RecyclerView源码学习笔记(四)滑动
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2022-06-14 11:17:19
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前几篇学习了RecyclerView的初始化和绘制过程，主要情景都是在静止状态下，没有手动操作，这篇开始就学习在人为操作下的代码流程，先从滑动开始
- RecyclerView源码学习笔记(一)构造函数和setLayoutManager方法
- RecyclerView源码学习笔记(二)setAdapter
- RecyclerView源码学习笔记(三)RecycleView的绘制过程onMeasure,onLayout,onDraw
onInterceptTouchEvent
当一个触摸事件传递到RecyclerView的时候，会调用onInterceptTouchEvent方法，判断当前viewGroup需不需要拦截这个touch事件，如果要拦截就返回true，否则false，onInterceptTouchEvent的源码
public boolean onInterceptTouchEvent(MotionEvent e) {
if (mLayoutFrozen) {
// When layout is frozen, RV does not intercept the motion event.
// A child view e.g. a button may still get the click.
return false;
}
if (dispatchOnItemTouchIntercept(e)) {
cancelTouch();
return true;
}
```
final int action = e.getActionMasked();
final int actionIndex = e.getActionIndex();
switch (action) {
case MotionEvent.ACTION_DOWN:
if (mIgnoreMotionEventTillDown) {
mIgnoreMotionEventTillDown = false;
}
mScrollPointerId = e.getPointerId(0);
mInitialTouchX = mLastTouchX = (int) (e.getX() + 0.5f);
mInitialTouchY = mLastTouchY = (int) (e.getY() + 0.5f);
if (mScrollState == SCROLL_STATE_SETTLING) {
getParent().requestDisallowInterceptTouchEvent(true);
setScrollState(SCROLL_STATE_DRAGGING);
}
// Clear the nested offsets
mNestedOffsets[0] = mNestedOffsets[1] = 0;
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;
}
final int x = (int) (e.getX(index) + 0.5f);
final int y = (int) (e.getY(index) + 0.5f);
if (mScrollState != SCROLL_STATE_DRAGGING) {
final int dx = x - mInitialTouchX;
final int dy = y - mInitialTouchY;
boolean startScroll = false;
if (canScrollHorizontally && Math.abs(dx) > mTouchSlop) {
mLastTouchX = x;
startScroll = true;
}
if (canScrollVertically && Math.abs(dy) > mTouchSlop) {
mLastTouchY = y;
startScroll = true;
}
if (startScroll) {
setScrollState(SCROLL_STATE_DRAGGING);
}
}
} break;
case MotionEvent.ACTION_POINTER_UP: {
onPointerUp(e);
} break;
case MotionEvent.ACTION_UP: {
mVelocityTracker.clear();
stopNestedScroll(TYPE_TOUCH);
} break;
case MotionEvent.ACTION_CANCEL: {
cancelTouch();
}
}
return mScrollState == SCROLL_STATE_DRAGGING;
}首先判断当前RecyclerView有没有被冻住，如果是则直接返回false，也就是说在frozen的情况下，RecyclerView的item还是可以收到touch事件。接下来调用dispatchOnItemTouchIntercept(e)方法，查看其返回值，如果是true，则取消当前touch事件，并返回true，看一下这个dispatchOnItemTouchIntercept(e)方法做了什么
private boolean dispatchOnItemTouchIntercept(MotionEvent e) {
final int action = e.getAction();
if (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_DOWN) {
mActiveOnItemTouchListener = null;
}
final int listenerCount = mOnItemTouchListeners.size();
for (int i = 0; i < listenerCount; i++) {
final OnItemTouchListener listener = mOnItemTouchListeners.get(i);
if (listener.onInterceptTouchEvent(this, e) && action != MotionEvent.ACTION_CANCEL) {
mActiveOnItemTouchListener = listener;
return true;
}
}
return false;
