RecycleView 源码解析

ListView 能局部刷新吗？
可以。。。

和ListView 的区别？强在哪里？
更加灵活

怎么布局的？怎么就实现了可以列表和网格的切换？

是怎样的回收和重复利用View?

不管recycleView 是何方神圣，最终都会调用到onMeasure、onLayout

androidx.recyclerview.widget.LinearLayoutManager#fill

RecycleView 缓存机制

1.从mAttachedScrap 里面获取
mAttachedScrap 好像没有被使用
真的没有看到这个的作用在哪里。好像没有用。

2.mCachedViews 获取
用于存放最常用的3个，包括将要显示的那个，预加载的，以及刚回收的
GapWorker 预创建View 完成之后就会放到cached 里面

"main"@12,125 in group "main": RUNNING
recycleViewHolderInternal(RecyclerView$ViewHolder):6487, RecyclerView$Recycler {androidx.recyclerview.widget}, RecyclerView.java
recycleView(View):6369, RecyclerView$Recycler {androidx.recyclerview.widget}, RecyclerView.java
prefetchPositionWithDeadline(RecyclerView, int, long):295, GapWorker {androidx.recyclerview.widget}, GapWorker.java
flushTaskWithDeadline(GapWorker$Task, long):345, GapWorker {androidx.recyclerview.widget}, GapWorker.java
flushTasksWithDeadline(long):361, GapWorker {androidx.recyclerview.widget}, GapWorker.java
prefetch(long):368, GapWorker {androidx.recyclerview.widget}, GapWorker.java
run():399, GapWorker {androidx.recyclerview.widget}, GapWorker.java
handleCallback(Message):873, Handler {android.os}, Handler.java
dispatchMessage(Message):99, Handler {android.os}, Handler.java
loop():193, Looper {android.os}, Looper.java

3.mRecyclerPool 获取
用于存在每一种类型的，5个。和CacheView 作用不太一样。

4.ViewCacheExtension获取

为什么做这么多层级的缓存？意义在哪里？
难道是这一层级的，不需要绑定数据？可以直接用？

5.GapWorker 到底是干什么用的？怎么干的？

6.绑定ViewHolder之后什么时候添加到View树上的？

androidx.recyclerview.widget.LinearLayoutManager#layoutChunk

里面会调用addView

7.怎么实现的？滑动就增加view? 怎么实现这么流畅的?
onMeasure,onLayout 不会调用吗？（不会）
为什么?

父容器RecycleView 是不会重新调用onMeasure，onLayout的，但是会调用item 的 onMeasure 和 on Layout.

androidx.recyclerview.widget.LinearLayoutManager#layoutChunk
但是不调用invalidate 会刷新吗？
他不会去刷新layout。

8.requestLayout 和 invalidate 什么区别？分别什么时候调用比较好？
invalidate 会导致重新布局吗？
requestLayout 会导致 onDraw 调用吗？
//invalidate 不会重新onLayout onMeasure
// invalidate();
// 重新布局 onMeasure onLayout onDraw 都会重新调用
requestLayout();
9.view 怎么移动的？肯定不是scroll, 应该是调用layout方法。

offsetTopAndBottom(int):15964, View {android.view}, View.java
offsetChildrenVertical(int):5042, RecyclerView {androidx.recyclerview.widget}, RecyclerView.java
offsetChildrenVertical(int):9136, RecyclerView$LayoutManager {androidx.recyclerview.widget}, RecyclerView.java
offsetChildren(int):369, OrientationHelper$2 {androidx.recyclerview.widget}, OrientationHelper.java
scrollBy(int, RecyclerView$Recycler, RecyclerView$State):1399, LinearLayoutManager {androidx.recyclerview.widget}, LinearLayoutManager.java
scrollVerticallyBy(int, RecyclerView$Recycler, RecyclerView$State):1128, LinearLayoutManager {androidx.recyclerview.widget}, LinearLayoutManager.java
scrollStep(int, int, int[]):1841, RecyclerView {androidx.recyclerview.widget}, RecyclerView.java
scrollByInternal(int, int, MotionEvent):1940, RecyclerView {androidx.recyclerview.widget}, RecyclerView.java

调用的offsetTopAndBottom 实现移动。调用requestLayout 其实会更改mTop 的，但是recycleView 没有问题是因为重写了onMeasure 方法，不会重新给mTop赋值。

10.offsetTopAndBottom 有什么优点？有什么作用？
快速移动？不需要重新布局？只移动自己，不需要麻烦parent.

