RecyclerView 滑动布局源码分析：带你深入掌握列表滑动机制
作者：maxcion
现在RV已经初始化好了,那当我们进行滑动交互时代码又是如何执行的呢?
RV优秀就优秀在他是动态布局的,与ScrollView不同在于:ScrollView是初始化时将所有child都inflate并add而RV是只inflate屏幕展示得下的child.
如果我们有100个child:
ScrollView便会在初始化时就inflate并add100个child,这样滑动的时候ScrollView不用执行过多的逻辑.RV由于只会inflate部分child,所以当滑动的时候就会涉及动态inflate、add、remove等逻辑.
RV再强大,他终究还是View,那他也逃离不了android触摸事件传递的限制,所以如果RV想要响应滑动事件,那一切的开始必然在onTouchEvent()中.
一切的开始
RV.onTouchEvent()
public boolean onTouchEvent(MotionEvent e) {if (mLayoutSuppressed || mIgnoreMotionEventTillDown) {return false;}if (dispatchToOnItemTouchListeners(e)) {cancelScroll();return true;}if (mLayout == null) {return false;}//判断滑动方向,这里我们选择以垂直方向滑动为例final boolean canScrollHorizontally = mLayout.canScrollHorizontally();final boolean canScrollVertically = mLayout.canScrollVertically();//这个就是创建一个 速度跟踪器if (mVelocityTracker == null) {mVelocityTracker = VelocityTracker.obtain();}boolean eventAddedToVelocityTracker = false;final int action = e.getActionMasked();final int actionIndex = e.getActionIndex();if (action == MotionEvent.ACTION_DOWN) {mNestedOffsets[0] = mNestedOffsets[1] = 0;}final MotionEvent vtev = MotionEvent.obtain(e);vtev.offsetLocation(mNestedOffsets[0], mNestedOffsets[1]);switch (action) {//DOWN 事件主要和嵌套滑动相关,所以这里可以跳过case MotionEvent.ACTION_DOWN: {mScrollPointerId = e.getPointerId(0);mInitialTouchX = mLastTouchX = (int) (e.getX() + 0.5f);mInitialTouchY = mLastTouchY = (int) (e.getY() + 0.5f);int nestedScrollAxis = ViewCompat.SCROLL_AXIS_NONE;if (canScrollHorizontally) {nestedScrollAxis |= ViewCompat.SCROLL_AXIS_HORIZONTAL;}if (canScrollVertically) {nestedScrollAxis |= ViewCompat.SCROLL_AXIS_VERTICAL;}startNestedScroll(nestedScrollAxis, TYPE_TOUCH);}break;case MotionEvent.ACTION_POINTER_DOWN: {mScrollPointerId = e.getPointerId(actionIndex);mInitialTouchX = mLastTouchX = (int) (e.getX(actionIndex) + 0.5f);mInitialTouchY = mLastTouchY = (int) (e.getY(actionIndex) + 0.5f);}break;case MotionEvent.ACTION_MOVE: {final int index = e.findPointerIndex(mScrollPointerId);if (index < 0) {Log.e(TAG, "Error processing scroll; pointer index for id "+ mScrollPointerId + " not found. Did any MotionEvents get skipped?");return false;}//这里主要计算手指滑动距离final int x = (int) (e.getX(index) + 0.5f);final int y = (int) (e.getY(index) + 0.5f);int dx = mLastTouchX - x;int dy = mLastTouchY - y;//mScrollState的默认值是SCROLL_STATE_IDLE,//从名字上能看出来跟滑动状态相关,现在还没开始滑动,所以是默认值//通过查看mScrollState所有赋值的地方都是在滑动的逻辑//所以这个if条件成立if (mScrollState != SCROLL_STATE_DRAGGING) {boolean startScroll = false;//我们以垂直滑动距离if (canScrollHorizontally) {if (dx > 0) {dx = Math.max(0, dx - mTouchSlop);} else {dx = Math.min(0, dx + mTouchSlop);}if (dx != 0) {startScroll = true;}}if (canScrollVertically) {//这里主要是过滤没有达到最小滑动距离的滑动if (dy > 0) {dy = Math.max(0, dy - mTouchSlop);} else {dy = Math.min(0, dy + mTouchSlop);}//滑动距离达标if (dy != 0) {startScroll = true;}}//判断当前是否应该滑动if (startScroll) {//把标志位设置为应该滑动的状态,下面紧接着会用到setScrollState(SCROLL_STATE_DRAGGING);}}//上面已经把mScrollState设置为SCROLL_STATE_DRAGGING//判断条件成立if (mScrollState == SCROLL_STATE_DRAGGING) {mReusableIntPair[0] = 0;mReusableIntPair[1] = 0;dx -= releaseHorizontalGlow(dx, e.getY());dy -= releaseVerticalGlow(dy, e.getX());//这里和嵌套滑动相关,可以跳过if (dispatchNestedPreScroll(canScrollHorizontally ? dx : 0,canScrollVertically ? dy : 0,mReusableIntPair, mScrollOffset, TYPE_TOUCH)) {dx -= mReusableIntPair[0];dy -= mReusableIntPair[1];// Updated the nested offsetsmNestedOffsets[0] += mScrollOffset[0];mNestedOffsets[1] += mScrollOffset[1];// Scroll has initiated, prevent parents from interceptinggetParent().requestDisallowInterceptTouchEvent(true);}mLastTouchX = x - mScrollOffset[0];mLastTouchY = y - mScrollOffset[1];//① 这里是重点了if (scrollByInternal(canScrollHorizontally ? dx : 0,canScrollVertically ? dy : 0,e, TYPE_TOUCH)) {getParent().requestDisallowInterceptTouchEvent(true);}if (mGapWorker != null && (dx != 0 || dy != 0)) {mGapWorker.postFromTraversal(this, dx, dy);}}}break;......}if (!eventAddedToVelocityTracker) {mVelocityTracker.addMovement(vtev);}vtev.recycle();return true;