}从代码可以看出，其实就是去调用了所有注册的OnItemTouchListener的onInterceptTouchEvent()方法，如果有一个返回true，则直接返回true，这说明OnItemTouchListener的实现者可以拦截RecyclerView的touch事件不再往下传递。我们这里假设没有注册OnItemTouchListener，代码继续往下走。
直接跳到switch部分，我们假设现在的操作是一次从底部往顶部的拖动动作，那么事件序列，首先是ACTION_DOWN事件，在ACTION_DOWN这个case里主要记录了触摸点的位置，然后调用startNestedScroll方法，这个方法是嵌套滑动体系里的，读者有兴趣可以另外研究，这里不再说明。然后就跳出switch，返回值由mScrollState == SCROLL_STATE_DRAGGING是否成立确定，这里是false，因为我们在点击之前，RecycleView是静止的，没有滑动，所以mScroolState是SCROLL_STATE_IDLE。
从onInterceptTouchEvent返回后，会调到onTouchEvent方法，这里再把源码贴上来
public boolean onTouchEvent(MotionEvent e) {
if (mLayoutFrozen || mIgnoreMotionEventTillDown) {
return false;
}
if (dispatchOnItemTouch(e)) {
cancelTouch();
return true;
}
if (mLayout == null) {
return false;
}
final boolean canScrollHorizontally = mLayout.canScrollHorizontally();
final boolean canScrollVertically = mLayout.canScrollVertically();
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
boolean eventAddedToVelocityTracker = false;
final MotionEvent vtev = MotionEvent.obtain(e);
final int action = e.getActionMasked();
final int actionIndex = e.getActionIndex();
if (action == MotionEvent.ACTION_DOWN) {
mNestedOffsets[0] = mNestedOffsets[1] = 0;
}
vtev.offsetLocation(mNestedOffsets[0], mNestedOffsets[1]);
switch (action) {
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;
}
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;
if (dispatchNestedPreScroll(dx, dy, mScrollConsumed, mScrollOffset, TYPE_TOUCH)) {
dx -= mScrollConsumed[0];
dy -= mScrollConsumed[1];
vtev.offsetLocation(mScrollOffset[0], mScrollOffset[1]);
// Updated the nested offsets
mNestedOffsets[0] += mScrollOffset[0];
mNestedOffsets[1] += mScrollOffset[1];
}
if (mScrollState != SCROLL_STATE_DRAGGING) {
boolean startScroll = false;
if (canScrollHorizontally && Math.abs(dx) > mTouchSlop) {
if (dx > 0) {
dx -= mTouchSlop;
} else {
dx += mTouchSlop;
}
startScroll = true;
}
if (canScrollVertically && Math.abs(dy) > mTouchSlop) {
if (dy > 0) {
dy -= mTouchSlop;
} else {
dy += mTouchSlop;
}
startScroll = true;
}
if (startScroll) {
setScrollState(SCROLL_STATE_DRAGGING);
}
}
if (mScrollState == SCROLL_STATE_DRAGGING) {
mLastTouchX = x - mScrollOffset[0];
mLastTouchY = y - mScrollOffset[1];
if (scrollByInternal(
canScrollHorizontally ? dx : 0,
canScrollVertically ? dy : 0,
vtev)) {
getParent().requestDisallowInterceptTouchEvent(true);
}
if (mGapWorker != null && (dx != 0 || dy != 0)) {
mGapWorker.postFromTraversal(this, dx, dy);
}
}
} break;
case MotionEvent.ACTION_POINTER_UP: {
onPointerUp(e);
} break;
case MotionEvent.ACTION_UP: {
mVelocityTracker.addMovement(vtev);
eventAddedToVelocityTracker = true;
mVelocityTracker.computeCurrentVelocity(1000, mMaxFlingVelocity);
final float xvel = canScrollHorizontally
? -mVelocityTracker.getXVelocity(mScrollPointerId) : 0;
final float yvel = canScrollVertically
? -mVelocityTracker.getYVelocity(mScrollPointerId) : 0;
if (!((xvel != 0 || yvel != 0) && fling((int) xvel, (int) yvel))) {
