RecyclerView与ListView对比浅析（三）：绘制
上一篇：RecyclerView与ListView对比浅析（二）：View缓存篇
（三）绘制篇
1. AbsListView（源码版本 4.4）
绘制就分三步来分析Measure、Layout和Draw
（1）onMeasure里未用到MeasureSpec的高宽，首先设Selector，然后根据SelectionPadding和mPadding算出ListPadding的上下左右。最后根据TranscriptMode，决定Scroll和Measure的先后顺序。 @Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mSelector == null) {
useDefaultSelector();
}
final Rect listPadding = mListPadding;
listPadding.left = mSelectionLeftPadding + mPaddingLeft;
listPadding.top = mSelectionTopPadding + mPaddingTop;
listPadding.right = mSelectionRightPadding + mPaddingRight;
listPadding.bottom = mSelectionBottomPadding + mPaddingBottom;
// Check if our previous measured size was at a point where we should scroll later.
if (mTranscriptMode == TRANSCRIPT_MODE_NORMAL) {
final int childCount = getChildCount();
final int listBottom = getHeight() - getPaddingBottom();
final View lastChild = getChildAt(childCount - 1);
final int lastBottom = lastChild != null ? lastChild.getBottom() : listBottom;
mForceTranscriptScroll = mFirstPosition + childCount >= mLastHandledItemCount &&
lastBottom <= listBottom;
}
}
（2）onLayout中，首先调super.onLayout，然后：
a. 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()：对所有缓存的View执行forceLayout()
b. 如果mFastScroller不为空且ItemCount变了，调mFastScroller的onItemCountChanged。
c. 调layoutChildren()，需由子类实现。
@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;
}
（3）draw调完super.draw后，处理了mEdgeGlowTop和mEdgeGlowBottom
2. ListView（源码版本 4.4）
同样分三步Measure、Layout和Draw继续分析：
（1）onMeasure中，先调父类的onMeasure，然后调父类的obtainView拿到 位置为0的View（mIsScrap作为入参记录是否为ScrapView），调measureScrapChild对它Measure，拿到childWidth、childHeight和childState，然后判断这个Type的View是否需要回收，需要就调addScrapView(child, -1)。最后根据childWidth和childHeight算出widthSize和heightSize（不同Mode用不同的计算方法），调setMeasuredDivision。 @Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// 设置mListPadding
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int childWidth = 0;
int childHeight = 0;
int childState = 0;
mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED ||
heightMode == MeasureSpec.UNSPECIFIED)) {
final View child = obtainView(0, mIsScrap);
measureScrapChild(child, 0, widthMeasureSpec);
childWidth = child.getMeasuredWidth();
childHeight = child.getMeasuredHeight();
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(
((LayoutParams) child.getLayoutParams()).viewType)) {
mRecycler.addScrapView(child, -1);
}
}
if (widthMode == MeasureSpec.UNSPECIFIED) {
widthSize = mListPadding.left + mListPadding.right + childWidth +
getVerticalScrollbarWidth();
} else {
widthSize |= (childState&MEASURED_STATE_MASK);
}
if (heightMode == MeasureSpec.UNSPECIFIED) {
heightSize = mListPadding.top + mListPadding.bottom + childHeight +
getVerticalFadingEdgeLength() * 2;
}
if (heightMode == MeasureSpec.AT_MOST) {
// TODO: 这里可以用首个可见View的位置，而不是0
heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}
setMeasuredDimension(widthSize , heightSize);
mWidthMeasureSpec = widthMeasureSpec;
}
（2）ListView没有重写onLayout，而是沿用父类的onLayout并实现了layoutChildren()，来看下layoutChildren的过程（源码见上一节）；
a. 如果Adapter为空，则重置List，移除Layout中的所有View。
b. 根据LayoutMode选择不同Layout方式，这里只看default的方式：用getChildAt方法拿到几个View备用：原来选择的oldSel、原来在首位的oldFirst和新选择的newSel。
c. 如果dataChanged为true，调handleDataChanged()处理选择和同步的相应变化。
d. 所有ChildView放入RecycleBin，数据变了就放ScrapViews，没变就放ActiveViews，然后detach所有旧View。
e. 根据LayoutMode，进行View填充，这里也是只看default方法：如果childCount==0，就根据List是自顶向下还是自底向上选择fill方法，正常是选择fillFromTop(childrenTop)进行填充；如果不为0，则根据先前计算的SelectionPosition调fillSpecific填充特定的View。
f. fill完毕后，把所有ActiveViews存入ScrapView
在这里重点分析下fillFromTop的源码：
// 从上向下在ListView中填充Item View
private View fillFromTop(int nextTop) {
mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
if (mFirstPosition < 0) {
mFirstPosition = 0;
}
// 具体操作在此
return fillDown(mFirstPosition, nextTop);
}
// 在位置pos下面填充Item View
private View fillDown(int pos, int nextTop) {
View selectedView = null;
// ListView getHeight也是这样计算的
int end = (mBottom - mTop);
……
// 初始化时pos = 0，如果item总数少于一屏幕，执行mItemCount - pos次。如果item多于一屏幕，执行end - nextTop次。
while (nextTop < end && pos < mItemCount) {
// is this the selected item?