}
RV.scrollByInternal()
boolean scrollByInternal(int x, int y, MotionEvent ev, int type) {int unconsumedX = 0;int unconsumedY = 0;int consumedX = 0;int consumedY = 0;//这个函数主要是为了处理当在滑动的时候,adapter数据更新的情况//滑动的时候默认数据是不变的,但是adapter实际修改了数据,就会有crashconsumePendingUpdateOperations();if (mAdapter != null) {mReusableIntPair[0] = 0;mReusableIntPair[1] = 0;//① 这里又是重点了,滑动时的layout逻辑都在这里scrollStep(x, y, mReusableIntPair);consumedX = mReusableIntPair[0];consumedY = mReusableIntPair[1];unconsumedX = x - consumedX;unconsumedY = y - consumedY;}//后面都是和滑动嵌套相关的逻辑,可以直接跳过if (!mItemDecorations.isEmpty()) {invalidate();}mReusableIntPair[0] = 0;mReusableIntPair[1] = 0;dispatchNestedScroll(consumedX, consumedY, unconsumedX, unconsumedY, mScrollOffset,type, mReusableIntPair);unconsumedX -= mReusableIntPair[0];unconsumedY -= mReusableIntPair[1];boolean consumedNestedScroll = mReusableIntPair[0] != 0 || mReusableIntPair[1] != 0;// Update the last touch co-ords, taking any scroll offset into accountmLastTouchX -= mScrollOffset[0];mLastTouchY -= mScrollOffset[1];mNestedOffsets[0] += mScrollOffset[0];mNestedOffsets[1] += mScrollOffset[1];if (getOverScrollMode() != View.OVER_SCROLL_NEVER) {if (ev != null && !MotionEventCompat.isFromSource(ev, InputDevice.SOURCE_MOUSE)) {pullGlows(ev.getX(), unconsumedX, ev.getY(), unconsumedY);// For rotary encoders, we release stretch EdgeEffects after they are pulled, to// avoid the effects being stuck pulled.if (Build.VERSION.SDK_INT >= 31&& MotionEventCompat.isFromSource(ev, InputDevice.SOURCE_ROTARY_ENCODER)) {releaseGlows();}}considerReleasingGlowsOnScroll(x, y);}if (consumedX != 0 || consumedY != 0) {dispatchOnScrolled(consumedX, consumedY);}if (!awakenScrollBars()) {invalidate();}return consumedNestedScroll || consumedX != 0 || consumedY != 0;
}
其实这里面大部分逻辑都是处理嵌套滑动的,有layout相关的逻辑都在scrollStep()
LLM.scrollStep()
void scrollStep(int dx, int dy, @Nullable int[] consumed) {startInterceptRequestLayout();onEnterLayoutOrScroll();TraceCompat.beginSection(TRACE_SCROLL_TAG);fillRemainingScrollValues(mState);int consumedX = 0;int consumedY = 0;//这里就是根据滑动方向进行滑动处理if (dx != 0) {consumedX = mLayout.scrollHorizontallyBy(dx, mRecycler, mState);}if (dy != 0) {consumedY = mLayout.scrollVerticallyBy(dy, mRecycler, mState);}TraceCompat.endSection();repositionShadowingViews();onExitLayoutOrScroll();stopInterceptRequestLayout(false);if (consumed != null) {consumed[0] = consumedX;consumed[1] = consumedY;}
}
scrollStep()主要处理两件事,将滑动委托给LayoutManager和其他…
LLM.scrollVerticallyBy
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler,RecyclerView.State state) {if (mOrientation == HORIZONTAL) {return 0;}//交给scrollBy()处理return scrollBy(dy, recycler, state);
}int scrollBy(int delta, RecyclerView.Recycler recycler, RecyclerView.State state) {if (getChildCount() == 0 || delta == 0) {return 0;}ensureLayoutState();mLayoutState.mRecycle = true;//detail > 0 代表往上滑或者往左滑final int layoutDirection = delta > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;final int absDelta = Math.abs(delta);//通过滑动方向来更新layoutState中的各种参数updateLayoutState(layoutDirection, absDelta, true, state);//开始填充childfinal 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 = absDelta > consumed ? layoutDirection * consumed : delta;mOrientationHelper.offsetChildren(-scrolled);if (DEBUG) {Log.d(TAG, "scroll req: " + delta + " scrolled: " + scrolled);}mLayoutState.mLastScrollDelta = scrolled;return scrolled;