setScrollState(SCROLL_STATE_IDLE);
}
resetTouch();
} break;
case MotionEvent.ACTION_CANCEL: {
cancelTouch();
} break;
}
if (!eventAddedToVelocityTracker) {
mVelocityTracker.addMovement(vtev);
}
vtev.recycle();
return true;
}这里会先进到ACTION_DOWN这个case，这个里面做的事情也是记录了坐标，调用startNestedScroll方法。接下来再回到onInterceptTouchEvent方法，因为我们的动作是一个拖动的动作，所以在ACTION_DOWN之后会紧跟着ACTION_MOVE事件，在onInterceptTouchEvent的ACTION_MOVE case中主要进行了滑动距离的判断，如果距离大于touchSlop则认为是拖动事件，则将mScrollState设置为SCROLL_STATE_DRAGGING，这时候因为mScrollState已经设置成SCROLL_STATE_DRAGGING。所以onInterceptTouchEvent将会返回true，这样touch事件就不会再传给item
接下来又会调到onTouchEvent方法，并进入case ACTION_MOVE，在case ACTION_MOVE里最重要的就是如下代码段
if (mScrollState == SCROLL_STATE_DRAGGING) {
mLastTouchX = x - mScrollOffset[0];
mLastTouchY = y - mScrollOffset[1];
if (scrollByInternal(
canScrollHorizontally ? dx : 0,
canScrollVertically ? dy : 0,
vtev)) {
getParent().requestDisallowInterceptTouchEvent(true);
}
if (mGapWorker != null && (dx != 0 || dy != 0)) {
mGapWorker.postFromTraversal(this, dx, dy);
}
}首先判断当前是不是在拖动，是的，然后就是调用scrollByInternal方法，这个方法内部会根据拖动的距离，添加item到RecyclerView，那么重点就研究这部分吧(省略部分代码)
boolean scrollByInternal(int x, int y, MotionEvent ev) {
int unconsumedX = 0, unconsumedY = 0;
int consumedX = 0, consumedY = 0;
consumePendingUpdateOperations();
if (mAdapter != null) {
eatRequestLayout();
onEnterLayoutOrScroll();
TraceCompat.beginSection(TRACE_SCROLL_TAG);
fillRemainingScrollValues(mState);
if (x != 0) {
consumedX = mLayout.scrollHorizontallyBy(x, mRecycler, mState);
unconsumedX = x - consumedX;
}
if (y != 0) {
consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState);
unconsumedY = y - consumedY;
}
。。。。
}做了如下事情
consumePendingUpdateOperations()将没有处理的item操作处理完，我们这里假设他没有，直接return
接下来直接跳到如下代码，因为是纵向滑动，所以y不等于0，然后调到mLayout.scrollVerticallyBy
if (y != 0) {
consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState);
unconsumedY = y - consumedY;
}mLayout.scrollVerticallyBy在LinearLayoutManager的实现主要就是调用了以下方法
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);
updateLayoutState(layoutDirection, absDy, true, state);
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;
}这里的dy是上次的点击位置减去这次的点击位置，如果大于0说明是从底部往顶部滑。所以这里layoutDirection的值就是LayoutState.LAYOUT_END，updateLayoutState()方法是在放item前做一些准备工作，代码如下
private void updateLayoutState(int layoutDirection, int requiredSpace,
boolean canUseExistingSpace, RecyclerView.State state) {
// If parent provides a hint, don't measure unlimited.
mLayoutState.mInfinite = resolveIsInfinite();
mLayoutState.mExtra = getExtraLayoutSpace(state);
mLayoutState.mLayoutDirection = layoutDirection;
int scrollingOffset;
if (layoutDirection == LayoutState.LAYOUT_END) {
mLayoutState.mExtra += mOrientationHelper.getEndPadding();
// get the first child in the direction we are going
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)
scrollingOffset = mOrientationHelper.getDecoratedEnd(child)
- mOrientationHelper.getEndAfterPadding();
} else {
...
