
}
mTransientStateViews.put(position, scrap);
} else {
// Otherwise, we'll have to remove the view and start over.
clearScrapForRebind(scrap);
getSkippedScrap().add(scrap);
}
} else {
clearScrapForRebind(scrap);
if (mViewTypeCount == 1) {
mCurrentScrap.add(scrap);
} else {
mScrapViews[viewType].add(scrap);
}
if (mRecyclerListener != null) {
mRecyclerListener.onMovedToScrapHeap(scrap);
}
}
}
private ArrayList<View> getSkippedScrap() {
if (mSkippedScrap == null) {
mSkippedScrap = new ArrayList<>();
}
return mSkippedScrap;
}
/**
* Finish the removal of any views that skipped the scrap heap.
*/
void removeSkippedScrap() {
if (mSkippedScrap == null) {
return;
}
final int count = mSkippedScrap.size();
for (int i = 0; i < count; i++) {
removeDetachedView(mSkippedScrap.get(i), false);
}
mSkippedScrap.clear();
}
/**
* Move all views remaining in mActiveViews to mScrapViews.
*/
void scrapActiveViews() {
final View[] activeViews = mActiveViews;
final boolean hasListener = mRecyclerListener != null;
final boolean multipleScraps = mViewTypeCount > 1;
ArrayList<View> scrapViews = mCurrentScrap;
final int count = activeViews.length;
for (int i = count - 1; i >= 0; i--) {
final View victim = activeViews[i];
if (victim != null) {
final AbsListView.LayoutParams lp
= (AbsListView.LayoutParams) victim.getLayoutParams();
final int whichScrap = lp.viewType;
activeViews[i] = null;
if (victim.hasTransientState()) {
// Store views with transient state for later use.
victim.dispatchStartTemporaryDetach();
if (mAdapter != null && mAdapterHasStableIds) {
if (mTransientStateViewsById == null) {
mTransientStateViewsById = new LongSparseArray<View>();
}
long id = mAdapter.getItemId(mFirstActivePosition + i);
mTransientStateViewsById.put(id, victim);
} else if (!mDataChanged) {
if (mTransientStateViews == null) {
mTransientStateViews = new SparseArray<View>();
}
mTransientStateViews.put(mFirstActivePosition + i, victim);
} else if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
// The data has changed, we can't keep this view.
removeDetachedView(victim, false);
}
} else if (!shouldRecycleViewType(whichScrap)) {
// Discard non-recyclable views except headers/footers.
if (whichScrap != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
removeDetachedView(victim, false);
}
} else {
// Store everything else on the appropriate scrap heap.
if (multipleScraps) {
scrapViews = mScrapViews[whichScrap];
}
lp.scrappedFromPosition = mFirstActivePosition + i;
removeDetachedView(victim, false);
scrapViews.add(victim);
if (hasListener) {
mRecyclerListener.onMovedToScrapHeap(victim);
}
}
}
}
pruneScrapViews();
}
/**
* At the end of a layout pass, all temp detached views should either be re-attached or
* completely detached. This method ensures that any remaining view in the scrap list is
* fully detached.
*/
void fullyDetachScrapViews() {
final int viewTypeCount = mViewTypeCount;
final ArrayList<View>[] scrapViews = mScrapViews;
for (int i = 0; i < viewTypeCount; ++i) {
final ArrayList<View> scrapPile = scrapViews[i];
for (int j = scrapPile.size() - 1; j >= 0; j--) {
final View view = scrapPile.get(j);
if (view.isTemporarilyDetached()) {
removeDetachedView(view, false);
}
}
}
}
/**
* Makes sure that the size of mScrapViews does not exceed the size of
* mActiveViews, which can happen if an adapter does not recycle its
* views. Removes cached transient state views that no longer have
* transient state.
