RecyclerView.onTouchEvent
@Override
public boolean onTouchEvent(MotionEvent e) {
case MotionEvent.ACTION_MOVE: {
if (mScrollState != SCROLL_STATE_DRAGGING) {
if (mGapWorker != null && (dx != 0 || dy != 0)) {
mGapWorker.postFromTraversal(this, dx, dy);
}
}
} break;
}
GapWorker.postFromTraversal
void postFromTraversal(RecyclerView recyclerView, int prefetchDx, int prefetchDy) {
if (recyclerView.isAttachedToWindow()) {
if (RecyclerView.DEBUG && !mRecyclerViews.contains(recyclerView)) {
throw new IllegalStateException("attempting to post unregistered view!");
}
// 第一次触发拖动的是否将该runnable提交到Mainhandler里面,等待UI thread执行完成再执行prefetch
if (mPostTimeNs == 0) {
mPostTimeNs = recyclerView.getNanoTime();
recyclerView.post(this);
}
}
// 后续的动作触发去更新最新的dx和dy,prefect会按照这个最新的dx和dy计算prefetch的item的position
recyclerView.mPrefetchRegistry.setPrefetchVector(prefetchDx, prefetchDy);
}
GapWorker.run
@Override
public void run() {
try {
TraceCompat.beginSection(RecyclerView.TRACE_PREFETCH_TAG);
// mRecyclerViews的原因是存在嵌套的recyclerView的情况
if (mRecyclerViews.isEmpty()) {
// abort - no work to do
return;
}
// Query most recent vsync so we can predict next one. Note that drawing time not yet
// valid in animation/input callbacks, so query it here to be safe.
final int size = mRecyclerViews.size();
long latestFrameVsyncMs = 0;
// 获取RecyclerView最进一次开始RenderThread的时间
for (int i = 0; i < size; i++) {
RecyclerView view = mRecyclerViews.get(i);
if (view.getWindowVisibility() == View.VISIBLE) {
latestFrameVsyncMs = Math.max(view.getDrawingTime(), latestFrameVsyncMs);
}
}
if (latestFrameVsyncMs == 0) {
// abort - either no views visible, or couldn't get last vsync for estimating next
return;
}
// 计算预加载的最后时间,如果能在截止日期之前完成预加载,那么就能成功完成ViewHolder的预加载,如果不能那么就预加载失败
long nextFrameNs = TimeUnit.MILLISECONDS.toNanos(latestFrameVsyncMs) + mFrameIntervalNs;
prefetch(nextFrameNs);
// TODO: consider rescheduling self, if there's more work to do
} finally {
mPostTimeNs = 0;
TraceCompat.endSection();
}
}
GapWorker.prefetch
void prefetch(long deadlineNs) {
// 计算任务列表
buildTaskList();
// 开始预加载
flushTasksWithDeadline(deadlineNs);
}
private void buildTaskList() {
// Update PrefetchRegistry in each view
final int viewCount = mRecyclerViews.size();
int totalTaskCount = 0;
for (int i = 0; i < viewCount; i++) {
RecyclerView view = mRecyclerViews.get(i);
if (view.getWindowVisibility() == View.VISIBLE) {
// 计算每一个RecyclerView预加载的数量,保存在LayoutPrefetchRegistryImpl.count里面
view.mPrefetchRegistry.collectPrefetchPositionsFromView(view, false);
totalTaskCount += view.mPrefetchRegistry.mCount;
}
}
// Populate task list from prefetch data...