}
mLayoutState.mAvailable = requiredSpace;
if (canUseExistingSpace) {
mLayoutState.mAvailable -= scrollingOffset;
}
mLayoutState.mScrollingOffset = scrollingOffset;
}-
mLayoutState.mInfinite表示当前RecyclerView对加载的item有没有数量限制 -
mLayoutState.mExtra表示提前加载那些还没有显示到屏幕上的item，一般是提前一个屏幕的长度，单位是px - 因为我们这里
layoutDirection等于LayoutState.LAYOUT_END，所以进入第一个if，先是给mLayoutState.mExtra加上底部padding的长度，因为有时候我们会给RecyclerView设置padding，接着找到屏幕上最靠近底部的那个item。 -
mLayoutState.mItemDirection的意思是指我们在添加itemview的时候，从adapter去取对应数据的时候，是从头开始取还是倒着取，我们这里不做特殊处理，所以是顺着取，mLayoutState.mItemDirection = LayoutState.ITEM_DIRECTION_TAIL -
mLayoutState.mCurrentPosition是指当前要添加的item对应到adapter中的位置 -
mLayoutState.mOffset表示开始画item的坐标位置，单位是像素 -
scrollingOffset表示的值是屏幕上最后一个item的下边缘的坐标减去RecycleView的下边缘坐标再减去底部padding的值，这个值表示的意思是，如果我们不添加item，那我们还可以滑动多少距离，如下图，蓝色代表RecyclerView，红色代表item，绿色代表RecyclerView的底部坐标减去buttom padding后的坐标，scrollingOffse的值就是第二个item的下边缘坐标减去绿色线的坐标的差。 -
mLayoutState.mAvailable代表滑动的距离，如果canUseExistingSpace为true，则mLayoutState.mAvailable再减去scrollingOffset,说明需要绘制填充的距离更短了，最后将scrollingOffset赋值给mLayoutState.mScrollingOffset
我们再回到scrollBy()方法，在调用完updateLayoutState()方法后会调用fill()方法，源码如下
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;
}
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;
}这边会判断layoutState.mAvailable是否小于0，如果是，就设置回原来的值，也就是在减去mScrollingOffset之前的值，这里是什么目的还不清楚，难道是因为layoutState.mAvailable不可以为负数？接下来会调用recycleByLayoutState()方法，将在接下来被滑出屏幕的item回收，就是将item view全部remove，并回收到cache或者pool中。
然后调用while循环添加item，循环继续的条件是item还没有加载完或者RecyclerView可以无限加载且当前还没有加载满需要填充的空间。具体实施item加载的是layoutChunk()方法，源码如下:
void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
LayoutState layoutState, LayoutChunkResult result) {
View view = layoutState.next(recycler);
if (view == null) {
if (DEBUG && layoutState.mScrapList == null) {
throw new RuntimeException("received null view when unexpected");
}
// if we are laying out views in scrap, this may return null which means there is
// no more items to layout.
result.mFinished = true;
return;
}
LayoutParams params = (LayoutParams) view.getLayoutParams();
if (layoutState.mScrapList == null) {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addView(view);
} else {
addView(view, 0);
}
} else {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addDisappearingView(view);
} else {
addDisappearingView(view, 0);
}
}
measureChildWithMargins(view, 0, 0);
result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view);
int left, top, right, bottom;
if (mOrientation == VERTICAL) {
if (isLayoutRTL()) {
right = getWidth() - getPaddingRight();
left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);
} else {
left = getPaddingLeft();
right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);
}
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
bottom = layoutState.mOffset;
top = layoutState.mOffset - result.mConsumed;
} else {
top = layoutState.mOffset;
bottom = layoutState.mOffset + result.mConsumed;
}
} else {
top = getPaddingTop();
bottom = top + mOrientationHelper.getDecoratedMeasurementInOther(view);
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
right = layoutState.mOffset;
left = layoutState.mOffset - result.mConsumed;
} else {
left = layoutState.mOffset;
right = layoutState.mOffset + result.mConsumed;
}
}
// We calculate everything with View's bounding box (which includes decor and margins)
// To calculate correct layout position, we subtract margins.
