解析RecyclerView中itemView的动画

 上篇文章主要讲的是itemView的绘制流程。

  分析完上篇文章后，我又开始疑惑了，itemView的动画是如何绘制的呢 ？ 

  从源码来分析这个问题，从上篇文章我们看到，RecyclerView 绘制的关键是onLayout 里面的position 位置确定，因此直接上源码：

     

 @Override    protected void onLayout(boolean changed , int l , int t , int r , int b) {        eatRequestLayout();        dispatchLayout();        resumeRequestLayout(false);        mFirstLayoutComplete = true;    }

 里面关键的是： 

  dispatchLayout（）

  

void dispatchLayout() {        if (mAdapter == null) {            Log.e(TAG, "No adapter attached; skipping layout");            return;        }        eatRequestLayout();        // simple animations are a subset of advanced animations (which will cause a        // prelayout step)        boolean animateChangesSimple = mItemAnimator != null && mItemsAddedOrRemoved && !mItemsChanged;        final boolean animateChangesAdvanced = ENABLE_PREDICTIVE_ANIMATIONS && animateChangesSimple && predictiveItemAnimationsEnabled();        mItemsAddedOrRemoved = mItemsChanged = false;        ArrayMap<View, Rect> appearingViewInitialBounds = null;        mState.mInPreLayout = animateChangesAdvanced;        mState.mItemCount = mAdapter.getItemCount();        if (animateChangesSimple) {            // Step 0: Find out where all non-removed items are, pre-layout            mState.mPreLayoutHolderMap.clear();            mState.mPostLayoutHolderMap.clear();            final int count = getChildCount();            for (int i = 0; i < count; ++i) {                final ViewHolder holder = getChildViewHolderInt(getChildAt(i));                final View view = holder.itemView;                mState.mPreLayoutHolderMap.put(holder, new ItemHolderInfo(holder, view.getLeft(), view.getTop(), view.getRight(), view.getBottom(), holder.mPosition));            }        }        if (animateChangesAdvanced) {            // Step 1: run prelayout: This will use the old positions of items. The layout manager            // is expected to layout everything, even removed items (though not to add removed            // items back to the container). This gives the pre-layout position of APPEARING views            // which come into existence as part of the real layout.            mInPreLayout = true;            final boolean didStructureChange = mState.mStructureChanged;            mState.mStructureChanged = false;            // temporarily disable flag because we are asking for previous layout            mLayout.onLayoutChildren(mRecycler, mState);            mState.mStructureChanged = didStructureChange;            mInPreLayout = false;            appearingViewInitialBounds = new ArrayMap<View, Rect>();            for (int i = 0; i < getChildCount(); ++i) {                boolean found = false;                final View child = getChildAt(i);                for (int j = 0; j < mState.mPreLayoutHolderMap.size(); ++j) {                    final ViewHolder holder = mState.mPreLayoutHolderMap.keyAt(j);                    if (holder.itemView == child) {                        found = true;                        continue;                    }                }                if (!found) {                    appearingViewInitialBounds.put(child, new Rect(child.getLeft(), child.getTop(), child.getRight(), child.getBottom()));                }            }        }        clearOldPositions();        dispatchLayoutUpdates();        mState.mItemCount = mAdapter.getItemCount();        // Step 2: Run layout        mState.mInPreLayout = false;        mLayout.onLayoutChildren(mRecycler, mState);        mState.mStructureChanged = false;        mPendingSavedState = null;        // onLayoutChildren may have caused client code to disable item animations; re-check        animateChangesSimple = animateChangesSimple && mItemAnimator != null;        if (animateChangesSimple) {            // Step 3: Find out where things are now, post-layout            int count = getChildCount();            for (int i = 0; i < count; ++i) {                final ViewHolder holder = getChildViewHolderInt(getChildAt(i));                final View view = holder.itemView;                mState.mPostLayoutHolderMap.put(holder, new ItemHolderInfo(holder, view.getLeft(), view.getTop(), view.getRight(), view.getBottom(), holder.mPosition));            }            // Step 4: Animate DISAPPEARING and REMOVED items            final int preLayoutCount = mState.mPreLayoutHolderMap.size();            for (int i = preLayoutCount - 1; i >= 0; i--) {                final ViewHolder itemHolder = mState.mPreLayoutHolderMap.keyAt(i);                if (!mState.mPostLayoutHolderMap.containsKey(itemHolder)) {                    final ItemHolderInfo disappearingItem = mState.mPreLayoutHolderMap.valueAt(i);                    mState.mPreLayoutHolderMap.removeAt(i);                    final View disappearingItemView = disappearingItem.holder.itemView;                    removeDetachedView(disappearingItemView, false);                    mRecycler.unscrapView(disappearingItem.holder);                    animateDisappearance(disappearingItem);                }            }            // Step 5: Animate APPEARING and ADDED items            final int postLayoutCount = mState.mPostLayoutHolderMap.size();            if (postLayoutCount > 0) {                for (int i = postLayoutCount - 1; i >= 0; i--) {                    final ViewHolder itemHolder = mState.mPostLayoutHolderMap.keyAt(i);                    final ItemHolderInfo info = mState.mPostLayoutHolderMap.valueAt(i);                    if ((mState.mPreLayoutHolderMap.isEmpty() || !mState.mPreLayoutHolderMap.containsKey(itemHolder))) {                        mState.mPostLayoutHolderMap.removeAt(i);                        final Rect initialBounds = (appearingViewInitialBounds != null) ? appearingViewInitialBounds.get(itemHolder.itemView) : null;                        animateAppearance(itemHolder, initialBounds, info.left, info.top);                    }                }            }            // Step 6: Animate PERSISTENT items            count = mState.mPostLayoutHolderMap.size();            for (int i = 0; i < count; ++i) {                final ViewHolder postHolder = mState.mPostLayoutHolderMap.keyAt(i);                final ItemHolderInfo postInfo = mState.mPostLayoutHolderMap.valueAt(i);                final ItemHolderInfo preInfo = mState.mPreLayoutHolderMap.get(postHolder);                if (preInfo != null && postInfo != null) {                    if (preInfo.left != postInfo.left || preInfo.top != postInfo.top) {                        postHolder.setIsRecyclable(false);                        if (DEBUG) {                            Log.d(TAG, "PERSISTENT: " + postHolder + " with view " + postHolder.itemView);                        }                        if (mItemAnimator.animateMove(postHolder, preInfo.left, preInfo.top, postInfo.left, postInfo.top)) {                            postAnimationRunner();                        }                    }                }            }        }        resumeRequestLayout(false);        mLayout.removeAndRecycleScrapInt(mRecycler, !animateChangesAdvanced);        mState.mPreviousLayoutItemCount = mState.mItemCount;        mState.mDeletedInvisibleItemCountSincePreviousLayout = 0;    }