boolean selected = pos == mSelectedPosition;
// 获取Item View对象，在View缓存中已经介绍过了，如果数据没变，从ActiveViews里拿，如果变了，调obtainView查找缓存拿View
View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
nextTop = child.getBottom() + mDividerHeight;
if (selected) {
selectedView = child;
}
pos++;
}
setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
return selectedView;
}
在makeAndAddView中，拿到View后，还会调setupChild进行状态设置
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
boolean selected, boolean recycled) {
// 判断当前Item View是否选中状态
final boolean isSelected = selected && shouldShowSelector();
final boolean updateChildSelected = isSelected != child.isSelected();
final int mode = mTouchMode;
// 是否处于按下状态
final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
mMotionPosition == position;
final boolean updateChildPressed = isPressed != child.isPressed();
// 是否需要重新measure与layout
final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();
// Respect layout params that are already in the view. Otherwise make some up...
// noinspection unchecked
AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
}
p.viewType = mAdapter.getItemViewType(position);
if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter &&
p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
attachViewToParent(child, flowDown ? -1 : 0, p);
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
p.recycledHeaderFooter = true;
}
// 向ListView（ViewGroup子类）添加当前Item View
addViewInLayout(child, flowDown ? -1 : 0, p, true);
}
// 更新选中状态
if (updateChildSelected) {
child.setSelected(isSelected);
}
// 更新按下状态
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 (needToMeasure) {
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);
}
// 与普通视图的measure流程不同，ListView是在此处执行具体的当前Item View measure
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;
// 大小改变肯定位置也会发生变化，当前Item View重新进行layout
child.layout(childrenLeft, childTop, childRight, childBottom);
} else {
child.offsetLeftAndRight(childrenLeft - child.getLeft());
child.offsetTopAndBottom(childTop - child.getTop());
}
if (mCachingStarted && !child.isDrawingCacheEnabled()) {
child.setDrawingCacheEnabled(true);
}
if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition)
!= position) {
child.jumpDrawablesToCurrentState();
}
}
（3）ListView沿用了父类的draw方法。
3. RecyclerView（源码版本 5.1.1）
同样分三步分析，Measure、Layout和Draw
（1）onMeasure
@Override
protected void onMeasure(int widthSpec, int heightSpec) {
//Adapter在Measure过程中执行了更新
if (mAdapterUpdateDuringMeasure) {
//中止Layout请求
eatRequestLayout();
//执行更新并且设置动画
processAdapterUpdatesAndSetAnimationFlags();
if (mState.mRunPredictiveAnimations) {
//处理Predictive动画的机制
// TODO: try to provide a better approach.
// When RV decides to run predictive animations, we need to measure in pre-layout
// state so that pre-layout pass results in correct layout.
