问题描述

原因分析

• dispatchLayoutStep1：处理adapter的更新、决定哪些view执行动画、保存view的信息
• dispatchLayoutStep2：真正执行childView的layout操作
• dispatchLayoutStep3：触发动画、保存状态、清理信息

需要注意的是，在onMeasure的过程中，如果传入的measureMode不是exactly，会去调用dispatchLayoutStep1和dispatchLayoutStep2从而取得真正需要的宽高。 所以在dispatchLayout会先判断是否需要重新执行dispatchLayoutStep1和dispatchLayoutStep2

重点分析dispatchLayoutStep2这一步： 核心操作在 mLayout.onLayoutChildren(mRecycler, mState)这一行。以LinearLayoutManager为例继续往下挖：

public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {

    ...
final View focused = getFocusedChild();
if (!mAnchorInfo.mValid || mPendingScrollPosition != RecyclerView.NO_POSITION

                || mPendingSavedState != null) {

            mAnchorInfo.reset();

            mAnchorInfo.mLayoutFromEnd = mShouldReverseLayout ^ mStackFromEnd;
// 关键步骤1，寻找锚点View位置

            updateAnchorInfoForLayout(recycler, state, mAnchorInfo);

            mAnchorInfo.mValid = true;

        } else if (focused != null && (mOrientationHelper.getDecoratedStart(focused)

                >= mOrientationHelper.getEndAfterPadding()

                || mOrientationHelper.getDecoratedEnd(focused)

                <= mOrientationHelper.getStartAfterPadding())) {

            mAnchorInfo.assignFromViewAndKeepVisibleRect(focused, getPosition(focused));

        }

        ...
// fill towards end

            updateLayoutStateToFillEnd(mAnchorInfo);

            mLayoutState.mExtraFillSpace = extraForEnd;
//关键步骤2，从锚点View位置往后填充

            fill(recycler, mLayoutState, state, false);

            endOffset = mLayoutState.mOffset;
final int lastElement = mLayoutState.mCurrentPosition;
if (mLayoutState.mAvailable > 0) {
//如果锚点位置后面数据不足，无法填满剩余的空间，那把剩余空间加到顶部

                extraForStart += mLayoutState.mAvailable;

            }
// fill towards start

            updateLayoutStateToFillStart(mAnchorInfo);

            mLayoutState.mExtraFillSpace = extraForStart;

            mLayoutState.mCurrentPosition += mLayoutState.mItemDirection;
//关键步骤3，从锚点View位置向前填充

            fill(recycler, mLayoutState, state, false);

            startOffset = mLayoutState.mOffset;

if (mLayoutState.mAvailable > 0) {
//如果锚点View位置前面数据不足，那把剩余空间加到尾部再做一次尝试

                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;

            }

}

先解释一下锚点View，锚点View在一次layout过程中的位置不会发生变化，即之前在哪里显示，这次layout完还在哪，从视觉上看没有位移。

总结一下，mLayout.onLayoutChildren主要做了以下几件事：

所以回到一开始的问题，RecyclerView在notify之后位置跳跃的关键在于锚点View的确定，也就是updateAnchorInfoForLayout方法，所以下面重点看下这个方法：

private void updateAnchorInfoForLayout(RecyclerView.Recycler recycler, RecyclerView.State state,

        AnchorInfo anchorInfo) {
if (updateAnchorFromPendingData(state, anchorInfo)) {
if (DEBUG) {

            Log.d(TAG, "updated anchor info from pending information");

        }
return;

    }

if (updateAnchorFromChildren(recycler, state, anchorInfo)) {
if (DEBUG) {

            Log.d(TAG, "updated anchor info from existing children");

        }
return;

    }
if (DEBUG) {

        Log.d(TAG, "deciding anchor info for fresh state");

    }

    anchorInfo.assignCoordinateFromPadding();

    anchorInfo.mPosition = mStackFromEnd ? state.getItemCount() - 1 : 0;

}

这个方法比较短，所以代码全贴出来了。如果是调用了scrollToPosition后的刷新，会通过updateAnchorFromPendingData方法确定锚点View位置，否则调用updateAnchorFromChildren来计算：

private boolean updateAnchorFromChildren(RecyclerView.Recycler recycler,

        RecyclerView.State state, AnchorInfo anchorInfo) {
if (getChildCount() == 0) {
return false;