*/
private void pruneScrapViews() {
final int maxViews = mActiveViews.length;
final int viewTypeCount = mViewTypeCount;
final ArrayList<View>[] scrapViews = mScrapViews;
for (int i = 0; i < viewTypeCount; ++i) {
final ArrayList<View> scrapPile = scrapViews[i];
int size = scrapPile.size();
while (size > maxViews) {
scrapPile.remove(--size);
}
}
final SparseArray<View> transViewsByPos = mTransientStateViews;
if (transViewsByPos != null) {
for (int i = 0; i < transViewsByPos.size(); i++) {
final View v = transViewsByPos.valueAt(i);
if (!v.hasTransientState()) {
removeDetachedView(v, false);
transViewsByPos.removeAt(i);
i--;
}
}
}
final LongSparseArray<View> transViewsById = mTransientStateViewsById;
if (transViewsById != null) {
for (int i = 0; i < transViewsById.size(); i++) {
final View v = transViewsById.valueAt(i);
if (!v.hasTransientState()) {
removeDetachedView(v, false);
transViewsById.removeAt(i);
i--;
}
}
}
}
/**
* Puts all views in the scrap heap into the supplied list.
*/
void reclaimScrapViews(List<View> views) {
if (mViewTypeCount == 1) {
views.addAll(mCurrentScrap);
} else {
final int viewTypeCount = mViewTypeCount;
final ArrayList<View>[] scrapViews = mScrapViews;
for (int i = 0; i < viewTypeCount; ++i) {
final ArrayList<View> scrapPile = scrapViews[i];
views.addAll(scrapPile);
}
}
}
/**
* Updates the cache color hint of all known views.
*
* @param color The new cache color hint.
*/
void setCacheColorHint(int color) {
if (mViewTypeCount == 1) {
final ArrayList<View> scrap = mCurrentScrap;
final int scrapCount = scrap.size();
for (int i = 0; i < scrapCount; i++) {
scrap.get(i).setDrawingCacheBackgroundColor(color);
}
} else {
final int typeCount = mViewTypeCount;
for (int i = 0; i < typeCount; i++) {
final ArrayList<View> scrap = mScrapViews[i];
final int scrapCount = scrap.size();
for (int j = 0; j < scrapCount; j++) {
scrap.get(j).setDrawingCacheBackgroundColor(color);
}
}
}
// Just in case this is called during a layout pass
final View[] activeViews = mActiveViews;
final int count = activeViews.length;
for (int i = 0; i < count; ++i) {
final View victim = activeViews[i];
if (victim != null) {
victim.setDrawingCacheBackgroundColor(color);
}
}
}
private View retrieveFromScrap(ArrayList<View> scrapViews, int position) {
final int size = scrapViews.size();
if (size > 0) {
// See if we still have a view for this position or ID.
// Traverse backwards to find the most recently used scrap view
for (int i = size - 1; i >= 0; i--) {
final View view = scrapViews.get(i);
final AbsListView.LayoutParams params =
(AbsListView.LayoutParams) view.getLayoutParams();
if (mAdapterHasStableIds) {
final long id = mAdapter.getItemId(position);
if (id == params.itemId) {
return scrapViews.remove(i);
}
} else if (params.scrappedFromPosition == position) {
final View scrap = scrapViews.remove(i);
clearScrapForRebind(scrap);
return scrap;
}
}
final View scrap = scrapViews.remove(size - 1);
clearScrapForRebind(scrap);
return scrap;
} else {
return null;
}
}
private void clearScrap(final ArrayList<View> scrap) {
final int scrapCount = scrap.size();
for (int j = 0; j < scrapCount; j++) {
removeDetachedView(scrap.remove(scrapCount - 1 - j), false);
}
}
private void clearScrapForRebind(View view) {
view.clearAccessibilityFocus();
view.setAccessibilityDelegate(null);
}
private void removeDetachedView(View child, boolean animate) {
child.setAccessibilityDelegate(null);
AbsListView.this.removeDetachedView(child, animate);
}
}源码不多只有300多行，咱们先看开头，google对这个的注释。
The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
* storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
* start of a layout. By construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to The RecycleBin facilitates reuse of views across layouts. The RecycleBin has two levels of
* storage: ActiveViews and ScrapViews. ActiveViews are those views which were onscreen at the
* start of a layout. By construction, they are displaying current information. At the end of
* layout, all views in ActiveViews are demoted to ScrapViews. ScrapViews are old views that
* could potentially be used by the adapter to avoid allocating views unnecessarily.. ScrapViews are old views that
* could potentially be used by the adapter to avoid allocating views unnecessarily.可知，这个内部类中最关键的是ActiveViews和ScrapViews。更据这两个关键词我们去找对应类，如下：
fillActiveViews()
* 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; } } }老样子，先从注释看起，这里还是要感叹一下自己平时，写代码要多加注释！！言归正传，注释上说，把AbsListView中的指定元素放到ActiveViews数组中。然后看传入的两个参数，childCount，firstActivePosition。@param中都有写，一个是传入需要储存view的数量，另一个是ListView中第一个可见view的position。调用这个函数主要是将view添加进ActiveViews数组。
getActiveView()
* 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; }调用这个函数，其实注释里面说的非常清楚了，获取一个position下的View然后它就从mActiveViews数组中移除了。意思是什么呢，一个view只能用一个，用完以后就被废弃了。那么新的view怎么加入，废弃的view怎么再利用呢？接着往下看。
- addScrapView()
Puts a view into the list of scrap views.