layoutDecoratedWithMargins(view, left, top, right, bottom);
if (DEBUG) {
Log.d(TAG, "laid out child at position " + getPosition(view) + ", with l:"
+ (left + params.leftMargin) + ", t:" + (top + params.topMargin) + ", r:"
+ (right - params.rightMargin) + ", b:" + (bottom - params.bottomMargin));
}
// Consume the available space if the view is not removed OR changed
if (params.isItemRemoved() || params.isItemChanged()) {
result.mIgnoreConsumed = true;
}
result.mFocusable = view.hasFocusable();
}首先会调用layoutState.next(recycler)方法获取到一个item view，这个next方法源码如下:
View next(RecyclerView.Recycler recycler) {
if (mScrapList != null) {
return nextViewFromScrapList();
}
final View view = recycler.getViewForPosition(mCurrentPosition);
mCurrentPosition += mItemDirection;
return view;
}先会判断mScrapList是不是null，这个对象只有在layoutForPredictiveAnimations方法中会被赋值，而layoutForPredictiveAnimations内部会判断要不要直接返回，我们这里现在是没有PredictiveAnimation所以layoutForPredictiveAnimations方法会直接返回，也就是mScrapList值等于null，这样的话就是会通过recycler.getViewForPosition方法获取itemview，getViewForPosition方法最后会调用到tryGetViewHolderForPositionByDeadline方法，此方法的第三个参数是一个long型的，表示需要在多长时间内完成item的创建和bind工作，如果是FOREVER_NS，表示没有时间限制，否则，就必须在规定时间内完成，否则就有可能返回null或者一个没有bind过的itemview。这里我们传进去的是FOREVER_NS，所以没有时间限制。tryGetViewHolderForPositionByDeadline方法内部会依次从mChangedScrap，mAttachedScrap，mCachedViews，mViewCacheExtension，RecycledViewPool去获取itemview，如果都没有获取到，则会调用adapter的createViewHolder方法来创建，创建view的代码如下
if (holder == null) {
long start = getNanoTime();
if (deadlineNs != FOREVER_NS
&& !mRecyclerPool.willCreateInTime(type, start, deadlineNs)) {
// abort - we have a deadline we can't meet
return null;
}
holder = mAdapter.createViewHolder(RecyclerView.this, type);
if (ALLOW_THREAD_GAP_WORK) {
// only bother finding nested RV if prefetching
RecyclerView innerView = findNestedRecyclerView(holder.itemView);
if (innerView != null) {
holder.mNestedRecyclerView = new WeakReference<>(innerView);
}
}
long end = getNanoTime();
mRecyclerPool.factorInCreateTime(type, end - start);
if (DEBUG) {
Log.d(TAG, "tryGetViewHolderForPositionByDeadline created new ViewHolder");
}
}可以看到最开始会判断deadlineNs是不是等于FOREVER_NS，如果不是就会先预测在规定时间内是不是可以完成创建，如果不能则直接返回null了，这里的预测时间是根据前面创建item的平均时间来确定的。在createViewHolder方法内部会调用Adapter子类实现的onCreateViewHolder方法
public final VH createViewHolder(ViewGroup parent, int viewType) {
TraceCompat.beginSection(TRACE_CREATE_VIEW_TAG);
final VH holder = onCreateViewHolder(parent, viewType);
holder.mItemViewType = viewType;
TraceCompat.endSection();
return holder;
}然后接下来有如下代码:
boolean bound = false;
if (mState.isPreLayout() && holder.isBound()) {
// do not update unless we absolutely have to.
