ListView 源码研究 带你从源码中赞叹谷歌大吊们的代码艺术

ListView有一个非常神奇的功能，即使在ListView中加载非常非常多的数据，比如达到成百上千条甚至更多，ListView都不会发生OOM或者崩溃，而且随着我们手指滑动来浏览更多数据时，程序所占用的内存竟然都不会跟着增长。

在ListView的父类AbsListView中有一个非常重要的机制叫做RecycleBin：

它的实现是一个RecycleBin 类

我们看下里面的一些主要的方法

class RecycleBin {
private RecyclerListener mRecyclerListener;

private int mFirstActivePosition;

private View[] mActiveViews = new View[0];

private ArrayList<View>[] mScrapViews;

private int mViewTypeCount;

private ArrayList<View> mCurrentScrap;

void fillActiveViews(int childCount, int firstActivePosition) {//.....}

View getActiveView(int position) {//.....}

void addScrapView(View scrap) {//.....}

View getScrapView(int position) {//...}

public void setViewTypeCount(int viewTypeCount) {//.....}
}

我们一个一个去研究：

fillActiveViews()

这个方法接收两个参数，第一个参数表示要存储的view的数量，第二个参数表示ListView中第一个可见元素的position值。RecycleBin当中使用mActiveViews这个数组来存储View，调用这个方法后就会根据传入的参数来将ListView中的指定元素存储到mActiveViews数组当中，除开的特例就是if 语句中我们不会将 Header和Footer加入到mActiveViews数组中。

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;
}
}
}

getActiveView()

fillActiveViews()是对应的，用于从mActiveViews数组当中获取数据。该方法接收一个position参数，表示元素在ListView当中的位置，方法内部会自动将position值转换成mActiveViews数组对应的下标值。需要注意的是，mActiveViews当中所存储的View，一旦被获取了之后就会从mActiveViews当中移除，下次获取同样位置的View将会返回null，也就是说mActiveViews不能被重复利用。注意一个小细节，我们是首先将传入的position 转换为了在mActiveViews数组中的下标：

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;
}

addScrapView()

用于将一个废弃的View进行缓存，该方法接收一个View参数，当有某个View确定要废弃掉的时候(比如滚动出了屏幕)，就应该调用这个方法来对View进行缓存，RecycleBin当中使用mScrapViews和mCurrentScrap这两个List来存储废弃View。

首先贴出源码：

void addScrapView(View scrap, int position) {
final AbsListView.LayoutParams lp = (AbsListView.LayoutParams) scrap.getLayoutParams();
if (lp == null) {
return;
}

lp.scrappedFromPosition = position;

final int viewType = lp.viewType;
if (!shouldRecycleViewType(viewType)) {
return;
}

scrap.dispatchStartTemporaryDetach();

notifyViewAccessibilityStateChangedIfNeeded(
AccessibilityEvent.CONTENT_CHANGE_TYPE_SUBTREE);

final boolean scrapHasTransientState = scrap.hasTransientState();
if (scrapHasTransientState) {
if (mAdapter != null && mAdapterHasStableIds) {

if (mTransientStateViewsById == null) {
mTransientStateViewsById = new LongSparseArray<View>();
}
mTransientStateViewsById.put(lp.itemId, scrap);
} else if (!mDataChanged) {
if (mTransientStateViews == null) {
mTransientStateViews = new SparseArray<View>();
}
mTransientStateViews.put(position, scrap);
} else {
if (mSkippedScrap == null) {
mSkippedScrap = new ArrayList<View>();
}
mSkippedScrap.add(scrap);
}
} else {
if (mViewTypeCount == 1) {
mCurrentScrap.add(scrap);
} else {
mScrapViews[viewType].add(scrap);
}

if (scrap.isAccessibilityFocused()) {
scrap.clearAccessibilityFocus();
}

scrap.setAccessibilityDelegate(null);

if (mRecyclerListener != null) {
mRecyclerListener.onMovedToScrapHeap(scrap);
}
}
}