* <p> * If the list data hasn't changed or the adapter has stable IDs, views * with transient state will be preserved for later retrieval. * * @param scrap The view to add * @param position The view's position within its parent */ void addScrapView(View scrap, int position) { final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams(); if (lp == null) { // Can't recycle, but we don't know anything about the view. // Ignore it completely. return; } lp.scrappedFromPosition = position; // Remove but don't scrap header or footer views, or views that // should otherwise not be recycled. final int viewType = lp.viewType; if (!shouldRecycleViewType(viewType)) { // Can't recycle. If it's not a header or footer, which have // special handling and should be ignored, then skip the scrap // heap and we'll fully detach the view later. if (viewType != ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { getSkippedScrap().add(scrap); } return; } scrap.dispatchStartTemporaryDetach(); // The the accessibility state of the view may change while temporary // detached and we do not allow detached views to fire accessibility // events. So we are announcing that the subtree changed giving a chance // to clients holding on to a view in this subtree to refresh it. notifyViewAccessibilityStateChangedIfNeeded( AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE); // Don't scrap views that have transient state. final boolean scrapHasTransientState = scrap.hasTransientState(); if (scrapHasTransientState) { if (mAdapter != null && mAdapterHasStableIds) { // If the adapter has stable IDs, we can reuse the view for // the same data. if (mTransientStateViewsById == null) { mTransientStateViewsById = new LongSparseArray<>(); } mTransientStateViewsById.put(lp.itemId, scrap); } else if (!mDataChanged) { // If the data hasn't changed, we can reuse the views at // their old positions. if (mTransientStateViews == null) { mTransientStateViews = new SparseArray<>(); } mTransientStateViews.put(position, scrap); } else { // Otherwise, we'll have to remove the view and start over. clearScrapForRebind(scrap); getSkippedScrap().add(scrap); } } else { clearScrapForRebind(scrap); if (mViewTypeCount == 1) { mCurrentScrap.add(scrap); } else { mScrapViews[viewType].add(scrap); } if (mRecyclerListener != null) { mRecyclerListener.onMovedToScrapHeap(scrap); } } }这个函数，更据他的代码量就知道他是recyclebin中很关键的函数，意思很简单，调用这个函数以后将view放进scrapviews数组。两个传入的变量，一个是废弃的view，一个是view的位置。
getScrapView()
* @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; }这个函数就很好理解了，就是将mScrapViews中最后一个取出来。
上面这四个函数超级重要，一定要牢记三、ListView源码流程
- 第一次layout
首先，ListView也是ViewGoup也是View，所以说ListView也是基本的三步绘制流程。onmeasure()测量View的大小,onlayout()确认View的布局,ondraw()绘制View到界面上。onmeasure()在ListView中没有什么特殊的。但是在onlayout中体现出不一样了，我们从onlayout的源码一层层看。
看源码可以发现ListView中并没有onlayout方法，那么我们去父类AbsListView中去看，果然我们发现了onlayout。
/**
* 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;
}方法代码量很少，主要是一个判断，如果View发生改变，那么就重新绘制所有的view。然后就是layoutChildren()方法，这个从名字上可以看出，是绘制子view的方法，我们跟进去看。
* Subclasses must override this method to layout their children.