// On the other hand, this will prevent the layout manager from resizing properly.
mState.mInPreLayout = true;
} else {
// consume remaining updates to provide a consistent state with the layout pass.
mAdapterHelper.consumeUpdatesInOnePass();
mState.mInPreLayout = false;
}
mAdapterUpdateDuringMeasure = false;
//恢复Layout请求
resumeRequestLayout(false);
}
if (mAdapter != null) {
mState.mItemCount = mAdapter.getItemCount();
} else {
mState.mItemCount = 0;
}
if (mLayout == null) {
//默认Measure方法
defaultOnMeasure(widthSpec, heightSpec);
} else {
//调LayoutManager的onMeasure
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
}
mState.mInPreLayout = false; // clear
}
首先看defaultOnMeasure，与普通View的Measure基本相似，都是根据Mode得出对应的高宽，最后setMeasuredDimision。而LayoutManager的onMeasure就是调mRecyclerView.defaultOnMeasure，就不贴代码了。
private void defaultOnMeasure(int widthSpec, int heightSpec) {
final int widthMode = MeasureSpec.getMode(widthSpec);
final int heightMode = MeasureSpec.getMode(heightSpec);
final int widthSize = MeasureSpec.getSize(widthSpec);
final int heightSize = MeasureSpec.getSize(heightSpec);
int width = 0;
int height = 0;
switch (widthMode) {
case MeasureSpec.EXACTLY:
case MeasureSpec.AT_MOST:
width = widthSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
width = ViewCompat.getMinimumWidth(this);
break;
}
switch (heightMode) {
case MeasureSpec.EXACTLY:
case MeasureSpec.AT_MOST:
height = heightSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
height = ViewCompat.getMinimumHeight(this);
break;
}
setMeasuredDimension(width, height);
}
（2）onLayout
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
//中止Layout请求
eatRequestLayout();
//分发Layout任务
dispatchLayout();
//恢复Layout请求
resumeRequestLayout(false);
mFirstLayoutComplete = true;
}
DispatchLayout()是其中的关键函数，首先看下函数描述：在layoutChildren()外面包一层，用于处理layout引发的动画变化。这里的layoutChildren()就是表示前面介绍的LayoutManager中的onLayoutChildren()，处理各个Child的Measure操作。源码很长，就分步看下吧：
void dispatchLayout() {
if (mAdapter == null) {
Log.e(TAG, "No adapter attached; skipping layout");
return;
}
if (mLayout == null) {
Log.e(TAG, "No layout manager attached; skipping layout");
return;
}
mDisappearingViewsInLayoutPass.clear();
//这里再调一次中止Layout请求
eatRequestLayout();
mRunningLayoutOrScroll = true;
//处理Adapter更新并设动画标志位
processAdapterUpdatesAndSetAnimationFlags();
//一些参数的初始化
mState.mOldChangedHolders = mState.mRunSimpleAnimations && mItemsChanged
&& supportsChangeAnimations() ? new ArrayMap<Long, ViewHolder>() : null;
mItemsAddedOrRemoved = mItemsChanged = false;
ArrayMap<View, Rect> appearingViewInitialBounds = null;
mState.mInPreLayout = mState.mRunPredictiveAnimations;
mState.mItemCount = mAdapter.getItemCount();
findMinMaxChildLayoutPositions(mMinMaxLayoutPositions);
if (mState.mRunSimpleAnimations) {
// 准备工作：找到所有免移除的item，执行预layout
mState.mPreLayoutHolderMap.clear();
mState.mPostLayoutHolderMap.clear();
int count = mChildHelper.getChildCount();
for (int i = 0; i < count; ++i) {
final ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