holder.mPreLayoutPosition = position;
} else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) {
if (DEBUG && holder.isRemoved()) {
throw new IllegalStateException("Removed holder should be bound and it should"
+ " come here only in pre-layout. Holder: " + holder
+ exceptionLabel());
}
final int offsetPosition = mAdapterHelper.findPositionOffset(position);
bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
}因为我们这里没有prelayout，所以会走elseif，其中会判断viewholder是不是需要重新bind，如果是就调用tryBindViewHolderByDeadline方法,需要bind的条件有三个，`.没有bind过，2.需要更新，3.当前holder已经不可用
private boolean tryBindViewHolderByDeadline(ViewHolder holder, int offsetPosition,
int position, long deadlineNs) {
holder.mOwnerRecyclerView = RecyclerView.this;
final int viewType = holder.getItemViewType();
long startBindNs = getNanoTime();
if (deadlineNs != FOREVER_NS
&& !mRecyclerPool.willBindInTime(viewType, startBindNs, deadlineNs)) {
// abort - we have a deadline we can't meet
return false;
}
mAdapter.bindViewHolder(holder, offsetPosition);
long endBindNs = getNanoTime();
mRecyclerPool.factorInBindTime(holder.getItemViewType(), endBindNs - startBindNs);
attachAccessibilityDelegateOnBind(holder);
if (mState.isPreLayout()) {
holder.mPreLayoutPosition = position;
}
return true;
}方法内部也会先判断能不能在要求的时间内完成bind，如果不能就返回false，我能这里是FOREVER_NS，所以会调用 mAdapter.bindViewHolder(holder,offsetPosition)，在bindViewHolder()内部会调用Adapter子类的实现的onBindViewHolder()方法
public final void bindViewHolder(VH holder, int position) {
holder.mPosition = position;
if (hasStableIds()) {
holder.mItemId = getItemId(position);
}
holder.setFlags(ViewHolder.FLAG_BOUND,
ViewHolder.FLAG_BOUND | ViewHolder.FLAG_UPDATE | ViewHolder.FLAG_INVALID
| ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN);
TraceCompat.beginSection(TRACE_BIND_VIEW_TAG);
onBindViewHolder(holder, position, holder.getUnmodifiedPayloads());
holder.clearPayload();
final ViewGroup.LayoutParams layoutParams = holder.itemView.getLayoutParams();
if (layoutParams instanceof RecyclerView.LayoutParams) {
((LayoutParams) layoutParams).mInsetsDirty = true;
}
TraceCompat.endSection();
}
再回到tryGetViewHolderForPositionByDeadline()方法，最后该方法会给viewholder设置一个LayoutParams，如果viewholder当前没有LayoutParams，就会调用LayoutManager的generateDefaultLayoutParams()方法来生成一个默认的LayoutParams，此方法需要子类实现。
再回到layoutChunk()方法，再贴一下代码
void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
LayoutState layoutState, LayoutChunkResult result) {
View view = layoutState.next(recycler);
if (view == null) {
if (DEBUG && layoutState.mScrapList == null) {
throw new RuntimeException("received null view when unexpected");
}
// if we are laying out views in scrap, this may return null which means there is
// no more items to layout.
result.mFinished = true;
return;
}
LayoutParams params = (LayoutParams) view.getLayoutParams();
if (layoutState.mScrapList == null) {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addView(view);
} else {
addView(view, 0);
}
} else {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addDisappearingView(view);
} else {
addDisappearingView(view, 0);
}
}
measureChildWithMargins(view, 0, 0);
result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view);
int left, top, right, bottom;
if (mOrientation == VERTICAL) {
if (isLayoutRTL()) {
right = getWidth() - getPaddingRight();
left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);
} else {
left = getPaddingLeft();
right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);
}
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
bottom = layoutState.mOffset;
top = layoutState.mOffset - result.mConsumed;
} else {
top = layoutState.mOffset;
bottom = layoutState.mOffset + result.mConsumed;
}
} else {
top = getPaddingTop();
bottom = top + mOrientationHelper.getDecoratedMeasurementInOther(view);
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
right = layoutState.mOffset;
left = layoutState.mOffset - result.mConsumed;
} else {
left = layoutState.mOffset;
right = layoutState.mOffset + result.mConsumed;
}
}
// We calculate everything with View's bounding box (which includes decor and margins)
// To calculate correct layout position, we subtract margins.
