【Android进阶】RecyclerView之绘制流程（三）
前言
上一篇，说了RecyclerView的回收复用，这一篇，我们来说说RecyclerView的绘制流程。
- 【Android进阶】RecyclerView之ItemDecoration（一）
- 【Android进阶】RecyclerView之缓存（二）
- 【Android进阶】RecyclerView之绘制流程（三）
onMeasure
我们先看看RecyclerView#onMeasure()方法
protected void onMeasure(int widthSpec, int heightSpec) {
if (this.mLayout == null) {
this.defaultOnMeasure(widthSpec, heightSpec);
} else {
if (!this.mLayout.isAutoMeasureEnabled()) {
if (this.mHasFixedSize) {
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
return;
}
if (this.mAdapterUpdateDuringMeasure) {
this.startInterceptRequestLayout();
this.onEnterLayoutOrScroll();
this.processAdapterUpdatesAndSetAnimationFlags();
this.onExitLayoutOrScroll();
if (this.mState.mRunPredictiveAnimations) {
this.mState.mInPreLayout = true;
} else {
this.mAdapterHelper.consumeUpdatesInOnePass();
this.mState.mInPreLayout = false;
}
this.mAdapterUpdateDuringMeasure = false;
this.stopInterceptRequestLayout(false);
} else if (this.mState.mRunPredictiveAnimations) {
this.setMeasuredDimension(this.getMeasuredWidth(), this.getMeasuredHeight());
return;
}
if (this.mAdapter != null) {
this.mState.mItemCount = this.mAdapter.getItemCount();
} else {
this.mState.mItemCount = 0;
}
this.startInterceptRequestLayout();
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
this.stopInterceptRequestLayout(false);
this.mState.mInPreLayout = false;
} else {
int widthMode = MeasureSpec.getMode(widthSpec);
int heightMode = MeasureSpec.getMode(heightSpec);
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
boolean measureSpecModeIsExactly = widthMode == 1073741824 && heightMode == 1073741824;
if (measureSpecModeIsExactly || this.mAdapter == null) {
return;
}
if (this.mState.mLayoutStep == 1) {
this.dispatchLayoutStep1();
}
this.mLayout.setMeasureSpecs(widthSpec, heightSpec);
this.mState.mIsMeasuring = true;
this.dispatchLayoutStep2();
this.mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
if (this.mLayout.shouldMeasureTwice()) {
this.mLayout.setMeasureSpecs(MeasureSpec.makeMeasureSpec(this.getMeasuredWidth(), 1073741824), MeasureSpec.makeMeasureSpec(this.getMeasuredHeight(), 1073741824));
this.mState.mIsMeasuring = true;
this.dispatchLayoutStep2();
this.mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
}
}
}
}
我们从上往下看，首先，mLayout即为LayoutManager，如果其为null会执行defaultOnMeasure方法
void defaultOnMeasure(int widthSpec, int heightSpec) {
int width = RecyclerView.LayoutManager.chooseSize(widthSpec, this.getPaddingLeft() + this.getPaddingRight(), ViewCompat.getMinimumWidth(this));
int height = RecyclerView.LayoutManager.chooseSize(heightSpec, this.getPaddingTop() + this.getPaddingBottom(), ViewCompat.getMinimumHeight(this));
this.setMeasuredDimension(width, height);
}
可以看到，这里没有测量item的高度就直接调用setMeasuredDimension方法设置宽高了
接着，是根据isAutoMeasureEnabled为true或false，会走2套逻辑，通过查看源码可以发现，isAutoMeasureEnabled即mAutoMeasure在LayoutManager中，默认为false，但在LinearLayoutManager中为true
LinearLayoutManager相关代码
public boolean isAutoMeasureEnabled() {
return true;
}
而onMeasure的主要逻辑也是在isAutoMeasureEnabled为true时，我们接着往下看