在进行itemView回收之前调用了shouldRecycleViewType进行判断这个itemView值不值得回收；满足回收标准就是ItemView.getLayoutParams().viewType>=0，viewType值是什么呢？追踪代码可以发现源代码中的注释是：它的值等于Adapter中getItemViewType(int)方法的返回值。这个方法在BaseAdapter中有实现，就简单的返回了0.所以在我们开发过程中如果让我们的Adapter继承BaseAdapter并且没有重写getItemViewType(int)方法的话，我们的Adapter产生的itemView是全部满足回收条件的。接下来会依据：

final boolean scrapHasTransientState = scrap.hasTransientState();

判断是否是瞬态 也就是transient state

如果是瞬态（transient state）那么会依据情况将view放入到mTransientStateViewsById中否则：

if (mViewTypeCount == 1) {
mCurrentScrap.add(scrap);
} else {
mScrapViews[viewType].add(scrap);
}

getScrapView

用于从废弃缓存中取出一个View:

a.第三种情况，这个最简单：

一开始，listview稳定后，此时mScrapView中是没有缓存view的，当我们向上滚动一小段距离（第一个此时仍显示部分），新的view将会显示，此时listview会调用Adapter.getView，但是缓存中没有，因此convertView是null，所以，我们得分配一块内存来创建新的convertView；

b.第二种情况：

在a中，我们继续向上滚动，直接第一个view完全移出屏幕（假设没有新的item），此时，第一个view就会被detach，并被加入到mScrapView中；然后，我们还继续向上滚动，直接后面又将要显示新的item view时，此时，系统会从mScrapView中找position对应的View，显然，是找不到的，则将从mScrapView中，取最后一个缓存的view传递给convertView；

c.第一种情况：

AbsListView中的LayoutParams继承了ViewGroup的LayoutParams，其中一个增加的成员变量是scrappedFromPosition，这个变量是由addScrapView时传递进来的position赋值，就是当时要回收的view的位置。所以scrappedFromPosition== position的判断就是，如果某个位置的view被回收了，此时该位置又要重新显示出来，那么优先显示原来的view，就是尽量保持原来位置的view不变。当然，如果原来的view已经用在别的位置了，那就没办法了。

View的执行流程无非就分为三步，onMeasure()用于测量View的大小，onLayout()用于确定View的布局，onDraw()用于将View绘制到界面上。而在ListView当中，onMeasure()并没有什么特殊的地方，因为它终归是一个View，占用的空间最多并且通常也就是整个屏幕。onDraw()在ListView当中也没有什么意义，因为ListView本身并不负责绘制，而是由ListView当中的子元素来进行绘制的。那么ListView大部分的神奇功能其实都是在onLayout()方法中进行的了，因此我们本篇文章也是主要分析的这个方法里的内容：

我们在ListView的源码中无法找到onLayout函数，其实实现实在它的父类AbsListView中：

首先看下onLayout：

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
mInLayout = true;
if (changed) {
int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
getChildAt(i).forceLayout();
}
mRecycler.markChildrenDirty();
}

if (mFastScroller != null && (mItemCount != mOldItemCount || mDataChanged)) {
mFastScroller.onItemCountChanged(mItemCount);
}

layoutChildren();
mInLayout = false;

mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;
}

如果changed为true，遍历拿到所有Child执行forceLayout()：清空MeasureCache，设mPrivateFlags为FORCE_LAYOUT，然后调mRecycler的markChildrenDirty()：

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();
}
}
}

通过调用这个函数可以很清楚的看到我们将mCurrentScrap以及mTransientStateViews 中的缓存全部执行一遍forceLayout()

回到onLayout中 接下来便是子类ListView的layoutChildren函数了：

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);

// Remember which child, if any, had accessibility focus.
final int accessibilityFocusPosition;
final View accessFocusedChild = getAccessibilityFocusedChild();
if (accessFocusedChild != null) {
accessibilityFocusPosition = getPositionForView(accessFocusedChild);
accessFocusedChild.setHasTransientState(true);
} else {
accessibilityFocusPosition = INVALID_POSITION;
}