layoutDecoratedWithMargins(view, left, top, right, bottom);
if (DEBUG) {
Log.d(TAG, "laid out child at position " + getPosition(view) + ", with l:"
+ (left + params.leftMargin) + ", t:" + (top + params.topMargin) + ", r:"
+ (right - params.rightMargin) + ", b:" + (bottom - params.bottomMargin));
}
// Consume the available space if the view is not removed OR changed
if (params.isItemRemoved() || params.isItemChanged()) {
result.mIgnoreConsumed = true;
}
result.mFocusable = view.hasFocusable();
}此时我们已经从next()方法获取到了一个view,此view不等于null，然后按照我们现在的场景，会进到addView(view)方法。此方法最终会调用到LayoutManager的addViewInt方法,此方法源码如下
private void addViewInt(View child, int index, boolean disappearing) {
final ViewHolder holder = getChildViewHolderInt(child);
if (disappearing || holder.isRemoved()) {
// these views will be hidden at the end of the layout pass.
mRecyclerView.mViewInfoStore.addToDisappearedInLayout(holder);
} else {
// This may look like unnecessary but may happen if layout manager supports
// predictive layouts and adapter removed then re-added the same item.
// In this case, added version will be visible in the post layout (because add is
// deferred) but RV will still bind it to the same View.
// So if a View re-appears in post layout pass, remove it from disappearing list.
mRecyclerView.mViewInfoStore.removeFromDisappearedInLayout(holder);
}
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (holder.wasReturnedFromScrap() || holder.isScrap()) {
if (holder.isScrap()) {
holder.unScrap();
} else {
holder.clearReturnedFromScrapFlag();
}
mChildHelper.attachViewToParent(child, index, child.getLayoutParams(), false);
if (DISPATCH_TEMP_DETACH) {
ViewCompat.dispatchFinishTemporaryDetach(child);
}
} else if (child.getParent() == mRecyclerView) { // it was not a scrap but a valid child
// ensure in correct position
int currentIndex = mChildHelper.indexOfChild(child);
if (index == -1) {
index = mChildHelper.getChildCount();
}
if (currentIndex == -1) {
throw new IllegalStateException("Added View has RecyclerView as parent but"
+ " view is not a real child. Unfiltered index:"
+ mRecyclerView.indexOfChild(child) + mRecyclerView.exceptionLabel());
}
if (currentIndex != index) {
mRecyclerView.mLayout.moveView(currentIndex, index);
}
} else {
mChildHelper.addView(child, index, false);
lp.mInsetsDirty = true;
if (mSmoothScroller != null && mSmoothScroller.isRunning()) {
mSmoothScroller.onChildAttachedToWindow(child);
}
}
if (lp.mPendingInvalidate) {
if (DEBUG) {
Log.d(TAG, "consuming pending invalidate on child " + lp.mViewHolder);
}
holder.itemView.invalidate();
lp.mPendingInvalidate = false;
}
}做了如下事情：
- 判断需要添加的view的viewholder是需要消失的或者被移除的，两者满足一个就将此viewholder添加到ViewInfoState的
mLayoutHolderMap中，用于之后的动画。 - 判断viewholder是不是scrap，如果是则清除相关标志，并将view重新attach到RecyclerView
- 如果当前view的parent是当前RecyclerView，也就是说没有dettach，则将此view移动到指定的index，这个过程是dettach到attach的过程