11.ainamator是怎么实现的？

12.具体的recycleView 的机制是什么？会提前加载上下一个列表项目吗？

13.滑动的时候，是先回收view还是先创建view 添加到底部？
首先，回收view, 然后添加下面的view，最后再回收view.

androidx.recyclerview.widget.RecyclerView#scrollBy

fill(RecyclerView$Recycler, LinearLayoutManager$LayoutState, RecyclerView$State, boolean):1576, LinearLayoutManager {androidx.recyclerview.widget}, LinearLayoutManager.java
scrollBy(int, RecyclerView$Recycler, RecyclerView$State):1391, LinearLayoutManager {androidx.recyclerview.widget}, LinearLayoutManager.java
scrollVerticallyBy(int, RecyclerView$Recycler, RecyclerView$State):1128, LinearLayoutManager {androidx.recyclerview.widget}, LinearLayoutManager.java
scrollStep(int, int, int[]):1841, RecyclerView {androidx.recyclerview.widget}, RecyclerView.java
scrollByInternal(int, int, MotionEvent):1940, RecyclerView {androidx.recyclerview.widget}, RecyclerView.java
onTouchEvent(MotionEvent):3391, RecyclerView {androidx.recyclerview.widget}, RecyclerView.java

    static class LayoutState {static final String TAG = "LLM#LayoutState";static final int LAYOUT_START = -1;static final int LAYOUT_END = 1;static final int INVALID_LAYOUT = Integer.MIN_VALUE;static final int ITEM_DIRECTION_HEAD = -1;static final int ITEM_DIRECTION_TAIL = 1;static final int SCROLLING_OFFSET_NaN = Integer.MIN_VALUE;/*** We may not want to recycle children in some cases (e.g. layout)*/boolean mRecycle = true;/*** Pixel offset where layout should start* 应该接着哪里布局，接着那里添加view*/int mOffset;/*** Number of pixels that we should fill, in the layout direction.* 当前需要填充的长度* 可以为负数 如果是负数 说明滑动的距离 不足够划出下一个展示的View*/int mAvailable;/*** Current position on the adapter to get the next item.*/int mCurrentPosition;/*** Defines the direction in which the data adapter is traversed.* Should be {@link #ITEM_DIRECTION_HEAD} or {@link #ITEM_DIRECTION_TAIL}*/int mItemDirection;/*** Defines the direction in which the layout is filled.* Should be {@link #LAYOUT_START} or {@link #LAYOUT_END}*/int mLayoutDirection;/*** Used when LayoutState is constructed in a scrolling state.* It should be set the amount of scrolling we can make without creating a new view.* Settings this is required for efficient view recycling.* 滚动多少范围内，不需要创建新的view*/int mScrollingOffset;/*** Used if you want to pre-layout items that are not yet visible.* The difference with {@link #mAvailable} is that, when recycling, distance laid out for* {@link #mExtraFillSpace} is not considered to avoid recycling visible children.*/int mExtraFillSpace = 0;/*** Contains the {@link #calculateExtraLayoutSpace(RecyclerView.State, int[])}  extra layout* space} that should be excluded for recycling when cleaning up the tail of the list during* a smooth scroll.*/int mNoRecycleSpace = 0;/*** Equal to {@link RecyclerView.State#isPreLayout()}. When consuming scrap, if this value* is set to true, we skip removed views since they should not be laid out in post layout* step.*/boolean mIsPreLayout = false;/*** The most recent {@link #scrollBy(int, RecyclerView.Recycler, RecyclerView.State)}* amount.*/int mLastScrollDelta;/*** When LLM needs to layout particular views, it sets this list in which case, LayoutState* will only return views from this list and return null if it cannot find an item.*/List<RecyclerView.ViewHolder> mScrapList = null;/*** Used when there is no limit in how many views can be laid out.*/boolean mInfinite;/*** @return true if there are more items in the data adapter*/boolean hasMore(RecyclerView.State state) {return mCurrentPosition >= 0 && mCurrentPosition < state.getItemCount();}/*** Gets the view for the next element that we should layout.* Also updates current item index to the next item, based on {@link #mItemDirection}** @return The next element that we should layout.*/View next(RecyclerView.Recycler recycler) {if (mScrapList != null) {return nextViewFromScrapList();}final View view = recycler.getViewForPosition(mCurrentPosition);mCurrentPosition += mItemDirection;return view;}/*** Returns the next item from the scrap list.* <p>* Upon finding a valid VH, sets current item position to VH.itemPosition + mItemDirection** @return View if an item in the current position or direction exists if not null.*/private View nextViewFromScrapList() {final int size = mScrapList.size();for (int i = 0; i < size; i++) {final View view = mScrapList.get(i).itemView;final RecyclerView.LayoutParams lp = (RecyclerView.LayoutParams) view.getLayoutParams();if (lp.isItemRemoved()) {continue;}if (mCurrentPosition == lp.getViewLayoutPosition()) {assignPositionFromScrapList(view);return view;}}return null;}public void assignPositionFromScrapList() {assignPositionFromScrapList(null);}public void assignPositionFromScrapList(View ignore) {final View closest = nextViewInLimitedList(ignore);if (closest == null) {mCurrentPosition = RecyclerView.NO_POSITION;} else {mCurrentPosition = ((RecyclerView.LayoutParams) closest.getLayoutParams()).getViewLayoutPosition();}}public View nextViewInLimitedList(View ignore) {int size = mScrapList.size();View closest = null;int closestDistance = Integer.MAX_VALUE;if (DEBUG && mIsPreLayout) {throw new IllegalStateException("Scrap list cannot be used in pre layout");}for (int i = 0; i < size; i++) {View view = mScrapList.get(i).itemView;final RecyclerView.LayoutParams lp = (RecyclerView.LayoutParams) view.getLayoutParams();if (view == ignore || lp.isItemRemoved()) {continue;}final int distance = (lp.getViewLayoutPosition() - mCurrentPosition)* mItemDirection;if (distance < 0) {continue; // item is not in current direction}if (distance < closestDistance) {closest = view;closestDistance = distance;if (distance == 0) {break;}}}return closest;}void log() {Log.d(TAG, "avail:" + mAvailable + ", ind:" + mCurrentPosition + ", dir:"+ mItemDirection + ", offset:" + mOffset + ", layoutDir:" + mLayoutDirection);}}

