measure
在分析onMeasure()前，需要介绍三个基本的layout方法和mState.mLayoutStep状态：
- State.STEP_START : 默认状态，表示RV此时还没有经过
dispatchLayoutStep1() - State.STEP_LAYOUT :
dispatchLayoutStep1()后会将状态转变为此状态，表示处于layout阶段，会调用dispatchLayoutStep2(); - State.STEP_ANIMATIONS:
dispatchLayoutStep2()会将状态转变为次状态，处于执行动画阶段，会调用dispatchLayoutStep3(),dispatchLayoutStep3()执行完毕会将状态重新转为初始默认状态STEP_START
dispatchLayoutStep1():
/**
* The first step of a layout where we;
* - process adapter updates
* - decide which animation should run
* - save information about current views
* - If necessary, run predictive layout and save its information
*/
private void dispatchLayoutStep1() {
processAdapterUpdatesAndSetAnimationFlags();
if (mState.mRunSimpleAnimations) {
// Step 0: Find out where all non-removed items are, pre-layout
final ItemHolderInfo animationInfo = mItemAnimator
.recordPreLayoutInformation(mState, holder,
ItemAnimator.buildAdapterChangeFlagsForAnimations(holder),
holder.getUnmodifiedPayloads());
mViewInfoStore.addToPreLayout(holder, animationInfo);
}
if (mState.mRunPredictiveAnimations) {
// Step 1: run prelayout: This will use the old positions of items.
// Save old positions so that LayoutManager can run its mapping logic.
saveOldPositions();
// temporarily disable flag because we are asking for previous layout
mLayout.onLayoutChildren(mRecycler, mState);
for (int i = 0; i < mChildHelper.getChildCount(); ++i) {
final View child = mChildHelper.getChildAt(i);
final ViewHolder viewHolder = getChildViewHolderInt(child);
final ItemHolderInfo animationInfo = mItemAnimator.recordPreLayoutInformation(
mState, viewHolder, flags, viewHolder.getUnmodifiedPayloads());
if (wasHidden) {
recordAnimationInfoIfBouncedHiddenView(viewHolder, animationInfo);
} else {
mViewInfoStore.addToAppearedInPreLayoutHolders(viewHolder, animationInfo);
}
}
}
}
mState.mLayoutStep = State.STEP_LAYOUT;
}
- 调用
processAdapterUpdatesAndSetAnimationFlags()处理adapter更新和设置动画标记 - 保存
animationInfo，run pre-layout：保存animationInfo到ViewInfoStore类中; - 改变state为
Step_Layout
dispatchLayoutStep2():
private void dispatchLayoutStep2() {
// Step 2: Run layout
mState.mInPreLayout = false;
mLayout.onLayoutChildren(mRecycler, mState);
mState.mLayoutStep = State.STEP_ANIMATIONS;
}
- 调用LayoutManager的
onLayoutChildren（）真正进行child的测量和布局 - 将state转为
Step_Animations
看看LinearLayoutManager的onLayoutChildren()实现
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
// layout algorithm:
// 1) by checking children and other variables, find an anchor coordinate and an anchor
// item position.
// 2) fill towards start, stacking from bottom
// 3) fill towards end, stacking from top
// 4) scroll to fulfill requirements like stack from bottom.
// create layout state
updateAnchorInfoForLayout(recycler, state, mAnchorInfo); // 计算锚点
detachAndScrapAttachedViews(recycler); // 一级缓存scrap缓存
fill(recycler, mLayoutState, state, false); //
}
-
调用
updateAnchorInfoForLayout()计算锚点
锚点 detachAndScrapAttachedViews()一级缓存，Scrap缓存fill()计算可填充空间，在fill()中调用layoutChunk()真正的去填充child
锚点布局的三种
void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
LayoutState layoutState, LayoutChunkResult result) {
// 如果是第一次，addView()
View view = layoutState.next(recycler);
LayoutParams params = (LayoutParams) view.getLayoutParams();
if (layoutState.mScrapList == null) {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addView(view);
} else {
addView(view, 0);
}
} else {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addDisappearingView(view);
} else {
addDisappearingView(view, 0);
}
}
// measure child
measureChildWithMargins(view, 0, 0);
// 计算child的layout参数
int left, top, right, bottom;
if (mOrientation == VERTICAL) {
if (isLayoutRTL()) {
right = getWidth() - getPaddingRight();
left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);
} else {
left = getPaddingLeft();
right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);
}
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
bottom = layoutState.mOffset;
top = layoutState.mOffset - result.mConsumed;
} else {
top = layoutState.mOffset;
bottom = layoutState.mOffset + result.mConsumed;
}
} else {
top = getPaddingTop();
bottom = top + mOrientationHelper.getDecoratedMeasurementInOther(view);
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
right = layoutState.mOffset;
left = layoutState.mOffset - result.mConsumed;
} else {
left = layoutState.mOffset;
right = layoutState.mOffset + result.mConsumed;
}
}
// layout child
layoutDecoratedWithMargins(view, left, top, right, bottom);
}
- 调用
layoutState.next()获取View,如果是第一次布局(Scrap缓存为null)，addView() - 调用
measureChildWithMargins()measure child - 根据LayoutManager的Orientation计算child 坐标
- 调用
layoutDecoratedWithMargins()layout child
dispatchLayoutStep3():
/**
* The final step of the layout where we save the information about views for animations,
* trigger animations and do any necessary cleanup.