- 如果不满足2,3两种情况，则这个view是新建的view，则将此view add到RecyclerView，如果此view的位置是
mSmoothScroller滑动的目标位置，则将targetview设为此view - 最后如果此view需要刷新，也就是有重新bind，则调用
invalidate()
layoutChunk()方法在调用完addview()方法后，会调用measureChildWithMargins()方法，此方法会用标准测量方法测量被添加的view，并将RecyclerView的滑动方向和itemdecoration考虑进去，这里不再分析源码。接下来把此item view消耗掉的空间数据保存到result的mConsumed变量中。接下来会根据之前的测量结果，决定view的摆放位置，也就是调用layout()方法，此过程中会将itemdecoration占用的空间也考虑进去。最后如果此view是被removed或者changed，则会忽略掉它所消耗的空间，因为remove的item不会消耗空间，反倒会增加空间，change的item所占空间不变，不会影响后续空间消耗的计算，这里的空间指的是根据用户滑动距离，RecyclerView需要填充的空间，也就是mLayoutState.mAvailable。到这里，·layoutChunk()·就结束了，接下来就又要回到fill()方法。再贴一遍code
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;
}
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;
}在调用完layoutChunk()方法之后，fill()继续做了如下事情：
- 判断此次填充动作是不是结束，也就是看
layoutChunkResult.mFinished这个变量，如果结束则break，我们这里假设它没有结束 - 接下来会根据item的填充方向和item消耗的空间更新
layoutState.mOffset - 然后更新
layoutState.mAvailable，更新此变量的条件是：1.所添加的view不需要忽略，2.正在布局scrap的view，3.没有进行pre-layout. - 更新
layoutState.mScrollingOffset，并调用recycleByLayoutState()方法回收暂时会被移除屏幕的item - 返回已经消耗的空间，单位是px
到这里fill()方法就结束了，再回到scrollBy()方法，判断返回的consumer是否小于0，小于0说明已经没有item可以再添加，直接返回0(其实这部分不明白，为什么consumer小于0就是没有item需要再添加)，否则就根据scroll的像素个数来移动child views的位置，实现滑动效果，然后将scroll的距离保存在mLayoutState.mLastScrollDelta，最后返回scroll。
scroll结束后再回到scrollByInternal()方法，后续的代码如下:
if (y != 0) {
consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState);
unconsumedY = y - consumedY;
}
...
repositionShadowingViews();
...
}
if (!mItemDecorations.isEmpty()) {
invalidate();
}
if (dispatchNestedScroll(consumedX, consumedY, unconsumedX, unconsumedY, mScrollOffset,
TYPE_TOUCH)) {
// Update the last touch co-ords, taking any scroll offset into account
mLastTouchX -= mScrollOffset[0];
mLastTouchY -= mScrollOffset[1];
if (ev != null) {
ev.offsetLocation(mScrollOffset[0], mScrollOffset[1]);
}
mNestedOffsets[0] += mScrollOffset[0];
mNestedOffsets[1] += mScrollOffset[1];
} else if (getOverScrollMode() != View.OVER_SCROLL_NEVER) {
if (ev != null && !MotionEventCompat.isFromSource(ev, InputDevice.SOURCE_MOUSE)) {
pullGlows(ev.getX(), unconsumedX, ev.getY(), unconsumedY);
}
considerReleasingGlowsOnScroll(x, y);
}
if (consumedX != 0 || consumedY != 0) {
dispatchOnScrolled(consumedX, consumedY);
}
if (!awakenScrollBars()) {
invalidate();
}
return consumedX != 0 || consumedY != 0;接下来会调用repositionShadowingViews方法，此方法里做的事情和itemChange的动画有关，我们这里先不涉及动画。然后判断itemdecoration是否有指定，如果有的话就重绘RecyclerView。后面就是嵌套滑动和边缘阴影的动作，然后再调用各个监听scroll的listener，不再深究，最后返回是否有滑动被消耗掉。到这里scrollByInternal()也结束了，终于又回到onTouchEvent了,在onTouchEvent会判断滑动是否有被消耗，如果有的话，就会调用getParent().requestDisallowInterceptTouchEvent(true)方法来阻止RecyclerView的父控件来拦截touch事件，说明RecyclerView需要继续处理后续事件。case Move后面的代码就不讲了，比较简单，都是一些资源回收，状态重置的动作。
最后就是调用onTouchEvent方法中Case Up的代码，这里主要是做了一个fling()的动作也就是快速滑动，这部分代码这里不讲了，等后面有机会再看，其实做的事情和case move差不多。
好了，滑动部分就到这里，我也是在学习源码，有些地方如果有错误还请各位读者提出