Android窗口管理框架：Android列表控件RecyclerView
A flexible view for providing a limited window into a large data set.
RecyclerView继承于ViewGroup，实现了ScrollingView与NestedScrollingChild接口，它是日常业务开发中使用频度非常高的一个组件，它被Android设计出来代替原来的ListView组件。RecyclerView的的 解耦与设计是十分精妙的，应用了适配器、观察者等多种模式。
提到RecyclerView，就不得不说一下早期使用的AdapterView（ListView），这里做下它们的简单对比：
- Item复用方面：RecyclerView内置了RecyclerViewPool、多级缓存、ViewHolder，而AdapterView需要手动添加ViewHolder且复用功能也没RecyclerView完善。
- 样式丰富方面：RecyclerView通过支持水平、垂直和表格列表及其他更复杂形式，而AdapterView只支持具体某一种。
- 效果增强方面：RecyclerView内置了ItemDecoration和ItemAnimator，可以自定义绘制itemView之间的一些特殊UI或item项数据变化时的动画效果，而用AdapterView实现时采取的做法是 将这些特殊UI作为itemView的一部分，设置可见不可见决定是否展现，且数据变化时的动画效果没有提供，实现较为繁琐。
- 代码内聚方面：RecyclerView将功能密切相关的类写成内部类（例如：ViewHolder，Adapter），而AdapterView则没有。
我们再来看看RecyclerView的渲染流程，一般说来，列表展示涉及两个角色：列表的数据DataSet以及最终显示在界面上的RecyclerView，这两者的转换流程如下所示：
RecyclerView渲染流程图
Adapter将数据DataSet翻译成RecyclerView可以理解的ViewHolder，Recycler负责对这些ViewHolder进行管理，LayoutManager从Recycler获取这些ViewHolder，然后在RecyclerView里对它们进行布局，在布局 的过程中还可以通过ItemDecoration、ItemAnimator为这些ViewHolder添加分隔条、转场动画等东西，让整个RecyclerView更加具有交互性。
接下来我们分别来看看每个角色的源码实现。
一 RecyclerView创建流程
首先我们还是从一个简单的例子引入，如何实现一个简单的RecyclerView列表。
RecyclerView recyclerView = findViewById(R.id.recycler_view);
RecyclerAdapter adapter = new RecyclerAdapter();
List<RecyclerModel> data = new ArrayList<>();
for(int i = 0; i < 30; i++){
data.add(new RecyclerModel());
}
adapter.setData(data);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setAdapter(adapter);我们可以从这段简短的代码提炼出三个关键点：
- RecyclerView()
- setLayoutManager()
- setAdapter()
我们首先来分析它的构造函数。
1.1 RecyclerView()
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild {
public RecyclerView(Context context, @Nullable AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
...
//设置当前View可以滑动
setScrollContainer(true);
//设置当前View可以获取焦点
setFocusableInTouchMode(true);
//从ViewConfiguration获取和View相关的信息，例如最小滑动距离TouchSlop等
final ViewConfiguration vc = ViewConfiguration.get(context);
mTouchSlop = vc.getScaledTouchSlop();
mMinFlingVelocity = vc.getScaledMinimumFlingVelocity();
mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity();
setWillNotDraw(getOverScrollMode() == View.OVER_SCROLL_NEVER);
//设置动画结束时的监听器，这里是实现了ItemAnimator.ItemAnimatorListener接口
//的ItemAnimatorRestoreListener
mItemAnimator.setListener(mItemAnimatorListener);
//初始化AdapterHelper，它是一个工具类，用来对Adapter里的操作进行排队和处理
initAdapterManager();
//初始化ChildHelper，它是一个工具类，用来向RecyclerView移除和添加子View
initChildrenHelper();
// If not explicitly specified this view is important for accessibility.
if (ViewCompat.getImportantForAccessibility(this)
== ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_AUTO) {
ViewCompat.setImportantForAccessibility(this,
ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_YES);
}
mAccessibilityManager = (AccessibilityManager) getContext()
.getSystemService(Context.ACCESSIBILITY_SERVICE);
setAccessibilityDelegateCompat(new RecyclerViewAccessibilityDelegate(this));
// Create the layoutManager if specified.
boolean nestedScrollingEnabled = true;
//从xml文件里读取参数信息
if (attrs != null) {
...