    /*** The magic functions :). Fills the given layout, defined by the layoutState. This is fairly* independent from the rest of the {@link LinearLayoutManager}* and with little change, can be made publicly available as a helper class.** @param recycler        Current recycler that is attached to RecyclerView* @param layoutState     Configuration on how we should fill out the available space.* @param state           Context passed by the RecyclerView to control scroll steps.* @param stopOnFocusable If true, filling stops in the first focusable new child* @return Number of pixels that it added. Useful for scroll functions.*/int fill(RecyclerView.Recycler recycler, LayoutState layoutState,RecyclerView.State state, boolean stopOnFocusable) {// max offset we should set is mFastScroll + available//记录最开始的时候  可用的空间final int start = layoutState.mAvailable;if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {// TODO ugly bug fix. should not happenif (layoutState.mAvailable < 0) {//如果可用空间小于0  那么mScrollingOffset 就减去这个数layoutState.mScrollingOffset += layoutState.mAvailable;}recycleByLayoutState(recycler, layoutState);}int remainingSpace = layoutState.mAvailable + layoutState.mExtraFillSpace;LayoutChunkResult layoutChunkResult = mLayoutChunkResult;while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {layoutChunkResult.resetInternal();if (RecyclerView.VERBOSE_TRACING) {TraceCompat.beginSection("LLM LayoutChunk");}layoutChunk(recycler, state, layoutState, layoutChunkResult);if (RecyclerView.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 || layoutState.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) {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;}

回收View 代码块解读

    private void recycleViewsFromStart(RecyclerView.Recycler recycler, int scrollingOffset,int noRecycleSpace) {if (scrollingOffset < 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 = scrollingOffset - noRecycleSpace;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 {for (int i = 0; i < childCount; i++) {View child = getChildAt(i);//limit 表示滑动多少 就会创建新View//如果说你的底部小于limit 说明你肯定要被回收//直到找到一个View的底部>limit 的地方if (mOrientationHelper.getDecoratedEnd(child) > limit|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {// stop hererecycleChildren(recycler, 0, i);return;}}}}

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