//判断条件：是否忽视、是否合法、Adapter是否有StableId
if (holder.shouldIgnore() || (holder.isInvalid() && !mAdapter.hasStableIds())) {
continue;
}
final View view = holder.itemView;
mState.mPreLayoutHolderMap.put(holder, new ItemHolderInfo(holder,
view.getLeft(), view.getTop(), view.getRight(), view.getBottom()));
}
}
if (mState.mRunPredictiveAnimations) {
// 第一步：执行预Layout，需要用到item的原位置。LayoutManager会对所有元素进行layout，包括已移除item（但不会把已移除项再加回来）。这就能让pre-layout拿到用户可见View的position，真正执行layout时不会变。
//存储原位置方便LayoutManager执行映射逻辑
saveOldPositions();
//前面的processAdapterUpdatesAndSetAnimationFlags已经对动画执行了预Layiout
//遍历mChildHelper，将里面已改变、未移除且不该忽略的holder移出预Layout容器
if (mState.mOldChangedHolders != null) {
int count = mChildHelper.getChildCount();
for (int i = 0; i < count; ++i) {
final ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
if (holder.isChanged() && !holder.isRemoved() && !holder.shouldIgnore()) {
long key = getChangedHolderKey(holder);
mState.mOldChangedHolders.put(key, holder);
mState.mPreLayoutHolderMap.remove(holder);
}
}
}
final boolean didStructureChange = mState.mStructureChanged;
//因为是预Layout，暂时把此标志位置false
mState.mStructureChanged = false;
//执行onLayoutChildren，不同的LayoutManager子类有不同的实现
mLayout.onLayoutChildren(mRecycler, mState);
mState.mStructureChanged = didStructureChange;
//设置即将显示View的对应Bounds
appearingViewInitialBounds = new ArrayMap<View, Rect>();
for (int i = 0; i < mChildHelper.getChildCount(); ++i) {
boolean found = false;
View child = mChildHelper.getChildAt(i);
if (getChildViewHolderInt(child).shouldIgnore()) {
continue;
}
for (int j = 0; j < mState.mPreLayoutHolderMap.size(); ++j) {
ViewHolder holder = mState.mPreLayoutHolderMap.keyAt(j);
if (holder.itemView == child) {
found = true;
break;
}
}
if (!found) {
appearingViewInitialBounds.put(child, new Rect(child.getLeft(), child.getTop(),
child.getRight(), child.getBottom()));
}
}
// 不处理disappearingList是因为它会在Post Layout的时候处理
clearOldPositions();
mAdapterHelper.consumePostponedUpdates();
} else {
//执行完预Layout但决定不执行Predictive动画的情况
clearOldPositions();
mAdapterHelper.consumeUpdatesInOnePass();
//遍历mChildHelper，对mState里的两容器的元素进行更新
if (mState.mOldChangedHolders != null) {
int count = mChildHelper.getChildCount();
for (int i = 0; i < count; ++i) {
final ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
if (holder.isChanged() && !holder.isRemoved() && !holder.shouldIgnore()) {
long key = getChangedHolderKey(holder);
mState.mOldChangedHolders.put(key, holder);
mState.mPreLayoutHolderMap.remove(holder);
}
}
}
}
mState.mItemCount = mAdapter.getItemCount();
mState.mDeletedInvisibleItemCountSincePreviousLayout = 0;
//第二步：执行Layout
mState.mInPreLayout = false;
mLayout.onLayoutChildren(mRecycler, mState);
mState.mStructureChanged = false;
mPendingSavedState = null;
// onLayoutChildren有可能导致禁用item动画，这里重新检查一下
mState.mRunSimpleAnimations = mState.mRunSimpleAnimations && mItemAnimator != null;
if (mState.mRunSimpleAnimations) {
// 第三步：执行后Layout
ArrayMap<Long, ViewHolder> newChangedHolders = mState.mOldChangedHolders != null ?
