RecyclerView缓存机制（回收些啥？）

这是RecyclerView缓存机制系列文章的第二篇，系列文章的目录如下：

RecyclerView缓存机制（咋复用？）
RecyclerView缓存机制（回收些啥？）
RecyclerView缓存机制（回收去哪？）
RecyclerView缓存机制（scrap view）

如果想直接看结论可以移步到第四篇末尾（你会后悔的，过程更加精彩）。

上一篇文章讲述了“从哪里获得回收的表项”，这一篇会结合实际回收场景分析下“回收哪些表项？”。

(ps: 下文中的粗斜体字表示引导源码阅读的内心戏)

回收场景

在众多回收场景中最显而易见的就是“滚动列表时移出屏幕的表项被回收”。滚动是由MotionEvent.ACTION_MOVE事件触发的，就以RecyclerView.onTouchEvent()为切入点寻觅“回收表项”的时机：

public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 {@Overridepublic boolean onTouchEvent(MotionEvent e) {...case MotionEvent.ACTION_MOVE: {...if (scrollByInternal(canScrollHorizontally ? dx : 0,canScrollVertically ? dy : 0,vtev)) {getParent().requestDisallowInterceptTouchEvent(true);}...}} break;...}
}
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去掉了大量位移赋值逻辑后，一个处理滚动的函数出现在眼前：

public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild2 {...@VisibleForTesting LayoutManager mLayout;...boolean scrollByInternal(int x, int y, MotionEvent ev) {...if (mAdapter != null) {...if (x != 0) {consumedX = mLayout.scrollHorizontallyBy(x, mRecycler, mState);unconsumedX = x - consumedX;}if (y != 0) {consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState);unconsumedY = y - consumedY;}...}...
}
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RecyclerView把滚动交给了LayoutManager来处理，于是移步到最熟悉的LinearLayoutManager：

public class LinearLayoutManager extends RecyclerView.LayoutManager implements ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider {...@Overridepublic int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler,RecyclerView.State state) {if (mOrientation == HORIZONTAL) {return 0;}return scrollBy(dy, recycler, state);}...int scrollBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {if (getChildCount() == 0 || dy == 0) {return 0;}mLayoutState.mRecycle = true;ensureLayoutState();final int layoutDirection = dy > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;final int absDy = Math.abs(dy);//更新LayoutState（这个函数对于“回收哪些表项”来说很关键，待会会提到）updateLayoutState(layoutDirection, absDy, true, state);//滚动时向列表中填充新的表项final int consumed = mLayoutState.mScrollingOffset+ fill(recycler, mLayoutState, state, false);if (consumed < 0) {if (DEBUG) {Log.d(TAG, "Don't have any more elements to scroll");}return 0;}final int scrolled = absDy > consumed ? layoutDirection * consumed : dy;mOrientationHelper.offsetChildren(-scrolled);if (DEBUG) {Log.d(TAG, "scroll req: " + dy + " scrolled: " + scrolled);}mLayoutState.mLastScrollDelta = scrolled;return scrolled;}...
}
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沿着调用链往下找，发现了一个上一篇中介绍过的函数LinearLayoutManager.fill()，原来列表滚动的同时也会不断的向其中填充表项（ 想想也是，不然怎么会不断有新的表项出现呢~ ）。上一遍只关注了其中填充的逻辑，但其实里面还有回收逻辑：

public class LinearLayoutManager extends RecyclerView.LayoutManager implements ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider {...int fill(RecyclerView.Recycler recycler, LayoutState layoutState,RecyclerView.State state, boolean stopOnFocusable) {...int remainingSpace = layoutState.mAvailable + layoutState.mExtra;LayoutChunkResult layoutChunkResult = mLayoutChunkResult;//不断循环获取新的表项用于填充，直到没有填充空间while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {layoutChunkResult.resetInternal();if (VERBOSE_TRACING) {TraceCompat.beginSection("LLM LayoutChunk");}//填充新的表项layoutChunk(recycler, state, layoutState, layoutChunkResult);if (VERBOSE_TRACING) {TraceCompat.endSection();}if (layoutChunkResult.mFinished) {break;}layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;if (!layoutChunkResult.mIgnoreConsumed || mLayoutState.mScrapList != null|| !state.isPreLayout()) {layoutState.mAvailable -= layoutChunkResult.mConsumed;// we keep a separate remaining space because mAvailable is important for recyclingremainingSpace -= layoutChunkResult.mConsumed;}if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {//在当前滚动偏移量基础上追加因新表项插入增加的像素（这句话对于“回收哪些表项”来说很关键，待会会提到）layoutState.mScrollingOffset += layoutChunkResult.mConsumed;if (layoutState.mAvailable < 0) {layoutState.mScrollingOffset += layoutState.mAvailable;}//回收表项recycleByLayoutState(recycler, layoutState);}if (stopOnFocusable && layoutChunkResult.mFocusable) {break;}...}...return start - layoutState.mAvailable;}
}
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在不断获取新表项用于填充的同时也在回收表项（想想也是，列表滚动的时候有表项插入的同时也有表项被移出），移步到回收表项的函数：