}
这里的重点逻辑又进入到fill()这里的fill()和静态布局时的逻辑差别不大,不同的是这里设计Viewholder的回收,这里的回收逻辑都在recycleByLayoutState()中
LLM.fill()
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,RecyclerView.State state, boolean stopOnFocusable) {// max offset we should set is mFastScroll + availablefinal int start = layoutState.mAvailable;//①这里的逻辑就是先判断哪些child会被划出屏幕,然后把他们进行回收if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {// TODO ugly bug fix. should not happenif (layoutState.mAvailable < 0) {layoutState.mScrollingOffset += layoutState.mAvailable;}recycleByLayoutState(recycler, layoutState);}//计算总共有多少空间可以用来摆放childint remainingSpace = layoutState.mAvailable + layoutState.mExtraFillSpace;//一个用来保存每次布局一个child的结果类,比如一个child消费了多少空间//是否应该真实的计算这个child消费的空间(预布局的时候有些child虽然消费了空间,// 但是不应该不参与真正的空间剩余空间的计算)LayoutChunkResult layoutChunkResult = mLayoutChunkResult;//只要还有空间和item就进行布局layoutchunkwhile ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {//重置上一次布局child的结果layoutChunkResult.resetInternal();if (RecyclerView.VERBOSE_TRACING) {TraceCompat.beginSection("LLM LayoutChunk");}//这里是真正layout child的逻辑layoutChunk(recycler, state, layoutState, layoutChunkResult);if (RecyclerView.VERBOSE_TRACING) {TraceCompat.endSection();}if (layoutChunkResult.mFinished) {break;}//layoutState.mLayoutDirection的值是 1或者-1 所以这里是 乘法//如果是从顶部往底部填充,当前填充的是第三个child 且每个高度是10dp,那么layoutState.mOffset的值//就是上次填充时的偏移量 + 这次填充child的高度//如果是从底部往顶部填充,那就是次填充时的偏移量 - 这次填充child的高度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*///判断是否要真正的消费当前child参与布局所消费的高度//从判断条件中可以看到预布局和这个有关,不过预布局等后面几章会详细说的//这里就是同步目前还剩多少空间可以用来布局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;}//在这个判断内执行滑出去的child进行回收if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {layoutState.mScrollingOffset += layoutChunkResult.mConsumed;if (layoutState.mAvailable < 0) {layoutState.mScrollingOffset += layoutState.mAvailable;}//执行回收相关逻辑recycleByLayoutState(recycler, layoutState);}if (stopOnFocusable && layoutChunkResult.mFocusable) {break;}}if (DEBUG) {validateChildOrder();}return start - layoutState.mAvailable;
}
这里的逻辑几乎与 Recyclerview源码分析：一、静态时如何布局 一致,但是他在执行逻辑的开始进行了回收逻辑,并且在每次布局完一个child也会再次判断哪些child需要回收.
LLM.recycleByLayoutState()
private void recycleByLayoutState(RecyclerView.Recycler recycler, LayoutState layoutState) {if (!layoutState.mRecycle || layoutState.mInfinite) {return;}int scrollingOffset = layoutState.mScrollingOffset;int noRecycleSpace = layoutState.mNoRecycleSpace;//这里我们还是以垂直布局手指向上滑动场景为例//因为手指向上滑动,就需要在底部填充child,所以layoutState.mLayoutDirection != LayoutState.LAYOUT_START//就会走到else逻辑中if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {recycleViewsFromEnd(recycler, scrollingOffset, noRecycleSpace);} else {//①recycleViewsFromStart(recycler, scrollingOffset, noRecycleSpace);}
}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.//举个栗子:屏幕高度100dp,每个child高度为15dp,此时屏幕会显示//7个child,但是第7个child没有全部显示,底部还有5dp的内容在屏幕//下方,这时候scrollingOffset=5dpfinal 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 {//从顶部第一个child开始找,找到第一个child的bottom>scrollingOffset(5dp)的child//那么这个child之前的所有child在这次滑动中都会划出屏幕//所以要把他们都回收掉for (int i = 0; i < childCount; i++) {View child = getChildAt(i);if (mOrientationHelper.getDecoratedEnd(child) > limit|| mOrientationHelper.getTransformedEndWithDecoration(child) > limit) {// stop hererecycleChildren(recycler, 0, i);return;}}}
}
总结一下就是:RV在滑动的时候会把滑动的距离交给LayoutManager处理和消费,LayoutManager会遍历当前所有的child并根据他们的位置加上这次滑动距离,判断哪些child会划出屏幕,然后就把他们给回收掉.
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