new ArrayMap<Long, ViewHolder>() : null;
int count = mChildHelper.getChildCount();
for (int i = 0; i < count; ++i) {
ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
if (holder.shouldIgnore()) {
continue;
}
final View view = holder.itemView;
long key = getChangedHolderKey(holder);
if (newChangedHolders != null && mState.mOldChangedHolders.get(key) != null) {
newChangedHolders.put(key, holder);
} else {
//需要后layout处理的Holder存入这个容器
mState.mPostLayoutHolderMap.put(holder, new ItemHolderInfo(holder,
view.getLeft(), view.getTop(), view.getRight(), view.getBottom()));
}
}
processDisappearingList(appearingViewInitialBounds);
//第四步：执行移除和消失动画
int preLayoutCount = mState.mPreLayoutHolderMap.size();
for (int i = preLayoutCount - 1; i >= 0; i--) {
ViewHolder itemHolder = mState.mPreLayoutHolderMap.keyAt(i);
if (!mState.mPostLayoutHolderMap.containsKey(itemHolder)) {
ItemHolderInfo disappearingItem = mState.mPreLayoutHolderMap.valueAt(i);
mState.mPreLayoutHolderMap.removeAt(i);
View disappearingItemView = disappearingItem.holder.itemView;
mRecycler.unscrapView(disappearingItem.holder);
animateDisappearance(disappearingItem);
}
}
// 第五步：执行新增动画
int postLayoutCount = mState.mPostLayoutHolderMap.size();
if (postLayoutCount > 0) {
for (int i = postLayoutCount - 1; i >= 0; i--) {
ViewHolder itemHolder = mState.mPostLayoutHolderMap.keyAt(i);
ItemHolderInfo info = mState.mPostLayoutHolderMap.valueAt(i);
if ((mState.mPreLayoutHolderMap.isEmpty() ||
!mState.mPreLayoutHolderMap.containsKey(itemHolder))) {
mState.mPostLayoutHolderMap.removeAt(i);
Rect initialBounds = (appearingViewInitialBounds != null) ?
appearingViewInitialBounds.get(itemHolder.itemView) : null;
animateAppearance(itemHolder, initialBounds,
info.left, info.top);
}
}
}
//第6步，执行永久项的动画
count = mState.mPostLayoutHolderMap.size();
for (int i = 0; i < count; ++i) {
ViewHolder postHolder = mState.mPostLayoutHolderMap.keyAt(i);
ItemHolderInfo postInfo = mState.mPostLayoutHolderMap.valueAt(i);
ItemHolderInfo preInfo = mState.mPreLayoutHolderMap.get(postHolder);
if (preInfo != null && postInfo != null) {
if (preInfo.left != postInfo.left || preInfo.top != postInfo.top) {
postHolder.setIsRecyclable(false);
if (DEBUG) {
Log.d(TAG, "PERSISTENT: " + postHolder +
" with view " + postHolder.itemView);
}
if (mItemAnimator.animateMove(postHolder,
preInfo.left, preInfo.top, postInfo.left, postInfo.top)) {
postAnimationRunner();
}
}
}
}
// 第7步：执行改变项的动画
count = mState.mOldChangedHolders != null ? mState.mOldChangedHolders.size() : 0;
// 逆向遍历以免View在遍历过程被回收
for (int i = count - 1; i >= 0; i--) {
long key = mState.mOldChangedHolders.keyAt(i);
ViewHolder oldHolder = mState.mOldChangedHolders.get(key);
View oldView = oldHolder.itemView;
if (oldHolder.shouldIgnore()) {
continue;
}
if (mRecycler.mChangedScrap != null &&
mRecycler.mChangedScrap.contains(oldHolder)) {
animateChange(oldHolder, newChangedHolders.get(key));
} else if (DEBUG) {
Log.e(TAG, "cannot find old changed holder in changed scrap :/" + oldHolder);
}
}
}
resumeRequestLayout(false);
mLayout.removeAndRecycleScrapInt(mRecycler);
mState.mPreviousLayoutItemCount = mState.mItemCount;
mDataSetHasChangedAfterLayout = false;
mState.mRunSimpleAnimations = false;
mState.mRunPredictiveAnimations = false;
mRunningLayoutOrScroll = false;
mLayout.mRequestedSimpleAnimations = false;
if (mRecycler.mChangedScrap != null) {
mRecycler.mChangedScrap.clear();
}
mState.mOldChangedHolders = null;
if (didChildRangeChange(mMinMaxLayoutPositions[0], mMinMaxLayoutPositions[1])) {
notifyOnScrolled(0, 0);
}
}
由以上代码可知，onLayout分为预Layout、正式Layout和后Layout三步，这一操作主要是为了实现item的出现、消失等效果。
（3）onDraw()
onDraw主要是调用的父类的onDraw，后面加入了itemDecorations的draw操作，它常用来画分隔线Divider。
@Override
public void onDraw(Canvas c) {
super.onDraw(c);
final int count = mItemDecorations.size();
for (int i = 0; i < count; i++) {
mItemDecorations.get(i).onDraw(c, this, mState);
}
}