前言
上一篇，說了RecyclerView的回收複用，這一篇，我們來說說RecyclerView的繪製流程。
- 【Android進階】RecyclerView之ItemDecoration（一）
- 【Android進階】RecyclerView之快取（二）
- 【Android進階】RecyclerView之繪製流程（三）
onMeasure
我們先看看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);
}
}
}
}
複製程式碼我們從上往下看，首先，mLayout即為LayoutManager，如果其為null會執行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);
}
複製程式碼可以看到，這裡沒有測量item的高度就直接呼叫setMeasuredDimension方法設定寬高了
接著，是根據isAutoMeasureEnabled，會走2套邏輯，通過檢視原始碼可以發現，isAutoMeasureEnabled即mAutoMeasure在LayoutManager中，預設為false，但在LinearLayoutManager中為true
public boolean isAutoMeasureEnabled() {
return true;
}
複製程式碼而onMeasure的主要邏輯也是在isAutoMeasureEnabled為true時，我們接著往下看
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;
}
複製程式碼如果寬和高的測量值是絕對值時，直接跳過onMeasure方法。
if (this.mState.mLayoutStep == 1) {
this.dispatchLayoutStep1();
}
複製程式碼mLayoutStep預設值是 State.STEP_START即為1，關於dispatchLayoutStep1方法，其實沒有必要過多分析，因為分析原始碼主要是對於繪製思想的理解，如果過多的糾結於每一行程式碼的含義，那麼會陷入很大的困擾中。執行完之後，是this.mState.mLayoutStep = 2;即STEP_LAYOUT狀態。
接下來，是真正執行LayoutManager繪製的地方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);
}
複製程式碼可以看到，RecyclerView將item的繪製交給了LayoutManager，即mLayout.onLayoutChildren(this.mRecycler, this.mState);，關於LayoutManager將會在下一篇中詳細介紹。
這裡執行完之後，是this.mState.mLayoutStep = 4;即STEP_ANIMATIONS狀態。
之前也說過，onMeasure的主要邏輯在isAutoMeasureEnabled為true時，那麼為什麼LayoutManager中預設值為false，如果isAutoMeasureEnabled為false，item能正常繪製嗎？讓我們做個嘗試
重新isAutoMeasureEnabled方法
class MyLinLayoutManager extends LinearLayoutManager {
public MyLinLayoutManager(Context context) {
super(context);
}
@Override
public boolean isAutoMeasureEnabled() {
return false;
}
}
複製程式碼然後將其設定給RecyclerView，執行時，會發現item還能正常顯示，這是為什麼？這裡就要說是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;
}
複製程式碼這裡的就比較簡單了，來看看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();
}
}
複製程式碼可以看到，這裡將onMeasure的主要邏輯重新執行了一遍，也解釋了之前，當我們給RecyclerView設定固定的寬高的時候，onMeasure是直接跳過了執行，而子view仍能顯示出來的原因。