public class LinearLayoutManager extends RecyclerView.LayoutManager implements ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider {...private void recycleByLayoutState(RecyclerView.Recycler recycler, LayoutState layoutState) {if (!layoutState.mRecycle || layoutState.mInfinite) {return;}if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {recycleViewsFromEnd(recycler, layoutState.mScrollingOffset);} else {recycleViewsFromStart(recycler, layoutState.mScrollingOffset);}}.../*** Recycles views that went out of bounds after scrolling towards the end of the layout.* 当向列表尾部滚动时回收滚出屏幕的表项* <p>* Checks both layout position and visible position to guarantee that the view is not visible.** @param recycler Recycler instance of {@link android.support.v7.widget.RecyclerView}* @param dt       This can be used to add additional padding to the visible area. This is used*                 to detect children that will go out of bounds after scrolling, without*                 actually moving them.（该参数被用于检测滚出屏幕的表项）*/private void recycleViewsFromStart(RecyclerView.Recycler recycler, int dt) {if (dt < 0) {if (DEBUG) {Log.d(TAG, "Called recycle from start with a negative value. This might happen"+ " during layout changes but may be sign of a bug");}return;}// ignore padding, ViewGroup may not clip children.final int limit = dt;final int childCount = getChildCount();if (mShouldReverseLayout) {for (int i = childCount - 1; i >= 0; i--) {View child = getChildAt(i);if (mOrientationHelper.getDecoratedEnd(child) > limit|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {// stop hererecycleChildren(recycler, childCount - 1, i);return;}}} else {//遍历LinearLayoutManager的孩子找出其中应该被回收的for (int i = 0; i < childCount; i++) {View child = getChildAt(i);if (mOrientationHelper.getDecoratedEnd(child) > limit|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {// stop here//回收索引为0到i-1的表项recycleChildren(recycler, 0, i);return;}}}}...
}
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原来RecyclerView的回收分两个方向：1. 从列表头回收 2.从列表尾回收。就以“从列表头回收”为研究对象分析下RecyclerView在滚动时到底是怎么判断“哪些表项应该被回收？”。（“从列表头回收表项”所对应的场景是：手指上滑，列表向下滚动，新的表项逐个插入到列表尾部，列表头部的表项逐个被回收。）

回收哪些表项

要回答这个问题，刚才那段代码中套在recycleChildren(recycler, 0, i)外面的判断逻辑是关键：mOrientationHelper.getDecoratedEnd(child) > limit。

/*** Helper class for LayoutManagers to abstract measurements depending on the View's orientation.* 该类用于帮助LayoutManger抽象出基于视图方向的测量* <p>* It is developed to easily support vertical and horizontal orientations in a LayoutManager but* can also be used to abstract calls around view bounds and child measurements with margins and* decorations.** @see #createHorizontalHelper(RecyclerView.LayoutManager)* @see #createVerticalHelper(RecyclerView.LayoutManager)*/
public abstract class OrientationHelper {.../*** Returns the end of the view including its decoration and margin.* <p>* For example, for the horizontal helper, if a View's right is at pixel 200, has 2px right* decoration and 3px right margin, returned value will be 205.** @param view The view element to check* @return The last pixel of the element* @see #getDecoratedStart(android.view.View)*/public abstract int getDecoratedEnd(View view);...public static OrientationHelper createVerticalHelper(RecyclerView.LayoutManager layoutManager) {return new OrientationHelper(layoutManager) {...@Overridepublic int getDecoratedEnd(View view) {final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)view.getLayoutParams();return mLayoutManager.getDecoratedBottom(view) + params.bottomMargin;}...
}
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结合注释和该方法的实现，原来mOrientationHelper.getDecoratedEnd(child)表示当前表项的尾部相对于列表头部的坐标，OrientationHelper这层抽象屏蔽了列表的方向，所以这句话在纵向列表中可以翻译成“当前表项的底部相对于列表顶部的纵坐标”。