*/
protected void layoutChildren() {
}发现并没有写什么，当然这句话是废话，因为这是父类，不会涉及到具体的view绘制。所以我们回到ListView中去找这个方法。
@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;
// 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;
}
}
}源码超级多，我们走大流程分析吧。首先先看invalidate();这个是在合适的时候出发ondarw()方法，就是在Adapter中数据有变化的时候调用，final int childCount = getChildCount();第一次ListView绘制的时候，是没有子view的，所以肯定是0。
再往后就是 final RecycleBin recycleBin = mRecycler;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}这里执行了一个判断如果dataChanged，那么会执行addScrapView把view全部废弃。如果没有chang那么执行fillActiveViews，把view添加进数组。这两个函数在recyclebin中有说过，忘了回看。但是现在还没有填充数据，所以子view中没有东西，所以这行代码暂时不起效果。
关于mLayoutMode这个值默认都是LAYOUT_NORMAL，所以会进入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);
}
}首先判断了childCount是否等于0，然后验证了mSelectedPosition是否合规。默认布局就是从上向下依次填充view，所以是fillTOP，然后FillFromtop的return是filldown()。
* 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);
}可知fillDown才是绘制的真正方法，我们进入方法内部。
* 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;
}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。
跳出循环中执行了makeAndAddView()方法，进入这个方法看一下。
* 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;
}首先看mDataChanged，前面getchildview是0，所以mDataChanged为FALSE，然后从ActiveView数组中拿出来一个view。但是数组为空，所以肯定view为空，所以跳出if，执行obtainView。
View obtainView(int position, boolean[] isScrap) {
isScrap[0] = false;
View scrapView;
scrapView = mRecycler.getScrapView(position);
View child;
if (scrapView != null) {
child = mAdapter.getView(position, scrapView, this);
if (child != scrapView) {
mRecycler.addScrapView(scrapView);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
} else {
isScrap[0] = true;
dispatchFinishTemporaryDetach(child);
}
} else {
child = mAdapter.getView(position, null, this);
if (mCacheColorHint != 0) {
child.setDrawingCacheBackgroundColor(mCacheColorHint);
}
}
return child;
}可以看得出来，这里肯定会返回一个child，一个view。这个view怎么来的呢，先看源码。首先尝试从scrapView数组中获取一个view，但是我们是第一次绘制listview所以肯定失败了。然后从else看，child = mAdapter.getView(position, null, this);从mAdapter中获取一个view。就是前文提到的mAdapter中的getview了。这个时候ListView和adapter联系上了。
现在我们去adapter中看getview方法。
* Get a View that displays the data at the specified position in the data set. You can either
* create a View manually or inflate it from an XML layout file. When the View is inflated, the
* parent View (GridView, ListView...) will apply default layout parameters unless you use
* {@link android.view.LayoutInflater#inflate(int, android.view.ViewGroup, boolean)}
* to specify a root view and to prevent attachment to the root.
*
* @param position The position of the item within the adapter's data set of the item whose view
* we want.
* @param convertView The old view to reuse, if possible. Note: You should check that this view
* is non-null and of an appropriate type before using. If it is not possible to convert
* this view to display the correct data, this method can create a new view.
* Heterogeneous lists can specify their number of view types, so that this View is
* always of the right type (see {@link #getViewTypeCount()} and
* {@link #getItemViewType(int)}).
* @param parent The parent that this view will eventually be attached to
* @return A View corresponding to the data at the specified position.
