这是RecyclerView缓存机制系列文章的第二篇，系列文章的目录如下。
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 {
@Override
public boolean onTouchEvent(MotionEvent e) {
...
case MotionEvent.ACTION_MOVE: {
...
if (scrollByInternal(
canScrollHorizontally ? dx : 0,
canScrollVertically ? dy : 0,
vtev)) {
getParent().requestDisallowInterceptTouchEvent(true);
}
...
}
} break;
...
}
}
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;
}
...
}
...
}
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 {
...
@Override
public 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;
}
...
}
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 recycling
remainingSpace -= 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;
}
}
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 here
recycleChildren(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;
}
}
}
}
...
}
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) {
...
@Override
public int getDecoratedEnd(View view) {
final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
view.getLayoutParams();
return mLayoutManager.getDecoratedBottom(view) + params.bottomMargin;
}
...
}
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;
}
}
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 children
mLayoutState.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;
}
}
一图胜千语：
屏幕快照 2019-02-16 下午7.23.51.png
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“怎么回收”将放到下一篇在讲。
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