上篇分析了下RecyclerView源码，本文继续讲ListView源码。
本文目录如下：
0x00 基本代码结构
（1）measure过程
（2）layout过程
（3）draw过程
0x01 复用原理
（1）ListView滚动时
（2）数据集发生变化时
（3）重新设置数据适配器Adapter
0x02 总结
0x00 基本代码结构
0. measure过程
@Override
public void onMeasure() {
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
//宽度
if (widthMode == MeasureSpec.UNSPECIFIED) {
widthSize = mListPadding.left + mListPadding.right + childWidth +
getVerticalScrollbarWidth();
} else {
widthSize |= (childState&MEASURED_STATE_MASK);
}
//高度。如果是UNSPECIFIED，则设置为itemHeight + ListPadding，如果是AT_MOST，则取Min(count * (itemHeight + dividerHeight), heightSize)
if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
} else if (heightMode == MeasureSpec.AT_MOST) {
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
setMeasuredDimension(widthSize , heightSize);
}
1. layout过程
onLayout关键代码如下：
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
super.onLayout(changed, l, t, r, b);
final int childCount = getChildCount();
if (changed) {
for (int i = 0; i < childCount; i++) {
getChildAt(i).forceLayout();
}
mRecycler.markChildrenDirty();
}
...
layoutChildren();
...
}
当数据发生变化时，首先界面上显示的子View重新layout，然后调用layoutChildren()布局子View。
layoutChildren()代码如下：
@Override
protected void layoutChildren() {
invalidate();
...
final int childrenTop = mListPadding.top;
final int childrenBottom = mBottom - mTop - mListPadding.bottom;
final int childCount = getChildCount();
...
boolean dataChanged = mDataChanged;
if (dataChanged) {
handleDataChanged();
}
...
// 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();
...
//填充：fillUp() / fillFromTop() / fillSpecific();
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);
}
}
...
// Flush any cached views that did not get reused above
recycleBin.scrapActiveViews();
}
上面代码拆分成3个部分来看： （1）填充子View前 （2）填充子View （3）填充子View后
（1）填充子View前：
（1）首次Layout，此时childCount == 0，不需要做特殊处理。 （2）非首次Layout： 若数据发生变化：如果是瞬态子View，将屏幕上显示的子View添加到mTransientStateViewsById/mTransientStateViews/mSkippedScrap；如果是稳态子View，将屏幕上显示的子View添加到mCurrentScrap/mScrapViews列表。 若数据未变化：将屏幕上显示的子View添加到mActiveViews列表。
（2）填充子View：
（1）首次Layout，若填充方向为从下往上（mStackFromBottom = true），则调用fillUp(mItemCount - 1, childrenBottom)，若填充方向为从上往下（mStackFromBottom = false），则调用fillFromTop(childrenTop) （2）非首次Layout，则从指定位置先依次往上往下填充子View，见fillSpecific(int position, int top)
三种填充方式：
fillUp() 从下往上填充
fillFromTop() 从上往下填充
fillSpecific(position) 从某个位置先往上填充再往下填充
以fillFromTop(int nextTop)为例，主要代码如下：
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);
}
继续关注fillDown(mFirstPosition, nextTop)，
private View fillDown(int pos, int nextTop) {
View selectedView = null;
int end = (mBottom - mTop);
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
end -= mListPadding.bottom;
}
while (nextTop < end && pos < mItemCount) {
// is this the selected item?
boolean selected = pos == mSelectedPosition;
View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
nextTop = child.getBottom() + mDividerHeight;
if (selected) {
selectedView = child;
}
pos++;
}
setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
return selectedView;
}
从mFirstPosition位置从上往下填充子View直至屏幕填满，子View的获取方法见makeAndAddView(pos, nextTop, true, mListPadding.left, selected)，主要代码如下：
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;
}
数据未变化时，先从mActiveViews中复用。若数据发生变化或者从mActiveViews中未获取到子View，则调用obtainView(position, mIsScrap)获取，获取到之后再调用setupChild来measure、layout子View。
obtainView(position, mIsScrap)按照优先级依次从transientViews、mScrapViews中根据position匹配View，并将其作为convertView，然后调用mAdapter.getView(position, convertView, this)绑定数据
View obtainView(int position, boolean[] isScrap) {
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);
}
return transientView;
}
...
final View scrapView = mRecycler.getScrapView(position);
final View child = mAdapter.getView(position, scrapView, this);
...
return child;
}
setupChild做的事情就是重新调整子View的位置，会涉及到子View的measure、layout操作
//Add a view as a child and make sure it is measured (if necessary) and positioned properly.
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) {
...
int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,
mListPadding.left + mListPadding.right, p.width);
int lpHeight = p.height;
int childHeightSpec;
if (lpHeight > 0) {
childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
}
child.measure(childWidthSpec, childHeightSpec);
...