*/
private void dispatchLayoutStep3() {
mState.mLayoutStep = State.STEP_START;
// Step 4: Process view info lists and trigger animations
mViewInfoStore.process(mViewInfoProcessCallback);
}
- 将state设置为
STEP_START，下次再判断state时肯定会重新经历Step1(),Step2(),Step3() -
trigger animations，开启动画
measure
@Override
protected void onMeasure(int widthSpec, int heightSpec) {
if (mLayout == null) { // 第一种情况，没有LayoutManager
defaultOnMeasure(widthSpec, heightSpec);
return;
}
if (mLayout.mAutoMeasure) { // 第二种情况,有layoutmanager，并且开启自动测量
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
if (mState.mLayoutStep == State.STEP_START) {
dispatchLayoutStep1();
}
dispatchLayoutStep2();
// now we can get the width and height from the children.
mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
} else { // 第三种情况，有layoutmanager，没有开启自动测量
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
}
}
1. 如果没有layoutManger:
void defaultOnMeasure(int widthSpec, int heightSpec) {
// calling LayoutManager here is not pretty but that API is already public and it is better
// than creating another method since this is internal.
final int width = LayoutManager.chooseSize(widthSpec,
getPaddingLeft() + getPaddingRight(),
ViewCompat.getMinimumWidth(this));
final int height = LayoutManager.chooseSize(heightSpec,
getPaddingTop() + getPaddingBottom(),
ViewCompat.getMinimumHeight(this));
setMeasuredDimension(width, height);
}
public static int chooseSize(int spec, int desired, int min) {
final int mode = View.MeasureSpec.getMode(spec);
final int size = View.MeasureSpec.getSize(spec);
switch (mode) {
case View.MeasureSpec.EXACTLY:
return size;
case View.MeasureSpec.AT_MOST:
return Math.min(size, Math.max(desired, min));
case View.MeasureSpec.UNSPECIFIED:
default:
return Math.max(desired, min);
}
}
当前ＲＶ如果没有LayoutManager，就调用defaultOnMeasure()根据MeasureSpec默认测量
2. 如果有layoutManger，并且开启了自动测量默认场景
上面已经介绍了
step1()和step2()的作用,在第二种情况下，onMeasure()首先对child进行measure和layout,然后再结合childsetMeasuredDimension()
3. 关闭自动测量模式：
和第一种情况一样，也是调用了defaultOnMeasure()，不过这是通过LayoutManger来完成调用的；
至此ＲＶ的measure过程分析完毕
layout
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
TraceCompat.beginSection(TRACE_ON_LAYOUT_TAG);
dispatchLayout();
TraceCompat.endSection();
mFirstLayoutComplete = true;
}
void dispatchLayout() {
if (mAdapter == null) {
Log.e(TAG, "No adapter attached; skipping layout");
// leave the state in START
return;
}
if (mLayout == null) {
Log.e(TAG, "No layout manager attached; skipping layout");
// leave the state in START
return;
}
mState.mIsMeasuring = false;
if (mState.mLayoutStep == State.STEP_START) {
dispatchLayoutStep1();
mLayout.setExactMeasureSpecsFrom(this);
dispatchLayoutStep2();
} else if (mAdapterHelper.hasUpdates() || mLayout.getWidth() != getWidth()
|| mLayout.getHeight() != getHeight()) {
// First 2 steps are done in onMeasure but looks like we have to run again due to
// changed size.
mLayout.setExactMeasureSpecsFrom(this);
dispatchLayoutStep2();
} else {
// always make sure we sync them (to ensure mode is exact)
mLayout.setExactMeasureSpecsFrom(this);
}
dispatchLayoutStep3();
}
在layout中的工作就变得非常简单了，首先判断了adapter和layoutManager不为null，然后再调用step3，因为step3会将state设置为start，所以下次再layout的时候就会执行step1,step2,step3了
如果开启了自动测量，就会在measure中测量，将state设置为animation，layout过程就只会执行step3;
如果没有开启自动测量，就会在layout过程执行step1,step2,step3;
draw
@Override
public void draw(Canvas c) {
super.draw(c);
for (int i = 0; i < count; i++) {
mItemDecorations.get(i).onDrawOver(c, this, mState);
}
}
@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);
}
}
在ＲＶ的draw过程中，调用了ItemDecoration的onDrawOver和onDraw为item添加装饰