*/
View getView(int position, View convertView, ViewGroup parent);通过注释我们可以看到getview在GridView, ListView...等地方都会用到，在LayoutInflater#inflate中实现,那么我们看getview()的三个参数，position view viewgroup。这里我们第一次执行getview，传入的参数为geview(position,null,this),主要说view为null的情况，我们会调LayoutInflater.inflate()。getview返回，当然obtainView也返回了一个child。这就是为什么第一次加载页面卡的原因，每一个view都需要通过LayoutInflater.inflate()调用出来。
然后obtainView返回了一个view，makeAndAddView的child那么也有view了，然后执行setupChild()方法。源码上面都有，可仔细看一下。如果不方便我争取做一个思维导图。
* 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);
}老样子，先看注释。作为一个子view添加确保这个view已经被测量过以及位置正确。代码比较多，我们看关键的。既然要添加view，所以我们看我们传进去的子view，什么时候被调用。attachViewToParent()和addViewInLayout()。这两个viewgroupo的方法有调用，好我们具体来看一下。主要是一个判断Header和Footer，还记得前面的代码注释么？第一次layout不添加Header和Footer，所以我们先执行addViewInLayout()，把view添加到listliew中，fillDown()，更据while的魂环我们可知，无论多少数据量，只会填充第一屏的数据。到此为止，第一次layout执行结束。
- 第2、3次onlayout
为什么要把第二次和第三次放在一起呢，这个我倒是没试过，但是前人的经验和结果告诉我，view中都会经历至少两次onMeasure()和两次onLayout()的过程。对于其他的view还行，但是对于listview这个可就是个问题了，listview如果经历了两次onMeasure()和onLayout()，这样的话就会有重复数据。
那么问题来了，如何保证不添加重复数据的呢。在第一次layout中的layoutchild()，第19行，geichild()，因为添加了子view，所以不为0，这样在第90行，第一次layout的时候因为当时没有view，代码不起作用。（忘了可以回看第一次layout）现在有了子view，所以就会被添加到mActiveViews数组中，然后关键来了！官方也为此添加了一份注释。
Clear out old views detachAllViewsFromParent();
他就是反复多次onMeasure()和onLayout不会有重复数据的原因。因为，我们在清除前以及做了一次view的缓存所以，并不会有多次加载浪费资源的情况出现。然后继续走141行if (childCount == 0) （没找到怎么加行数=。=尴尬），
因为有子view了，所以childCount不等于0，然后走else，就不是filldown了，而是fillSpecific()，我们进入这个方法。
/**
* 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;
}
}老规矩，我们先看一下官方对他的注解：添加一个特殊的item在屏幕中特殊的位置，然后在从他的上或者下添item。然后我们一层层看，看到了个老朋友makeAndAddView();这个肯定要进去看一下走什么逻辑了。
/**
* 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;
}又是在19行，还是一样的方法，获取getActiveView(),这下肯定没问题了，进入if执行setupChild(),然后就直接return了。进去setupChild()，注意，这里的setupchild()，最后一个参数是true。
/**
* 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);
}最后一个boolean值是isAttachedToWindow，不一样的地方咱们打高亮看，看到了if，我们进去，因为是TRUE，所以执行attachViewToParent();因为，我们第二次layout的view是被清除掉的，状态为death的view，所以我们使用attachViewToParent()方法，为什么第一次layout使用addViewInLayout呢？
是因为viewgroup中创建view使用addViewInLayout，重用也就是状态为death的view用attachViewToParent()方法。再深层次就不细究了。等以后有时间再说吧。。。。。这里两次onlayout，才可以说Listview真正的展现出来了。
四、滑动LIsetView
因为我是做机顶盒TV的所以我想探究一下用按键事件ListView的数据是怎么变化的。这块都是自己找的源码，有不对请指正！
一、首先确定监听器，按键事件是onkey和onfocus。首先在AbsListView中查看。
@Override
protected void onFocusChanged(boolean gainFocus, int direction, Rect previouslyFocusedRect) {
super.onFocusChanged(gainFocus, direction, previouslyFocusedRect);
if (gainFocus && mSelectedPosition < 0 && !isInTouchMode()) {
if (!isAttachedToWindow() && mAdapter != null) {
// Data may have changed while we were detached and it's valid
// to change focus while detached. Refresh so we don't die.