final int childRight = childrenLeft + w;
final int childBottom = childTop + h;
child.layout(childrenLeft, childTop, childRight, childBottom);
}
（3）填充子View后：
调用mRecycler.scrapActiveViews()，将其余还存在于mActiveViews中的子View移到mScrapViews中。
RecycleBin负责layout过程的子View复用机制，复用的详细过程可以跳到后面小节的0x01 复用原理。
2. draw过程
主要代码如下：
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
//绘制上下边缘
mEdgeGlowTop.draw(canvas);
mEdgeGlowBottom.draw(canvas);
}
@Override
protected void dispatchDraw(Canvas canvas) {
if (!mStackFromBottom) {//从上往下
//绘制OverscrollHeader或header divider
drawOverscrollHeader(canvas, overscrollHeader, bounds) or drawDivider(canvas, bounds, -1);
//绘制元素间的divider
for (int i = 0; i < count; i++) {
drawDivider(canvas, bounds, i);
}
//绘制OverscrollFooter
drawOverscrollFooter(canvas, overscrollFooter, bounds);
} else {//从下往上
//绘制OverscrollHeader
drawOverscrollHeader(canvas, overscrollHeader, bounds);
//绘制元素间的divider
for (int i = 0; i < count; i++) {
drawDivider(canvas, bounds, i);
}
//绘制OverscrollFooter或footer divider
drawOverscrollFooter(canvas, overscrollFooter, bounds) or drawDivider(canvas, bounds, -1);
}
//drawSelector等
//绘制子View
super.dispatchDraw(canvas);
}
绘制的主要逻辑集中在dispatchDraw(Canvas canvas)，绘制顺序是：
（1）绘制OverscrollHeader或header divider
（2）绘制元素间的divider
（3）绘制OverscrollFooter
（4）绘制子View
0x01 复用原理
RecycleBin数据结构如下：
class RecycleBin {
private View[] mActiveViews;
private ArrayList<View>[] mScrapViews;
private int mViewTypeCount;
ArrayList<View> mCurrentScrap;
private SparseArray<View> mTransientStateViews;
private LongSparseArray<View> mTransientStateViewsById;
}
mActiveViews：子View数组。若数据集没发生变化，在布局子View之前，先将当前界面显示的子View添加到mActiveViews。mActiveViews生命周期只存在于onLayout过程。
mScrapViews：key为viewtype，value为ArrayList< View >的键值对。在子View布局前，若数据集发生变化，将当前界面显示的子View添加到mScrapViews；在子View布局之后，将mAcviteViews中剩余的子View添加到mScrapViews。
mTransientStateViews/mTransientStateViewsById：瞬态View列表的键值对，key为position/itemId，value为List< View >。如果hasStableId=true，则key为itemId。
填充子View的过程中，子View复用逻辑：
若数据集没变化，则先从mActiveViews中匹配，然后直接布局子View，子VIew不需要重新绑定数据。
若数据集发生变化，则从transientView或mScrapViews中匹配，并将匹配到的子View作为convertView，重新绑定数据后，再布局子View。如果没匹配到子View，则在getView中新创建一个子View。
首次Layout时，mActiveViews和mScrapViews都为空。
继续结合以下场景分析： （1）ListView滚动时 （2）数据集发生变化时 （3）重新设置数据适配器Adapter
0. ListView滚动时：
ListView滚动时，调用路线依次为：
onTouchEvent()
scrollIfNeeded()
trackMotionScroll()
fillGap(down);
fillDown(mFirstPosition + count, startOffset); 或fillUp() 注：往上滑是fillUp，往下滑是fillDown
makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
child = obtainView(position, mIsScrap);
mAdapter.getView(position, transientView, this);
对于滑出屏幕的View，会添加到mScrapViews，见trackMotionScroll代码。
对于滑入屏幕的View，会从mScrapViews或mTransientStateViews中匹配出convertView，然后调用getView进行数据绑定；若没匹配到则调用getView创建View并绑定数据。
对于既没滑入也没滑出屏幕的View，且数据集未发生变化时，直接从mActiveViews中获取现成的View进行展现，不需要进行数据绑定。反之若数据集发生变化则还是走mScrapViews、getView()复用那一套逻辑。
boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
...
final boolean down = incrementalDeltaY < 0;
if (down) {//从下向上滑动时
int top = -incrementalDeltaY;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
top += listPadding.top;
}
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (child.getBottom() >= top) {
break;
} else {
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
mRecycler.addScrapView(child, position);
}
}
}
} else {//从上向下滑动时
int bottom = getHeight() - incrementalDeltaY;
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
bottom -= listPadding.bottom;
}
for (int i = childCount - 1; i >= 0; i--) {
final View child = getChildAt(i);
if (child.getTop() <= bottom) {
break;
} else {
start = i;
count++;
int position = firstPosition + i;
if (position >= headerViewsCount && position < footerViewsStart) {
// The view will be rebound to new data, clear any
// system-managed transient state.
child.clearAccessibilityFocus();
mRecycler.addScrapView(child, position);
}
}
}
}
...
}
1. 数据集发生变化时：
以class BaseAdapter implements ListAdapter为例，当数据集发生变化时，会调用mAdapter.notifyDataSetChanged()或mAdapter.notifyDataSetInvalidated()。
调用路线依次是：
BaseAdapter.notifyDataSetChanged() -> DataSetObservable.notifyChanged() -> DataSetObserver.onChanged() -> AdapterDataSetObserver.onChanged() -> requestLayout()
所以还是会走layout那一套逻辑，整体刷新ListView。ListView没直接提供局部刷新的方法。
2. 重新设置数据适配器Adapter：
重新设置数据适配器时，会调到requestLayout()，重新走一遍layout逻辑。
@Override
public void setAdapter(ListAdapter adapter) {
mAdapter = adapter;
...
requestLayout();
}
0x02 其他
getChildCount() vs. mAdapter.getCount() getChildCount()屏幕上显示的子View个数 mAdapter.getCount() Adapter数据集的个数