  从开始进行分析，boolean animateChangesSimple = mItemAnimator != null && mItemsAddedOrRemoved && !mItemsChanged;   mItemAnimator 这个参数不为空，mItemsAddedOrRemoved  表示是添加和删除了itemView追踪一下，发现他在

 

 void updateChildViews() {        final int opCount = mPendingUpdates.size();        for (int i = 0; i < opCount; i++) {            final UpdateOp op = mPendingUpdates.get(i);            switch (op.cmd) {                case UpdateOp.ADD:                    if (DEBUG) {                        Log.d(TAG, "UpdateOp.ADD start=" + op.positionStart + " count=" + op.itemCount);                    }                    offsetPositionRecordsForInsert(op.positionStart, op.itemCount);                    mItemsAddedOrRemoved = true;                    break;                case UpdateOp.REMOVE:                    if (DEBUG) {                        Log.d(TAG, "UpdateOp.REMOVE start=" + op.positionStart + " count=" + op.itemCount);                    }                    for (int j = 0; j < op.itemCount; ++j) {                        final ViewHolder holder = findViewHolderForPosition(op.positionStart + j, true);                        if (holder != null) {                            holder.setIsRecyclable(false);                        }                        else {                            mState.mDeletedInvisibleItemCountSincePreviousLayout++;                        }                    }                    offsetPositionRecordsForRemove(op.positionStart, op.itemCount);                    mItemsAddedOrRemoved = true;                    break;                case UpdateOp.UPDATE:                    if (DEBUG) {                        Log.d(TAG, "UpdateOp.UPDATE start=" + op.positionStart + " count=" + op.itemCount);                    }                    viewRangeUpdate(op.positionStart, op.itemCount);                    mItemsChanged = true;                    break;            }            mPendingLayoutUpdates.add(op);            // TODO: recycle the op if no animator (also don't bother stashing in pending layout updates?)        }        mPendingUpdates.clear();    }

 发现在这个里面赋值为true，很明显的是UpdateOp操作为 ADD REMOVEDE 的时候，赋值为true，这里涉及到观察者模式，下篇文章会具体讲这个问题，在这里我们知道是我们手动通知了adapter 数据改变了。接着上面的讲，animateChangesSimple 为true，下面的变量animateChangesAdvanced 为true，mItemsAddedOrRemoved = mItemsChanged = false 避免下次重复执行动画的处理。

  if (animateChangesSimple) {            // Step 0: Find out where all non-removed items are, pre-layout            mState.mPreLayoutHolderMap.clear();            mState.mPostLayoutHolderMap.clear();            final int count = getChildCount();            for (int i = 0; i < count; ++i) {                final ViewHolder holder = getChildViewHolderInt(getChildAt(i));                final View view = holder.itemView;                mState.mPreLayoutHolderMap.put(holder, new ItemHolderInfo(holder, view.getLeft(), view.getTop(), view.getRight(), view.getBottom(), holder.mPosition));            }        }

