void dispatchLayout(){
  ...
  dispatchLayoutStep1();
  dispatchLayoutStep2();
  dispatchLayoutStep3();
  ...
}
复制代码

private void dispatchLayoutStep1() {
  ...
  //第一步 判断是否需要开启动画功能
  processAdapterUpdatesAndSetAnimationFlags();
  ...
  if (mState.mRunSimpleAnimations) {
    ...
    //第二步  将当前屏幕上的Item相关信息保存起来供后续动画使用
    int count = mChildHelper.getChildCount();
    for (int i = 0; i < count; ++i) {
        final ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
        final ItemHolderInfo animationInfo = mItemAnimator
                        .recordPreLayoutInformation(mState, holder,
                                ItemAnimator.buildAdapterChangeFlagsForAnimations(holder),
                                holder.getUnmodifiedPayloads());
        mViewInfoStore.addToPreLayout(holder, animationInfo);
    }
    ...
    if (mState.mRunPredictiveAnimations) {
          saveOldPositions();
          //第三步 调用onLayoutChildren方法预布局
          mLayout.onLayoutChildren(mRecycler, mState);
          mState.mStructureChanged = didStructureChange;
          for (int i = 0; i < mChildHelper.getChildCount(); ++i) {
              final View child = mChildHelper.getChildAt(i);
              final ViewHolder viewHolder = getChildViewHolderInt(child);
              if (viewHolder.shouldIgnore()) {
                  continue;
              }
                        //第四步 预布局后，对比预布局前后，哪些item需要放入到Appeared中
              if (!mViewInfoStore.isInPreLayout(viewHolder)) {
                  if (wasHidden) {
                      recordAnimationInfoIfBouncedHiddenView(viewHolder, animationInfo);
                  } else {
                      mViewInfoStore.addToAppearedInPreLayoutHolders(viewHolder, animationInfo);
                  }
              }
          }
          clearOldPositions();
      } else {
          clearOldPositions();
      }
  }
}

private void dispatchLayoutStep2() {
    ...
    // Step 2: Run layout
    mState.mInPreLayout = false;//此处关闭预布局模式
    mLayout.onLayoutChildren(mRecycler, mState);
    ...
}

private void dispatchLayoutStep3() {
    ...
    if (mState.mRunSimpleAnimations) {
        // Step 3: Find out where things are now, and process change animations.
        // traverse list in reverse because we may call animateChange in the loop which may
        // remove the target view holder.
        for (int i = mChildHelper.getChildCount() - 1; i >= 0; i--) {
            ViewHolder holder = getChildViewHolderInt(mChildHelper.getChildAt(i));
            if (holder.shouldIgnore()) {
                continue;
            }
            long key = getChangedHolderKey(holder);
            final ItemHolderInfo animationInfo = mItemAnimator
                    .recordPostLayoutInformation(mState, holder);
            ViewHolder oldChangeViewHolder = mViewInfoStore.getFromOldChangeHolders(key);
            if (oldChangeViewHolder != null && !oldChangeViewHolder.shouldIgnore()) {
                // run a change animation
                ...
            } else {
                mViewInfoStore.addToPostLayout(holder, animationInfo);
            }
        }
        // Step 4: Process view info lists and trigger animations
        //触发动画
        mViewInfoStore.process(mViewInfoProcessCallback);
    }
  ...
    }

public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
    ...
    //1. 寻找填充的锚点
    updateAnchorInfoForLayout(recycler, state, mAnchorInfo);
    ...
    //2. 移除屏幕上的Views
    detachAndScrapAttachedViews(recycler);
    ...
    //3. 从锚点处从上往下填充
    updateLayoutStateToFillEnd(mAnchorInfo);
    mLayoutState.mExtraFillSpace = extraForEnd;
    fill(recycler, mLayoutState, state, false);
    ...
    //4. 从锚点处从下往上填充
    // fill towards start
    updateLayoutStateToFillStart(mAnchorInfo);
    mLayoutState.mExtraFillSpace = extraForStart;
    mLayoutState.mCurrentPosition += mLayoutState.mItemDirection;
    fill(recycler, mLayoutState, state, false);
    ...
    //5. 如果还有多余的空间，继续填充
    if (mLayoutState.mAvailable > 0) {
        extraForEnd = mLayoutState.mAvailable;
        // start could not consume all it should. add more items towards end
        updateLayoutStateToFillEnd(lastElement, endOffset);
        mLayoutState.mExtraFillSpace = extraForEnd;
        fill(recycler, mLayoutState, state, false);
        endOffset = mLayoutState.mOffset;
    }
  }
    ...
    //6. 非预布局，将scrapList中多余的ViewHolder填充
    layoutForPredictiveAnimations(recycler, state, startOffset, endOffset);
    ...

private void layoutForPredictiveAnimations(RecyclerView.Recycler recycler,
            RecyclerView.State state, int startOffset,
            int endOffset) {
        //判断是否满足条件，如果是预布局直接返回
        if (!state.willRunPredictiveAnimations() ||  getChildCount() == 0 || state.isPreLayout()
                || !supportsPredictiveItemAnimations()) {
            return;
        }
        // 遍历scrapList，步骤2中屏幕中被移除的View
        int scrapExtraStart = 0, scrapExtraEnd = 0;
        final List<RecyclerView.ViewHolder> scrapList = recycler.getScrapList();
        final int scrapSize = scrapList.size();
        final int firstChildPos = getPosition(getChildAt(0));
        for (int i = 0; i < scrapSize; i++) {
            RecyclerView.ViewHolder scrap = scrapList.get(i);
            //如果被remove掉了，跳过
            if (scrap.isRemoved()) {
                continue;
            }
            //计算额外的控件
                scrapExtraEnd += mOrientationHelper.getDecoratedMeasurement(scrap.itemView);
        }
        mLayoutState.mScrapList = scrapList;
        ...
        // 步骤6 继续填充
        if (scrapExtraEnd > 0) {
            View anchor = getChildClosestToEnd();
            updateLayoutStateToFillEnd(getPosition(anchor), endOffset);
            mLayoutState.mExtraFillSpace = scrapExtraEnd;
            mLayoutState.mAvailable = 0;
            mLayoutState.assignPositionFromScrapList();
            fill(recycler, mLayoutState, state, false);
        }
        mLayoutState.mScrapList = null;
    }

if (getChildCount() > 0) {
            // because layout from end may be changed by scroll to position
            // we re-calculate it.
            // find which side we should check for gaps.
            if (mShouldReverseLayout ^ mStackFromEnd) {
                int fixOffset = fixLayoutEndGap(endOffset, recycler, state, true);
                startOffset += fixOffset;
                endOffset += fixOffset;
                fixOffset = fixLayoutStartGap(startOffset, recycler, state, false);
                startOffset += fixOffset;
                endOffset += fixOffset;
            } else {
                int fixOffset = fixLayoutStartGap(startOffset, recycler, state, true);
                startOffset += fixOffset;
                endOffset += fixOffset;
                fixOffset = fixLayoutEndGap(endOffset, recycler, state, false);
                startOffset += fixOffset;
                endOffset += fixOffset;
            }
        }