RecycleView Layout 详解

前提

我们以RecycleView 的宽高都是Match_Parent作为前提去分析，这样onMeasure方法就不会调用dispatchStep1、dispatchStep2.我们只看onLayout 方法

ViewHolder 一些属性

    public abstract static class ViewHolder {@NonNullpublic final View itemView;WeakReference<RecyclerView> mNestedRecyclerView;int mPosition = NO_POSITION;int mOldPosition = NO_POSITION;long mItemId = NO_ID;int mItemViewType = INVALID_TYPE;//之前int mPreLayoutPosition = NO_POSITION;

mPreLayoutPosition 用于dispatchLayoutStep1，表示当前ViewHolder的位置，mPosition 用于dispatchLayoutStep2 时表示ViewHolder的位置。

getLayoutPosition

 public final int getLayoutPosition() {return mPreLayoutPosition == NO_POSITION ? mPosition : mPreLayoutPosition;}

getLayoutPosition 如果当前是PreLayout阶段(dispatchLayoutStep1方法)，那么该方法返回的就是mPreLayoutPosition，如果不是PreLayout阶段(dispatchLayoutStep2方法)，就会返回mPosition.

首次进去RecycleView 页面

androidx.recyclerview.widget.RecyclerView#onLayout

    @Overrideprotected void onLayout(boolean changed, int l, int t, int r, int b) {dispatchLayout();mFirstLayoutComplete = true;}

里面调用了dispatchLayout 方法

androidx.recyclerview.widget.RecyclerView#dispatchLayout

    void dispatchLayout() {if (mLayout == null) {Log.e(TAG, "No layout manager attached; skipping layout");// leave the state in STARTreturn;}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();}

里面layout 会分成三步。

dispatchLayoutStep1

    private void dispatchLayoutStep1() {mState.assertLayoutStep(State.STEP_START);mState.mIsMeasuring = false;//用于控制在layout期间 requestLayout不会生效startInterceptRequestLayout();//首次进去页面不会有任何处理  当我们调用notifyItemRemove 会修改oldpositionprocessAdapterUpdatesAndSetAnimationFlags();saveFocusInfo();mState.mTrackOldChangeHolders = mState.mRunSimpleAnimations && mItemsChanged;mItemsAddedOrRemoved = mItemsChanged = false;mState.mInPreLayout = mState.mRunPredictiveAnimations;mState.mItemCount = mAdapter.getItemCount();if (mState.mRunPredictiveAnimations) {clearOldPositions();} else {//清楚OldPosition PreLayoutPositionclearOldPositions();}onExitLayoutOrScroll();stopInterceptRequestLayout(false);mState.mLayoutStep = State.STEP_LAYOUT;}

第一步主要进行ViewHolder的一些数据更新操作，比如ViewHolder 某个元素被移除，那么后面的ViewHolder的Position 都会被减一，同时如果RecycleView 有动画，那么会进行PreLayout，拿到动画位置的信息。

dispatchLayoutStep2

    private void dispatchLayoutStep2() {// Step 2: Run layoutmState.mInPreLayout = false;mLayout.onLayoutChildren(mRecycler, mState);mState.mStructureChanged = false;mPendingSavedState = null;}

真正的进行布局，如果在dispatchLayoutStep1进行过PreLayout，那么本次的布局所有的ViewHolder都是存在的，因为之前PreLayout 都已经进行了创建。
dispatchLayoutStep3

    private void dispatchLayoutStep3() {mState.assertLayoutStep(State.STEP_ANIMATIONS);onEnterLayoutOrScroll();mState.mLayoutStep = State.STEP_START;if (mState.mRunSimpleAnimations) {//执行动画}mLayout.removeAndRecycleScrapInt(mRecycler);mState.mPreviousLayoutItemCount = mState.mItemCount;}

主要进行动画的处理，回收Scrap的ViewHolder到RecycleViewPool中，比如被移除掉的ViewHolder.