判断条件mOrientationHelper.getDecoratedEnd(child) > limit中的limit又是什么鬼？在纵向列表中，“表项底部纵坐标 > 某个值”意味着表项位于某条线的下方，回看一眼“回收表项”的逻辑：

//遍历LinearLayoutManager的孩子找出其中应该被回收的
for (int i = 0; i < childCount; i++) {View child = getChildAt(i);//直到表项底部纵坐标大于某个值后，回收该表项以上的所有表项if (mOrientationHelper.getDecoratedEnd(child) > limit|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {// stop here//回收索引为0到索引为i-1的表项recycleChildren(recycler, 0, i);return;}
}
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隐约觉得limit应该等于0，这样不正好是回收所有从列表头移出的表项吗？ 不知道这样YY到底对不对，还是沿着调用链向上找一下limit被赋值的地方吧~，调用链很长，就不全部罗列了，但其中有两个关键点，其实我在上面的代码中埋了伏笔，现在再罗列一下：

public class LinearLayoutManager extends RecyclerView.LayoutManager implements ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider {   ...int scrollBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {if (getChildCount() == 0 || dy == 0) {return 0;}mLayoutState.mRecycle = true;ensureLayoutState();final int layoutDirection = dy > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;final int absDy = Math.abs(dy);//1. 更新LayoutState（这个函数对于“回收哪些表项”来说很关键，待会会提到）updateLayoutState(layoutDirection, absDy, true, state);//滚动时向列表中填充新的表项final int consumed = mLayoutState.mScrollingOffset+ fill(recycler, mLayoutState, state, false);...}...int fill(RecyclerView.Recycler recycler, LayoutState layoutState,RecyclerView.State state, boolean stopOnFocusable) {...//不断循环获取新的表项用于填充，直到没有填充空间while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {...if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {//2. 在当前滚动偏移量基础上追加因新表项插入增加的像素（这句话对于“回收哪些表项”来说很关键，待会会提到）layoutState.mScrollingOffset += layoutChunkResult.mConsumed;if (layoutState.mAvailable < 0) {layoutState.mScrollingOffset += layoutState.mAvailable;}//回收表项recycleByLayoutState(recycler, layoutState);}...}...return start - layoutState.mAvailable;}...private void updateLayoutState(int layoutDirection, int requiredSpace,boolean canUseExistingSpace, RecyclerView.State state) {...int scrollingOffset;if (layoutDirection == LayoutState.LAYOUT_END) {mLayoutState.mExtra += mOrientationHelper.getEndPadding();//获得当前方向上里列表尾部最近的孩子（最后一个孩子）final View child = getChildClosestToEnd();// the direction in which we are traversing childrenmLayoutState.mItemDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_HEAD: LayoutState.ITEM_DIRECTION_TAIL;mLayoutState.mCurrentPosition = getPosition(child) + mLayoutState.mItemDirection;mLayoutState.mOffset = mOrientationHelper.getDecoratedEnd(child);// calculate how much we can scroll without adding new children (independent of layout)// 获得一个滚动偏移量，如果只滚动了这个数值那不需要添加新的孩子scrollingOffset = mOrientationHelper.getDecoratedEnd(child)- mOrientationHelper.getEndAfterPadding();} else {...}...//对mLayoutState.mScrollingOffset赋值mLayoutState.mScrollingOffset = scrollingOffset;}
}
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一图胜千语：

关于limit等于0的YY破灭了，其实limit是一根横谓语列表中间的横线，它的值表示这一次滚动的总距离。（图中是一种理想情况，即当滚动结束后新插入表项的底部正好和列表底部重叠）其实回收表项的时机是在滚动真正发生之前，此时我们预先计算出滚动的偏移量，根据偏移量筛选出滚动发生后应该被删除的表项。即 limit这根线也可以表述为：当滚动发生后，列表当前 limit这个位置会成为列表的头部

分析完“回收哪些表项”后，一不小心发现篇幅有点长了，那关于回收去哪里？将放到下一篇在讲。