} else {
setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
}
// Re-set whether nested scrolling is enabled so that it is set on all API levels
setNestedScrollingEnabled(nestedScrollingEnabled);
}
}RecyclerView的构造方法很简单，它主要初始化了两个重要的工具类：
- AdapterHelper：用来对Adapter里的操作进行排队和处理
- ChildHelper：用来向RecyclerView移除和添加子View
我们接着来看看setLayoutManager()方法。
1.2 setLayoutManager()
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild {
public void setLayoutManager(LayoutManager layout) {
if (layout == mLayout) {
return;
}
//停止滑动
stopScroll();
// TODO We should do this switch a dispatchLayout pass and animate children. There is a good
// chance that LayoutManagers will re-use views.
//处理一些清空的操作，准备好重新进行布局
if (mLayout != null) {
//停止所有动画
if (mItemAnimator != null) {
mItemAnimator.endAnimations();
}
//移除当前LayoutManager里的View，并稍后在mRecycler里复用
mLayout.removeAndRecycleAllViews(mRecycler);
//移除Scrap View，Scrap View指的是那些暂时处理分离状态的View
//后面可能还会重新使用
mLayout.removeAndRecycleScrapInt(mRecycler);
//清空mRecycler
mRecycler.clear();
if (mIsAttached) {
mLayout.dispatchDetachedFromWindow(this, mRecycler);
}
mLayout.setRecyclerView(null);
mLayout = null;
} else {
mRecycler.clear();
}
//移除所有子View（包括处于hidden状态的View）
mChildHelper.removeAllViewsUnfiltered();
mLayout = layout;
if (layout != null) {
if (layout.mRecyclerView != null) {
throw new IllegalArgumentException("LayoutManager " + layout +
" is already attached to a RecyclerView: " + layout.mRecyclerView);
}
//从新给mLayout设置当前RecyclerView的引用
mLayout.setRecyclerView(this);
if (mIsAttached) {
//处理attach到Window
mLayout.dispatchAttachedToWindow(this);
}
}
//更新View的缓存数目
mRecycler.updateViewCacheSize();
//重新布局
requestLayout();
}
}前面我们已经提到LayoutManager只负责对View进行布局，而承担管理View责任的是Recycler，Recycler实现了View复用的功能，让列表变得流程。再回到这个方法，它主要做了两件事：
- 处理一些清空的操作，准备好重新进行布局。
- 重新赋值mLayout，并给mLayout设置当前RecyclerView的引用，然后请求进行重新布局。
我们接着来看看setAdapter()方法。
1.2 setAdapter()
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild {
public void setAdapter(Adapter adapter) {
//停止当前的布局和滑动
setLayoutFrozen(false);
//设置Adapter，这里有两个boolean，第一个表示是否使用当前相同的ViewHolder
//第二个表示是否移除和复用当前所有的View，当第一个boolean为false时，第二个
//boolean会被忽略
setAdapterInternal(adapter, false, true);
//请求重新进行布局
requestLayout();
}
private void setAdapterInternal(Adapter adapter, boolean compatibleWithPrevious,
boolean removeAndRecycleViews) {
//如果mAdapter不空，先释放掉原来数据和View
if (mAdapter != null) {
mAdapter.unregisterAdapterDataObserver(mObserver);
mAdapter.onDetachedFromRecyclerView(this);
}
//停止动画以及做一些清空操作，准备重新设置Adapter
if (!compatibleWithPrevious || removeAndRecycleViews) {
//停止所有动画
if (mItemAnimator != null) {
mItemAnimator.endAnimations();
}
if (mLayout != null) {
mLayout.removeAndRecycleAllViews(mRecycler);
mLayout.removeAndRecycleScrapInt(mRecycler);
}
mRecycler.clear();
}
mAdapterHelper.reset();
final Adapter oldAdapter = mAdapter;
//重新设置新的Adapter
mAdapter = adapter;
//重新注册数据监测DataObserver以及attach到RecyclerView
if (adapter != null) {