// Ensure the child containing focus, if any, has transient state.
// If the list data hasn't changed, or if the adapter has stable
// IDs, this will maintain focus.
final View focusedChild = getFocusedChild();
if (focusedChild != null) {
focusedChild.setHasTransientState(true);
}

// Pull all children into the RecycleBin.
// These views will be reused if possible
final int firstPosition = mFirstPosition;
final RecycleBin recycleBin = mRecycler;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}

// Clear out old views
detachAllViewsFromParent();
recycleBin.removeSkippedScrap();

switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
if (newSel != null) {
sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
} else {
sel = fillFromMiddle(childrenTop, childrenBottom);
}
break;
case LAYOUT_SYNC:
sel = fillSpecific(mSyncPosition, mSpecificTop);
break;
case LAYOUT_FORCE_BOTTOM:
sel = fillUp(mItemCount - 1, childrenBottom);
adjustViewsUpOrDown();
break;
case LAYOUT_FORCE_TOP:
mFirstPosition = 0;
sel = fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
case LAYOUT_SPECIFIC:
sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
break;
case LAYOUT_MOVE_SELECTION:
sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
break;
default:
if (childCount == 0) {
if (!mStackFromBottom) {
final int position = lookForSelectablePosition(0, true);
setSelectedPositionInt(position);
sel = fillFromTop(childrenTop);
} else {
final int position = lookForSelectablePosition(mItemCount - 1, false);
setSelectedPositionInt(position);
sel = fillUp(mItemCount - 1, childrenBottom);
}
} else {
if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
sel = fillSpecific(mSelectedPosition,
oldSel == null ? childrenTop : oldSel.getTop());
} else if (mFirstPosition < mItemCount) {
sel = fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
} else {
sel = fillSpecific(0, childrenTop);
}
}
break;
}

// Flush any cached views that did not get reused above
recycleBin.scrapActiveViews();

if (sel != null) {
final boolean shouldPlaceFocus = mItemsCanFocus && hasFocus();
final boolean maintainedFocus = focusedChild != null && focusedChild.hasFocus();
if (shouldPlaceFocus && !maintainedFocus && !sel.hasFocus()) {
if (sel.requestFocus()) {
// Successfully placed focus, clear selection.
sel.setSelected(false);
mSelectorRect.setEmpty();
} else {
// Failed to place focus, clear current (invalid) focus.
final View focused = getFocusedChild();
if (focused != null) {
focused.clearFocus();
}
positionSelector(INVALID_POSITION, sel);
}
} else {
positionSelector(INVALID_POSITION, sel);
}
mSelectedTop = sel.getTop();
} else {
// If the user's finger is down, select the motion position.
// Otherwise, clear selection.
if (mTouchMode == TOUCH_MODE_TAP || mTouchMode == TOUCH_MODE_DONE_WAITING) {
final View child = getChildAt(mMotionPosition - mFirstPosition);
if (child != null)  {
positionSelector(mMotionPosition, child);
}
} else {
mSelectedTop = 0;
mSelectorRect.setEmpty();
}
}

if (accessFocusedChild != null) {
accessFocusedChild.setHasTransientState(false);

// If we failed to maintain accessibility focus on the previous
// view, attempt to restore it to the previous position.
if (!accessFocusedChild.isAccessibilityFocused()
&& 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();
}
}
}

if (focusedChild != null) {
focusedChild.setHasTransientState(false);
}

mLayoutMode = LAYOUT_NORMAL;
mDataChanged = false;
if (mPositionScrollAfterLayout != null) {
post(mPositionScrollAfterLayout);
mPositionScrollAfterLayout = null;
}
mNeedSync = false;
setNextSelectedPositionInt(mSelectedPosition);

updateScrollIndicators();

if (mItemCount > 0) {
checkSelectionChanged();
}

invokeOnItemScrollListener();
} finally {
if (!blockLayoutRequests) {
mBlockLayoutRequests = false;
}
}
}

代码接近300行 我们一行一行分析：

首先调用:

invalidate();