    }
final View focused = getFocusedChild();
if (focused != null && anchorInfo.isViewValidAsAnchor(focused, state)) {

        anchorInfo.assignFromViewAndKeepVisibleRect(focused, getPosition(focused));
return true;

    }
if (mLastStackFromEnd != mStackFromEnd) {
return false;

    }

    View referenceChild =

            findReferenceChild(

                    recycler,

                    state,

                    anchorInfo.mLayoutFromEnd,

                    mStackFromEnd);
if (referenceChild != null) {

        anchorInfo.assignFromView(referenceChild, getPosition(referenceChild));

        ...
return true;

    }
return false;

}

代码比较简单，如果有焦点View，并且焦点View没被remove，则使用焦点View作为锚点。否则调用findReferenceChild来查找：

View findReferenceChild(RecyclerView.Recycler recycler, RecyclerView.State state,
boolean layoutFromEnd, boolean traverseChildrenInReverseOrder) {

    ensureLayoutState();

// Determine which direction through the view children we are going iterate.
int start = 0;
int end = getChildCount();
int diff = 1;
if (traverseChildrenInReverseOrder) {

        start = getChildCount() - 1;

        end = -1;

        diff = -1;

    }

int itemCount = state.getItemCount();

final int boundsStart = mOrientationHelper.getStartAfterPadding();
final int boundsEnd = mOrientationHelper.getEndAfterPadding();


    View invalidMatch = null;

    View bestFirstFind = null;

    View bestSecondFind = null;

for (int i = start; i != end; i += diff) {
final View view = getChildAt(i);
final int position = getPosition(view);
final int childStart = mOrientationHelper.getDecoratedStart(view);
final int childEnd = mOrientationHelper.getDecoratedEnd(view);
if (position >= 0 && position < itemCount) {
if (((RecyclerView.LayoutParams) view.getLayoutParams()).isItemRemoved()) {
if (invalidMatch == null) {

                    invalidMatch = view; // removed item, least preferred

                }

            } else {
// b/148869110: usually if childStart >= boundsEnd the child is out of
// bounds, except if the child is 0 pixels!
boolean outOfBoundsBefore = childEnd <= boundsStart && childStart < boundsStart;
boolean outOfBoundsAfter = childStart >= boundsEnd && childEnd > boundsEnd;
if (outOfBoundsBefore || outOfBoundsAfter) {
// The item is out of bounds.
// We want to find the items closest to the in bounds items and because we
// are always going through the items linearly, the 2 items we want are the
// last out of bounds item on the side we start searching on, and the first
// out of bounds item on the side we are ending on.  The side that we are
// ending on ultimately takes priority because we want items later in the
// layout to move forward if no in bounds anchors are found.
if (layoutFromEnd) {
if (outOfBoundsAfter) {

                            bestFirstFind = view;

                        } else if (bestSecondFind == null) {

                            bestSecondFind = view;

                        }

                    } else {
if (outOfBoundsBefore) {

                            bestFirstFind = view;

                        } else if (bestSecondFind == null) {

                            bestSecondFind = view;

                        }

                    }

                } else {
// We found an in bounds item, greedily return it.
return view;

                }

            }

        }

    }
// We didn't find an in bounds item so we will settle for an item in this order:
// 1. bestSecondFind
// 2. bestFirstFind
// 3. invalidMatch
return bestSecondFind != null ? bestSecondFind :

            (bestFirstFind != null ? bestFirstFind : invalidMatch);

}

解释一下，查找过程会遍历RecyclerView当前可见的所有childView，找到第一个没被notifyRemove的childView就停止查找，否则会把遍历过程中找到的第一个被notifyRemove的childView作为锚点View返回。

这里需要注意final int position = getPosition(view);这一行代码，getPosition返回的是经过校正的最终position，如果ViewHolder被notifyRemove了，这里的position会是0，所以如果可见的childView都被remove了，那最终定位的锚点View是第一个childView，锚点的position是0，偏移量offset是这个被删除的childView的top值，这就会导致后面fill操作时从位置0开始填充，先把position=0的view填充到偏移量offset的位置，再往后依次填满剩余空间，这也是导致画面上的跳动的根本原因。