int widthMode = MeasureSpec.getMode(widthSpec);
int heightMode = MeasureSpec.getMode(heightSpec);
this.mLayout.onMeasure(this.mRecycler, this.mState, widthSpec, heightSpec);
boolean measureSpecModeIsExactly = widthMode == 1073741824 && heightMode == 1073741824;
if (measureSpecModeIsExactly || this.mAdapter == null) {
return;
}
如果宽和高的测量值是绝对值时，直接跳过onMeasure方法。
if (this.mState.mLayoutStep == 1) {
this.dispatchLayoutStep1();
}
mLayoutStep默认值是 State.STEP_START即为1，关于dispatchLayoutStep1方法，其实没有必要过多分析，因为分析源码主要是对于绘制思想的理解，如果过多的纠结于每一行代码的含义，那么会陷入很大的困扰中。执行完之后，是this.mState.mLayoutStep = 2;即STEP_LAYOUT状态。
接下来，是真正执行LayoutManager绘制的地方dispatchLayoutStep2。
private void dispatchLayoutStep2() {
this.startInterceptRequestLayout();
this.onEnterLayoutOrScroll();
this.mState.assertLayoutStep(6);
this.mAdapterHelper.consumeUpdatesInOnePass();
this.mState.mItemCount = this.mAdapter.getItemCount();
this.mState.mDeletedInvisibleItemCountSincePreviousLayout = 0;
this.mState.mInPreLayout = false;
this.mLayout.onLayoutChildren(this.mRecycler, this.mState);
this.mState.mStructureChanged = false;
this.mPendingSavedState = null;
this.mState.mRunSimpleAnimations = this.mState.mRunSimpleAnimations && this.mItemAnimator != null;
this.mState.mLayoutStep = 4;
this.onExitLayoutOrScroll();
this.stopInterceptRequestLayout(false);
}
可以看到，RecyclerView将item的绘制交给了LayoutManager，即mLayout.onLayoutChildren(this.mRecycler, this.mState);，关于LayoutManager将会在下一篇中详细介绍。
这里执行完之后，是this.mState.mLayoutStep = 4;即STEP_ANIMATIONS状态。
之前也说过，onMeasure的主要逻辑在isAutoMeasureEnabled为true时，那么为什么LayoutManager中默认值为false？
如果isAutoMeasureEnabled为false，item能正常绘制吗？让我们做个尝试
我们重写isAutoMeasureEnabled方法，返回false
class MyLinLayoutManager extends LinearLayoutManager {
public MyLinLayoutManager(Context context) {
super(context);
}
@Override
public boolean isAutoMeasureEnabled() {
return false;
}
}
然后将其设置给RecyclerView，运行时，会发现item还能正常显示，这是为什么？这里就要说是onLayout方法
onLayout
protected void onLayout(boolean changed, int l, int t, int r, int b) {
TraceCompat.beginSection("RV OnLayout");
this.dispatchLayout();
TraceCompat.endSection();
this.mFirstLayoutComplete = true;
}
这里的就比较简单了，来看看dispatchLayout方法
void dispatchLayout() {
if (this.mAdapter == null) {
Log.e("RecyclerView", "No adapter attached; skipping layout");
} else if (this.mLayout == null) {
Log.e("RecyclerView", "No layout manager attached; skipping layout");
} else {
this.mState.mIsMeasuring = false;
if (this.mState.mLayoutStep == 1) {
this.dispatchLayoutStep1();
this.mLayout.setExactMeasureSpecsFrom(this);
this.dispatchLayoutStep2();
} else if (!this.mAdapterHelper.hasUpdates() && this.mLayout.getWidth() == this.getWidth() && this.mLayout.getHeight() == this.getHeight()) {
this.mLayout.setExactMeasureSpecsFrom(this);
} else {
this.mLayout.setExactMeasureSpecsFrom(this);
this.dispatchLayoutStep2();
}
this.dispatchLayoutStep3();
}
}
可以看到，这里将onMeasure的主要逻辑重新执行了一遍，也解释了之前，当我们给RecyclerView设置固定的宽高的时候，onMeasure是直接跳过了执行，而子view仍能显示出来的原因。
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