调用adapter.notifyItemRemoved()方法 onLayout执行分析

注意，本篇我们以有动画的RecyclerView 进行分析，如果RecyclerView 没有动画，那么不会执行PreLayout，也就是在dispatchLayoutStep1里面并不会进行布局的填充。

第一步，更新Position

   at androidx.recyclerview.widget.RecyclerView$ViewHolder.offsetPosition(RecyclerView.java:11106)at androidx.recyclerview.widget.RecyclerView$ViewHolder.flagRemovedAndOffsetPosition(RecyclerView.java:11095)at androidx.recyclerview.widget.RecyclerView.offsetPositionRecordsForRemove(RecyclerView.java:4635)at androidx.recyclerview.widget.RecyclerView$6.offsetPositionsForRemovingLaidOutOrNewView(RecyclerView.java:996)at androidx.recyclerview.widget.AdapterHelper.postponeAndUpdateViewHolders(AdapterHelper.java:448)at androidx.recyclerview.widget.AdapterHelper.applyRemove(AdapterHelper.java:183)at androidx.recyclerview.widget.AdapterHelper.preProcess(AdapterHelper.java:102)at androidx.recyclerview.widget.RecyclerView.processAdapterUpdatesAndSetAnimationFlags(RecyclerView.java:3793)at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:4039)at androidx.recyclerview.widget.RecyclerView.dispatchLayout(RecyclerView.java:3849)at androidx.recyclerview.widget.RecyclerView.onLayout(RecyclerView.java:4404)

在dispatchLayoutStep1 方法里面，会进行更新oldposition、preLayoutPostion，preLayoutPosition 有了之后，就可以执行dispatchLayoutStep1 里面的布局.

offsetPosition() 更新oldposition、preLayoutPostion

androidx.recyclerview.widget.RecyclerView.ViewHolder#offsetPositionvoid offsetPosition(int offset, boolean applyToPreLayout) {if (mOldPosition == NO_POSITION) {mOldPosition = mPosition;}if (mPreLayoutPosition == NO_POSITION) {mPreLayoutPosition = mPosition;}if (applyToPreLayout) {mPreLayoutPosition += offset;}mPosition += offset;if (itemView.getLayoutParams() != null) {((LayoutParams) itemView.getLayoutParams()).mInsetsDirty = true;}}

比如当一个item 移除掉，如果RecycleView 里面还有多余的元素，prelayout会加载没有下一个ViewHolder.

在布局的时候，移除的view 不会被计算占用的空间

androidx.recyclerview.widget.LinearLayoutManager#layoutChunk// Consume the available space if the view is not removed OR changedif (params.isItemRemoved() || params.isItemChanged()) {result.mIgnoreConsumed = true;}

         androidx.recyclerview.widget.LinearLayoutManager#fillif (!layoutChunkResult.mIgnoreConsumed || layoutState.mScrapList != null|| !state.isPreLayout()) {layoutState.mAvailable -= layoutChunkResult.mConsumed;// we keep a separate remaining space because mAvailable is important for recyclingremainingSpace -= layoutChunkResult.mConsumed;}

当dispatchLayoutStep1 执行完毕，清除preLayoutPosition，进行dispatchLayoutStep2

androidx.recyclerview.widget.RecyclerView#dispatchLayoutStep1
androidx.recyclerview.widget.RecyclerView.ViewHolder#clearOldPosition

            // we don't process disappearing list because they may re-appear in post layout pass.clearOldPositions();

dispatchLayoutStep3 回收被移除的ViewHolder

 androidx.recyclerview.widget.RecyclerView.Recycler#recycleViewHolderInternalat androidx.recyclerview.widget.RecyclerView$Recycler.recycleViewHolderInternal(RecyclerView.java:6429)at androidx.recyclerview.widget.RecyclerView$Recycler.quickRecycleScrapView(RecyclerView.java:6554)at androidx.recyclerview.widget.RecyclerView$LayoutManager.removeAndRecycleScrapInt(RecyclerView.java:9249)at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep3(RecyclerView.java:4207)

总结：

首页进去RecycleView，只会在dispatchLayoutStep2 方法里面，对RecycleView 的布局进行填充。
如果我们调用了notifyItemRemoved 方法，在dispatchLayoutStep1 方法里面，会先更新OldPosition、PreLayoutPosition. 然后执行layout，layout之前会先调用mLayout.onLayoutChildren(mRecycler, mState); 在这个方法里面，会根据PreLayoutPosition 进行布局，因为有item 被移除，那么肯定会多出来空间，这时候，就会调用androidx.recyclerview.widget.LinearLayoutManager#fill 方法就会填充View，就会创建新的ViewHolder，在dispatchLayoutStep1 的最后，会把PreLayoutPosition 置为-1,这样就不影响dispatchLayoutStep2的布局。dispatchLayoutStep2 布局的时候，就会使用diapatchLayoutStep1创建的ViewHolder. 在dispatchStep3中，进行动画的处理，动画结束后，会把移除的ViewHolder 放到RecycleViewPool中重复利用。