mDataChanged = true;
mOldItemCount = mItemCount;
mItemCount = mAdapter.getCount();
}
resurrectSelection();
}
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
return false;
}
@Override
public boolean onKeyUp(int keyCode, KeyEvent event) {
if (KeyEvent.isConfirmKey(keyCode)) {
if (!isEnabled()) {
return true;
}
if (isClickable() && isPressed() &&
mSelectedPosition >= 0 && mAdapter != null &&
mSelectedPosition < mAdapter.getCount()) {
final View view = getChildAt(mSelectedPosition - mFirstPosition);
if (view != null) {
performItemClick(view, mSelectedPosition, mSelectedRowId);
view.setPressed(false);
}
setPressed(false);
return true;
}
}
return super.onKeyUp(keyCode, event);
}然而并没有什么发现，然后去ListView中寻找。
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
return commonKey(keyCode, 1, event);
}
@Override
public boolean onKeyMultiple(int keyCode, int repeatCount, KeyEvent event) {
return commonKey(keyCode, repeatCount, event);
}
@Override
public boolean onKeyUp(int keyCode, KeyEvent event) {
return commonKey(keyCode, 1, event);
}
private boolean commonKey(int keyCode, int count, KeyEvent event) {
if (mAdapter == null || !isAttachedToWindow()) {
return false;
}
if (mDataChanged) {
layoutChildren();
}
boolean handled = false;
int action = event.getAction();
if (KeyEvent.isConfirmKey(keyCode)
&& event.hasNoModifiers() && action != KeyEvent.ACTION_UP) {
handled = resurrectSelectionIfNeeded();
if (!handled && event.getRepeatCount() == 0 && getChildCount() > 0) {
keyPressed();
handled = true;
}
}
if (!handled && action != KeyEvent.ACTION_UP) {
switch (keyCode) {
case KeyEvent.KEYCODE_DPAD_UP:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded();
if (!handled) {
while (count-- > 0) {
if (arrowScroll(FOCUS_UP)) {
handled = true;
} else {
break;
}
}
}
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_UP);
}
break;
case KeyEvent.KEYCODE_DPAD_DOWN:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded();
if (!handled) {
while (count-- > 0) {
if (arrowScroll(FOCUS_DOWN)) {
handled = true;
} else {
break;
}
}
}
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_DOWN);
}
break;
case KeyEvent.KEYCODE_DPAD_LEFT:
if (event.hasNoModifiers()) {
handled = handleHorizontalFocusWithinListItem(View.FOCUS_LEFT);
}
break;
case KeyEvent.KEYCODE_DPAD_RIGHT:
if (event.hasNoModifiers()) {
handled = handleHorizontalFocusWithinListItem(View.FOCUS_RIGHT);
}
break;
case KeyEvent.KEYCODE_PAGE_UP:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || pageScroll(FOCUS_UP);
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_UP);
}
break;
case KeyEvent.KEYCODE_PAGE_DOWN:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || pageScroll(FOCUS_DOWN);
} else if (event.hasModifiers(KeyEvent.META_ALT_ON)) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_DOWN);
}
break;
case KeyEvent.KEYCODE_MOVE_HOME:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_UP);
}
break;
case KeyEvent.KEYCODE_MOVE_END:
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || fullScroll(FOCUS_DOWN);
}
break;
case KeyEvent.KEYCODE_TAB:
// This creates an asymmetry in TAB navigation order. At some
// point in the future we may decide that it's preferable to
// force the list selection to the top or bottom when receiving
// TAB focus from another widget, but for now this is adequate.
if (event.hasNoModifiers()) {
handled = resurrectSelectionIfNeeded() || arrowScroll(FOCUS_DOWN);
} else if (event.hasModifiers(KeyEvent.META_SHIFT_ON)) {
handled = resurrectSelectionIfNeeded() || arrowScroll(FOCUS_UP);
}
break;
}
}
if (handled) {
return true;
}
if (sendToTextFilter(keyCode, count, event)) {
return true;
}
switch (action) {
case KeyEvent.ACTION_DOWN:
return super.onKeyDown(keyCode, event);
case KeyEvent.ACTION_UP:
return super.onKeyUp(keyCode, event);
case KeyEvent.ACTION_MULTIPLE:
return super.onKeyMultiple(keyCode, count, event);
default: // shouldn't happen
return false;
}
}在ListView中onkey事件被分成了3种事件：onKeyDown、onKeyMultiple、onKeyUp。都是调用commonKey()方法，结果没有什么发现。我们再看onfocus()事件。
@Override
protected void onFocusChanged(boolean gainFocus, int direction, Rect previouslyFocusedRect) {
super.onFocusChanged(gainFocus, direction, previouslyFocusedRect);
final ListAdapter adapter = mAdapter;
int closetChildIndex = -1;
int closestChildTop = 0;
if (adapter != null && gainFocus && previouslyFocusedRect != null) {
previouslyFocusedRect.offset(mScrollX, mScrollY);
// Don't cache the result of getChildCount or mFirstPosition here,
// it could change in layoutChildren.