adapter.registerAdapterDataObserver(mObserver);
adapter.onAttachedToRecyclerView(this);
}
if (mLayout != null) {
mLayout.onAdapterChanged(oldAdapter, mAdapter);
}
//触发Recycler的onAdapterChanged()方法。
mRecycler.onAdapterChanged(oldAdapter, mAdapter, compatibleWithPrevious);
mState.mStructureChanged = true;
//将所有视图标记为无效，因为要开始进行数据刷新和重绘
markKnownViewsInvalid();
}
}这个方法的逻辑和setLayoutManager()有点相似，我们知道当我们设置了Adapter，列表就可以显示出来了。它也做了两件事情：
- 对以前的Adapter（如果有的话）做一些清空操作，停止滑动和动画，准备好重新设置Adapter。
- 重新设置Adapter，重新注册AdapterDataObserver以及关联到RecyclerView，并触发Recycler的onAdapterChanged()方法，通知 Adapter已经发生改变，最终触发重新布局的操作。
以上就是这三个方法的全部内容，通过对三个方法的源码分析，我们抓住了一些关键类类以及他们的关键函数，具体说来：
LayoutManager
- removeAndRecycleAllViews(mRecycler); 移除当前LayoutManager里的View，并稍后在mRecycler里复用
- removeAndRecycleScrapInt(mRecycler); 移除Scrap View，Scrap View指的是那些暂时处理分离状态的View后面可能还会重新使用
Adapter
- unregisterAdapterDataObserver(mObserver); 取消注册RecyclerViewDataObserver
- onDetachedFromRecyclerView(this); 取消关联到RecyclerView
- registerAdapterDataObserver(mObserver); 重新注册RecyclerViewDataObserver
- onAttachedToRecyclerView(this); 重新关联到RecyclerView
Recycler
- onAdapterChanged(oldAdapter, mAdapter, compatibleWithPrevious); 通知Adapter已经发生改变
下面我们就带着对这些方法的疑问逐个去分类每个关键类的作用。
二 RecyclerView布局策略管理器LayoutManager
LayoutManager是一个抽象类。它主要用来测量和布局子View，滚动子View以及决定何时回收用户不可见的子View。
它有三个子类：
- LinearLayoutManager：线性布局，分为水平方向和竖直方向两种。
- GridLayoutManager：网络布局，分为水平方向和竖直方向两种。
- StaggeredGridLayoutManager：瀑布流布局，分为水平方向和竖直方向两种。
上面提到LayoutManager主要用来测量和布局子View，滚动子View以及决定何时回收用户不可见的子View。那么处理这些内容的关键方法就是我们 重点关注的对象，具体说来：
- onLayoutChildren(Recycler recycler, State state)：对子View进行布局。
- scrollHorizontallyBy(int dx, Recycler recycler, State state) :处理水平方向的滑动。
- scrollVerticallyBy(int dy, Recycler recycler, State state)：处理竖直方向的滑动。
关于滑动需要说明一下，RecyclerView已经处理了和触摸相关的事件，当RecyclerView上下滑动的时候，滑动的偏移量会传入这两个方法，就是上面的dx/dy，这个时候 LayoutManager的子类完成了以下三件事情：
- 将子View移动适当的位置
- 处理子View移动后的添加/删除逻辑
- 返回移动后的实际距离，RecyclerView会根据这个距离判断是否触屏到边界。
好，理解了哪些是关键方法，我们就来分析这三个字类对这些方法的实现。
2.1 LinearLayoutManager
onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state)
public class LinearLayoutManager extends RecyclerView.LayoutManager implements
ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider {
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
if (DEBUG) {
Log.d(TAG, "is pre layout:" + state.isPreLayout());
}
if (mPendingSavedState != null || mPendingScrollPosition != NO_POSITION) {
if (state.getItemCount() == 0) {
removeAndRecycleAllViews(recycler);
return;
}
}
if (mPendingSavedState != null && mPendingSavedState.hasValidAnchor()) {
mPendingScrollPosition = mPendingSavedState.mAnchorPosition;
}
ensureLayoutState();
mLayoutState.mRecycle = false;
// resolve layout direction
resolveShouldLayoutReverse();
final View focused = getFocusedChild();
if (!mAnchorInfo.mValid || mPendingScrollPosition != NO_POSITION
|| mPendingSavedState != null) {
mAnchorInfo.reset();
mAnchorInfo.mLayoutFromEnd = mShouldReverseLayout ^ mStackFromEnd;