目的是在合适时候触发OnDraw方法，其实在初始化ListView时自然在onLayout后会调用onDraw方法，此处是在Adapter中的数据有所改变时重绘视图。

if (mAdapter == null) {
resetList();
invokeOnItemScrollListener();
return;
}

在此处做了一个判断，如果数据源为空会调用 resetList() 方法，这个方法会移除ListView中的全部视图。

final RecycleBin recycleBin = mRecycler;
if (dataChanged) {
for (int i = 0; i < childCount; i++) {
recycleBin.addScrapView(getChildAt(i), firstPosition+i);
}
} else {
recycleBin.fillActiveViews(childCount, firstPosition);
}

此处会执：

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，因此第19行getChildCount()方法得到的值肯定是0。接着在第81行会根据dataChanged这个布尔型的值来判断执行逻辑，dataChanged只有在数据源发生改变的情况下才会变成true，其它情况都是false，因此这里会进入到第90行的执行逻辑，调用RecycleBin的fillActiveViews()方法。按理来说，调用fillActiveViews()方法是为了将ListView的子View进行缓存的，可是目前ListView中还没有任何的子View，因此这一行暂时还起不了任何作用。

switch (mLayoutMode) {
case LAYOUT_SET_SELECTION:
if (newSel != null) {
sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
} else {
sel = fillFromMiddle(childrenTop, childrenBottom);
}
break;
case LAYOUT_SYNC:
sel = fillSpecific(mSyncPosition, mSpecificTop);
break;
case LAYOUT_FORCE_BOTTOM:
sel = fillUp(mItemCount - 1, childrenBottom);
adjustViewsUpOrDown();
break;
case LAYOUT_FORCE_TOP:
mFirstPosition = 0;
sel = fillFromTop(childrenTop);
adjustViewsUpOrDown();
break;
case LAYOUT_SPECIFIC:
sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
break;
case LAYOUT_MOVE_SELECTION:
sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
break;
default:
if (childCount == 0) {
if (!mStackFromBottom) {
final int position = lookForSelectablePosition(0, true);
setSelectedPositionInt(position);
sel = fillFromTop(childrenTop);
} else {
final int position = lookForSelectablePosition(mItemCount - 1, false);
setSelectedPositionInt(position);
sel = fillUp(mItemCount - 1, childrenBottom);
}
} else {
if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
sel = fillSpecific(mSelectedPosition,
oldSel == null ? childrenTop : oldSel.getTop());
} else if (mFirstPosition < mItemCount) {
sel = fillSpecific(mFirstPosition,
oldFirst == null ? childrenTop : oldFirst.getTop());
} else {
sel = fillSpecific(0, childrenTop);
}
}
break;
}

接下来会根据mLayoutMode的值来决定布局模式，默认情况下都是普通模式LAYOUT_NORMAL，因此会进入到第的default语句当中。而下面又会紧接着进行两次if判断，childCount目前是等于0的，并且默认的布局顺序是从上往下，因此会进入到第145行的fillFromTop()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？循环中调用的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) {
View child;

if (!mDataChanged) {
// Try to use an existing view for this position
child = mRecycler.getActiveView(position);
if (child != null) {
// Found it -- we're using an existing child
// This just needs to be positioned
setupChild(child, position, y, flow, childrenLeft, selected, true);

return child;
}
}

// Make a new view for this position, or convert an unused view if possible
child = obtainView(position, mIsScrap);

// This needs to be positioned and measured
setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);

return child;
}

我们可以看到在这个函数中 首先如果mDataChanged为false，也就是说数据没有发生改变，那么我们首先会从缓存中拿出一个View，如果找到了这个View那么就可以返回了， 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，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[] 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;
}

调用了RecycleBin的getScrapView()方法来尝试获取一个废弃缓存中的View，同样的道理，这里肯定是获取不到的，getScrapView()方法会返回一个null。这时该怎么办呢？没有关系，代码会调用mAdapter的getView()方法来去获取一个View。那么mAdapter是什么呢？当然就是当前ListView关联的适配器了。