 因为上面我们已经确定animateChangesSimple 为true，注意: 这段 代码就是把所有的itemView的现状保存一下。

   if (animateChangesAdvanced) {            // Step 1: run prelayout: This will use the old positions of items. The layout manager            // is expected to layout everything, even removed items (though not to add removed            // items back to the container). This gives the pre-layout position of APPEARING views            // which come into existence as part of the real layout.            mInPreLayout = true;            final boolean didStructureChange = mState.mStructureChanged;            mState.mStructureChanged = false;            // temporarily disable flag because we are asking for previous layout            mLayout.onLayoutChildren(mRecycler, mState);            mState.mStructureChanged = didStructureChange;            mInPreLayout = false;            appearingViewInitialBounds = new ArrayMap<View, Rect>();            for (int i = 0; i < getChildCount(); ++i) {                boolean found = false;                final View child = getChildAt(i);                for (int j = 0; j < mState.mPreLayoutHolderMap.size(); ++j) {                    final ViewHolder holder = mState.mPreLayoutHolderMap.keyAt(j);                    if (holder.itemView == child) {                        found = true;                        continue;                    }                }                if (!found) {                    appearingViewInitialBounds.put(child, new Rect(child.getLeft(), child.getTop(), child.getRight(), child.getBottom()));                }            }        }

  里面有一段关键代码， 
 

mLayout.onLayoutChildren(mRecycler, mState);

这个主要是重新布局，后面的for循环是为了获取新增加的itemView，存储在了 appearingViewInitialBounds这个集合中

clearOldPositions()是清除itemView的位置信息；

  mState.mInPreLayout = false;        mLayout.onLayoutChildren(mRecycler, mState);

主要是调用LayoutManager 中的onlayoutChidren 方法，来为itemview 确定位置；

下面重点来了，

 

 // Step 3: Find out where things are now, post-layout            int count = getChildCount();            for (int i = 0; i < count; ++i) {                final ViewHolder holder = getChildViewHolderInt(getChildAt(i));                final View view = holder.itemView;                mState.mPostLayoutHolderMap.put(holder, new ItemHolderInfo(holder, view.getLeft(), view.getTop(), view.getRight(), view.getBottom(), holder.mPosition));            }

保存itemView的最终位置 

下面就开始执行“删除itemView ” 时候的动画了 

  

// Step 4: Animate DISAPPEARING and REMOVED items            final int preLayoutCount = mState.mPreLayoutHolderMap.size();            for (int i = preLayoutCount - 1; i >= 0; i--) {                final ViewHolder itemHolder = mState.mPreLayoutHolderMap.keyAt(i);                if (!mState.mPostLayoutHolderMap.containsKey(itemHolder)) {                    final ItemHolderInfo disappearingItem = mState.mPreLayoutHolderMap.valueAt(i);                    mState.mPreLayoutHolderMap.removeAt(i);                    final View disappearingItemView = disappearingItem.holder.itemView;                    removeDetachedView(disappearingItemView, false);                    mRecycler.unscrapView(disappearingItem.holder);                    animateDisappearance(disappearingItem);                }            }

   上下的是执行那些新增加的 ItemView的动画

 

// Step 5: Animate APPEARING and ADDED items            final int postLayoutCount = mState.mPostLayoutHolderMap.size();            if (postLayoutCount > 0) {                for (int i = postLayoutCount - 1; i >= 0; i--) {                    final ViewHolder itemHolder = mState.mPostLayoutHolderMap.keyAt(i);                    final ItemHolderInfo info = mState.mPostLayoutHolderMap.valueAt(i);                    if ((mState.mPreLayoutHolderMap.isEmpty() || !mState.mPreLayoutHolderMap.containsKey(itemHolder))) {                        mState.mPostLayoutHolderMap.removeAt(i);                        final Rect initialBounds = (appearingViewInitialBounds != null) ? appearingViewInitialBounds.get(itemHolder.itemView) : null;                        animateAppearance(itemHolder, initialBounds, info.left, info.top);                    }                }            }

 下面执行的剩余itemView的动画（位置有可能发生了变化）

  // Step 6: Animate PERSISTENT items            count = mState.mPostLayoutHolderMap.size();            for (int i = 0; i < count; ++i) {                final ViewHolder postHolder = mState.mPostLayoutHolderMap.keyAt(i);                final ItemHolderInfo postInfo = mState.mPostLayoutHolderMap.valueAt(i);                final ItemHolderInfo preInfo = mState.mPreLayoutHolderMap.get(postHolder);                if (preInfo != null && postInfo != null) {                    if (preInfo.left != postInfo.left || preInfo.top != postInfo.top) {                        postHolder.setIsRecyclable(false);                        if (DEBUG) {                            Log.d(TAG, "PERSISTENT: " + postHolder + " with view " + postHolder.itemView);                        }                        if (mItemAnimator.animateMove(postHolder, preInfo.left, preInfo.top, postInfo.left, postInfo.top)) {                            postAnimationRunner();                        }                    }                }            }

   至于每个itemView的动画交给了  DefaultItemAnimator 这个类进行处理，不停的更新的动画itemView的位置或者是透明度以及其他的数据。

   主要的逻辑就是：

       增加/删除itemView -  保留itemView 的位置 -进行预处理（测量所有itemView的位置），获取新增的itemView-   清除所有itemView的位置信息-  测量以及定位所有的itemView- 保存itemview的最终动画的位置以及其他信息- 处理删除remove的itemView- 处理被add的itemview的动画-处理剩余itemview 位置改变的itemview的动画。

   上面就是动画绘制的主要过程。