// calculate anchor position and coordinate
updateAnchorInfoForLayout(recycler, state, mAnchorInfo);
mAnchorInfo.mValid = true;
} else if (focused != null && (mOrientationHelper.getDecoratedStart(focused)
>= mOrientationHelper.getEndAfterPadding()
|| mOrientationHelper.getDecoratedEnd(focused)
<= mOrientationHelper.getStartAfterPadding())) {
// This case relates to when the anchor child is the focused view and due to layout
// shrinking the focused view fell outside the viewport, e.g. when soft keyboard shows
// up after tapping an EditText which shrinks RV causing the focused view (The tapped
// EditText which is the anchor child) to get kicked out of the screen. Will update the
// anchor coordinate in order to make sure that the focused view is laid out. Otherwise,
// the available space in layoutState will be calculated as negative preventing the
// focused view from being laid out in fill.
// Note that we won't update the anchor position between layout passes (refer to
// TestResizingRelayoutWithAutoMeasure), which happens if we were to call
// updateAnchorInfoForLayout for an anchor that's not the focused view (e.g. a reference
// child which can change between layout passes).
mAnchorInfo.assignFromViewAndKeepVisibleRect(focused);
}
if (DEBUG) {
Log.d(TAG, "Anchor info:" + mAnchorInfo);
}
// LLM may decide to layout items for "extra" pixels to account for scrolling target,
// caching or predictive animations.
int extraForStart;
int extraForEnd;
final int extra = getExtraLayoutSpace(state);
// If the previous scroll delta was less than zero, the extra space should be laid out
// at the start. Otherwise, it should be at the end.
if (mLayoutState.mLastScrollDelta >= 0) {
extraForEnd = extra;
extraForStart = 0;
} else {
extraForStart = extra;
extraForEnd = 0;
}
extraForStart += mOrientationHelper.getStartAfterPadding();
extraForEnd += mOrientationHelper.getEndPadding();
if (state.isPreLayout() && mPendingScrollPosition != NO_POSITION
&& mPendingScrollPositionOffset != INVALID_OFFSET) {
// if the child is visible and we are going to move it around, we should layout
// extra items in the opposite direction to make sure new items animate nicely
// instead of just fading in
final View existing = findViewByPosition(mPendingScrollPosition);
if (existing != null) {
final int current;
final int upcomingOffset;
if (mShouldReverseLayout) {
current = mOrientationHelper.getEndAfterPadding()
- mOrientationHelper.getDecoratedEnd(existing);
upcomingOffset = current - mPendingScrollPositionOffset;
} else {
current = mOrientationHelper.getDecoratedStart(existing)
- mOrientationHelper.getStartAfterPadding();
upcomingOffset = mPendingScrollPositionOffset - current;
}
if (upcomingOffset > 0) {
extraForStart += upcomingOffset;
} else {
extraForEnd -= upcomingOffset;
}
}
}
int startOffset;
int endOffset;
final int firstLayoutDirection;
if (mAnchorInfo.mLayoutFromEnd) {
firstLayoutDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_TAIL
: LayoutState.ITEM_DIRECTION_HEAD;
} else {
firstLayoutDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_HEAD
: LayoutState.ITEM_DIRECTION_TAIL;
}
onAnchorReady(recycler, state, mAnchorInfo, firstLayoutDirection);
detachAndScrapAttachedViews(recycler);
mLayoutState.mInfinite = resolveIsInfinite();
mLayoutState.mIsPreLayout = state.isPreLayout();
if (mAnchorInfo.mLayoutFromEnd) {
// fill towards start
updateLayoutStateToFillStart(mAnchorInfo);
mLayoutState.mExtra = extraForStart;
fill(recycler, mLayoutState, state, false);
startOffset = mLayoutState.mOffset;
final int firstElement = mLayoutState.mCurrentPosition;
if (mLayoutState.mAvailable > 0) {
extraForEnd += mLayoutState.mAvailable;
}
// fill towards end
updateLayoutStateToFillEnd(mAnchorInfo);
mLayoutState.mExtra = extraForEnd;
mLayoutState.mCurrentPosition += mLayoutState.mItemDirection;
fill(recycler, mLayoutState, state, false);
endOffset = mLayoutState.mOffset;
if (mLayoutState.mAvailable > 0) {
// end could not consume all. add more items towards start
extraForStart = mLayoutState.mAvailable;
updateLayoutStateToFillStart(firstElement, startOffset);
mLayoutState.mExtra = extraForStart;
fill(recycler, mLayoutState, state, false);
startOffset = mLayoutState.mOffset;
}
} else {
// fill towards end
updateLayoutStateToFillEnd(mAnchorInfo);
mLayoutState.mExtra = extraForEnd;
fill(recycler, mLayoutState, state, false);
endOffset = mLayoutState.mOffset;
final int lastElement = mLayoutState.mCurrentPosition;
if (mLayoutState.mAvailable > 0) {
extraForStart += mLayoutState.mAvailable;
}
// fill towards start
updateLayoutStateToFillStart(mAnchorInfo);
mLayoutState.mExtra = extraForStart;
mLayoutState.mCurrentPosition += mLayoutState.mItemDirection;
fill(recycler, mLayoutState, state, false);
startOffset = mLayoutState.mOffset;
if (mLayoutState.mAvailable > 0) {
extraForEnd = mLayoutState.mAvailable;
// start could not consume all it should. add more items towards end
updateLayoutStateToFillEnd(lastElement, endOffset);
mLayoutState.mExtra = extraForEnd;
fill(recycler, mLayoutState, state, false);
endOffset = mLayoutState.mOffset;
}
}
// changes may cause gaps on the UI, try to fix them.
// TODO we can probably avoid this if neither stackFromEnd/reverseLayout/RTL values have
// changed
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;
}
}
layoutForPredictiveAnimations(recycler, state, startOffset, endOffset);
if (!state.isPreLayout()) {
mOrientationHelper.onLayoutComplete();
} else {
mAnchorInfo.reset();
}
mLastStackFromEnd = mStackFromEnd;
if (DEBUG) {
validateChildOrder();
}
}
} // layout algorithm:
// 1) by checking children and other variables, find an anchor coordinate and an anchor
// item position.
// 2) fill towards start, stacking from bottom
// 3) fill towards end, stacking from top
// 4) scroll to fulfill requirements like stack from bottom.