if (adapter.getCount() < getChildCount() + mFirstPosition) {
mLayoutMode = LAYOUT_NORMAL;
layoutChildren();
}
// figure out which item should be selected based on previously
// focused rect
Rect otherRect = mTempRect;
int minDistance = Integer.MAX_VALUE;
final int childCount = getChildCount();
final int firstPosition = mFirstPosition;
for (int i = 0; i < childCount; i++) {
// only consider selectable views
if (!adapter.isEnabled(firstPosition + i)) {
continue;
}
View other = getChildAt(i);
other.getDrawingRect(otherRect);
offsetDescendantRectToMyCoords(other, otherRect);
int distance = getDistance(previouslyFocusedRect, otherRect, direction);
if (distance < minDistance) {
minDistance = distance;
closetChildIndex = i;
closestChildTop = other.getTop();
}
}
}
if (closetChildIndex >= 0) {
setSelectionFromTop(closetChildIndex + mFirstPosition, closestChildTop);
} else {
requestLayout();
}
}代码量很少，古话说得好，有备注就有希望，结果真被我发现了。
// Don't cache the result of getChildCount or mFirstPosition here,
// it could change in layoutChildren.
if (adapter.getCount() < getChildCount() + mFirstPosition) {
mLayoutMode = LAYOUT_NORMAL;
layoutChildren();
}备注是不要缓存这里获得子节点个数或mfirstposition结果，它可以改变layoutchildren。
这里的if判断就是如果adapter.getCount()小于getChildCount() + mFirstPosition。
意思就是在最后一个item变成倒数第二个item了，这个时候就要新增一个item，并且回收。我们继续往里面看，前面也有说mLayoutMode的默认方式是LAYOUT_NORMAL，然后进入 layoutChildren()方法。
又回到熟悉的方法了。老样子前面的就不说了，不过要变得是datachange了，所以我们要把最后一个item前面的所有item加入scrpview数组，然后直接去关键点，fillSpecific()方法中makeAndAddView()，返回一个view就是新增的item，然后往下走。在第二次layout没有往下走是因为我们新增的是第一个item，所以后面的无所谓，但是现在不一样了。
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);
}
}以前都是走上面，现在是走else，先filldown，加载下一个数据，然后关键（注释）来了adjustViewsUpOrDown()，这将更正最后一个视图的底部，而不是在列表的底部。意思是将屏幕外的第一个item加载到视图底部，然后再用fillup()加载现在最后一个item上面的其余item。
fillup()和filldown()都是通过循坏去对listview进行加载，但是涉及到加载就会使用makeAndAddView()方法。这次我们再看一下流程，具体可以参考第一次layout。到了obtainView()这个方法的时候就不一样了。我们看一下源码。
/**
* 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);
}
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;
}scrapView，这里是第一次layout没有调到的地方，这次scrapView数组里面是有值了。我们一旦有新的数据需要显示，就会从scrapView数组中取出。所以关于LIstview，不管有多少条数据，永远只有那么几个，这个就是OOM不会出现的原因。
这个是图片总结，从康志大大的博客上取来的，偷了点懒
五、总结
这篇博客从准备到学习花了有快1个星期的事件，都是下班时间一点点看，一点点学，才毕业没多久不想让自己荒废。后续我还会学习其他的源码，还有一些实践app希望与大家分享。在这个知识共享的年代一起进步学习！
不是打广告，是我下一步的学习目标。
HenCoder Android 开发进阶: 自定义 View
随便看了看，真的受益匪浅。