// create layout state- 从子View中查找出一个锚点坐标和锚点位置。
- 从底部开始堆积，直到填充到首端。
- 从顶部开始堆积，知道填充到尾部。
scrollHorizontallyBy()/scrollVerticallyBy()
RecyclerView在滚动的时候主要涉及两个问题：
- 旧View的回收与新View的添加
- 其他View重新布局，偏移到合适的位置。
整个流程的序列图如下所示：
注意观察图中标记红色和绿色的方法，它分别完成了上面说的两件事。
- recycleByLayoutState()：回收不再显示的View。
- layoutChunk()：其他View重新布局，偏移到合适的位置。。
这两个方法都在fill()方法里被调用，它是LinearLayoutManager最为关键的方法，如下所示：
public class LinearLayoutManager extends RecyclerView.LayoutManager implements
ItemTouchHelper.ViewDropHandler, RecyclerView.SmoothScroller.ScrollVectorProvider {
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
RecyclerView.State state, boolean stopOnFocusable) {
// max offset we should set is mFastScroll + available
final int start = layoutState.mAvailable;
if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
// TODO ugly bug fix. should not happen
if (layoutState.mAvailable < 0) {
layoutState.mScrollingOffset += layoutState.mAvailable;
}
//回收不再显示的子View
recycleByLayoutState(recycler, layoutState);
}
int remainingSpace = layoutState.mAvailable + layoutState.mExtra;
LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
layoutChunkResult.resetInternal();
if (VERBOSE_TRACING) {
TraceCompat.beginSection("LLM LayoutChunk");
}
layoutChunk(recycler, state, layoutState, layoutChunkResult);
if (VERBOSE_TRACING) {
TraceCompat.endSection();
}
if (layoutChunkResult.mFinished) {
break;
}
layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
/**
* Consume the available space if:
* * layoutChunk did not request to be ignored
* * OR we are laying out scrap children
* * OR we are not doing pre-layout
*/
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) {
layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
if (layoutState.mAvailable < 0) {
layoutState.mScrollingOffset += layoutState.mAvailable;
}
recycleByLayoutState(recycler, layoutState);
}
if (stopOnFocusable && layoutChunkResult.mFocusable) {
break;
}
}
if (DEBUG) {
validateChildOrder();
}
return start - layoutState.mAvailable;
}
}这里面有个LayoutState类，它保存了与布局相关的状态，它里面相关变量的含义如下所示：
- boolean mRecycle = true; 子View是否可以被回收
- nt mOffset; 布局从何处开始
- int mAvailable; 滚动完成后，空出多少空间需要被填充
- int mCurrentPosition; Adapter上当前的位置
- int mItemDirection; Adapter遍历数据的方向，有ITEM_DIRECTION_HEAD与ITEM_DIRECTION_TAIL两种。
- int mLayoutDirection; 布局填充的方向，有LAYOUT_START与LAYOUT_END两种。
- int mScrollingOffset; 添加一个新View后，还可以滑动多少空间。
- boolean mInfinite; 无限滚动，没有新View添加限制。
2.2 GridLayoutManager
2.3 StaggeredGridLayoutManager
三 RecyclerView视图描述者ViewHolder
A ViewHolder describes an item view and metadata about its place within the RecyclerView.
ViewHolder正如它的名字那样，它是一个Item View 视图容器。
四 RecyclerView视图复用器Recycler
A Recycler is responsible for managing scrapped or detached item views for reuse.
Recycler实现了Item View的回收与复用，也就是说它是用来做缓存的，我们首先来关注下Recycler里面几个缓存列表：
Scrap View指的是那些已经滚动出可视范围，但还没有detach掉的Item View。
- final ArrayList
mAttachedScrap = new ArrayList<>(); 存放已经滚动出可视范围，但还没有detach掉的ViewHolder。 - ArrayList
mChangedScrap = null; 存放那些被标记为Scrap View的ViewHolder - final ArrayList
mCachedViews = new ArrayList (); 存放缓存的ViewHolder - private final List
mUnmodifiableAttachedScrap = Collections.unmodifiableList(mAttachedScrap); 一个不可变列表，用来存放mAttachedScrap。
此外，Recycler里还有个缓存池的概念，也就是RecycledViewPool，它可以让我们在多个RecyclerView之间复用View，大大提高了渲染效率。使用的方式也是创建一个RecycledViewPool实例，然后 调用RecyclerView.setRecyclerPool()方法来进行设置即可，当然RecyclerView默认会创建一个RecycledViewPool。
五 RecyclerView视图适配器Adapter
六 RecyclerView视图动画ItemAnimator
七 RecyclerView视图分隔条ItemDecoration
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