mTasks.ensureCapacity(totalTaskCount);
int totalTaskIndex = 0;
for (int i = 0; i < viewCount; i++) {
RecyclerView view = mRecyclerViews.get(i);
if (view.getWindowVisibility() != View.VISIBLE) {
// Invisible view, don't bother prefetching
continue;
}
LayoutPrefetchRegistryImpl prefetchRegistry = view.mPrefetchRegistry;
final int viewVelocity = Math.abs(prefetchRegistry.mPrefetchDx)
+ Math.abs(prefetchRegistry.mPrefetchDy);
for (int j = 0; j < prefetchRegistry.mCount * 2; j += 2) {
final Task task;
if (totalTaskIndex >= mTasks.size()) {
// 针对每一个预加载的ViewHolder创建一个Task
task = new Task();
mTasks.add(task);
} else {
task = mTasks.get(totalTaskIndex);
}
final int distanceToItem = prefetchRegistry.mPrefetchArray[j + 1];
task.immediate = distanceToItem <= viewVelocity;
task.viewVelocity = viewVelocity;
task.distanceToItem = distanceToItem;
task.view = view;
task.position = prefetchRegistry.mPrefetchArray[j];
totalTaskIndex++;
}
}
// ... and priority sort
Collections.sort(mTasks, sTaskComparator);
}
GapWorker.flushTasksWithDeadline
private void flushTasksWithDeadline(long deadlineNs) {
// 便利所有Task开始预加载
for (int i = 0; i < mTasks.size(); i++) {
final Task task = mTasks.get(i);
if (task.view == null) {
break; // done with populated tasks
}
flushTaskWithDeadline(task, deadlineNs);
task.clear();
}
}
GapWorker.flushTaskWithDeadline
private void flushTaskWithDeadline(Task task, long deadlineNs) {
long taskDeadlineNs = task.immediate ? RecyclerView.FOREVER_NS : deadlineNs;
// 如果没能在deadlineNs之前构造好ViewHolder,那么次次预加载就失败了
RecyclerView.ViewHolder holder = prefetchPositionWithDeadline(task.view,
task.position, taskDeadlineNs);
if (holder != null
&& holder.mNestedRecyclerView != null
&& holder.isBound()
&& !holder.isInvalid()) {
prefetchInnerRecyclerViewWithDeadline(holder.mNestedRecyclerView.get(), deadlineNs);
}
}
RecyclerView.tryGetViewHolderForPositionByDeadline
// 如果不能在预测的预加载时间内预加载出来,那么就不会去预加载
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;
}
RecyclerView.willCreateInTime
boolean willCreateInTime(int viewType, long approxCurrentNs, long deadlineNs) {
long expectedDurationNs = getScrapDataForType(viewType).mCreateRunningAverageNs;
return expectedDurationNs == 0 || (approxCurrentNs + expectedDurationNs < deadlineNs);
}
collectPrefetchPositionsFromView的调用流程图
LayoutPrefetchRegistryImpl.collectPrefetchPositionsFromView
// nested:是否嵌套的,执行RecyclerView里面嵌套的RecyclerView的预加载
void collectPrefetchPositionsFromView(RecyclerView view, boolean nested) {
mCount = 0;
if (mPrefetchArray != null) {
Arrays.fill(mPrefetchArray, -1);
}
final RecyclerView.LayoutManager layout = view.mLayout;
if (view.mAdapter != null
&& layout != null
&& layout.isItemPrefetchEnabled()) {
if (nested) {
// nested prefetch, only if no adapter updates pending. Note: we don't query
// view.hasPendingAdapterUpdates(), as first layout may not have occurred
if (!view.mAdapterHelper.hasPendingUpdates()) {
layout.collectInitialPrefetchPositions(view.mAdapter.getItemCount(), this);
}
} else {
// momentum based prefetch, only if we trust current child/adapter state
if (!view.hasPendingAdapterUpdates()) {
layout.collectAdjacentPrefetchPositions(mPrefetchDx, mPrefetchDy,
view.mState, this);
}
}
if (mCount > layout.mPrefetchMaxCountObserved) {
layout.mPrefetchMaxCountObserved = mCount;
layout.mPrefetchMaxObservedInInitialPrefetch = nested;
view.mRecycler.updateViewCacheSize();
}
}
}
LinearLyoutManager.collectInitialPrefetchPositions
@Override
public void collectInitialPrefetchPositions(int adapterItemCount,
LayoutPrefetchRegistry layoutPrefetchRegistry) {
final boolean fromEnd;
final int anchorPos;
if (mPendingSavedState != null && mPendingSavedState.hasValidAnchor()) {
// use restored state, since it hasn't been resolved yet
fromEnd = mPendingSavedState.mAnchorLayoutFromEnd;
anchorPos = mPendingSavedState.mAnchorPosition;
} else {
resolveShouldLayoutReverse();
fromEnd = mShouldReverseLayout;
if (mPendingScrollPosition == NO_POSITION) {
anchorPos = fromEnd ? adapterItemCount - 1 : 0;
} else {
anchorPos = mPendingScrollPosition;
}
}
final int direction = fromEnd
? LayoutState.ITEM_DIRECTION_HEAD
: LayoutState.ITEM_DIRECTION_TAIL;
// 计算目标预加载位置=当前方向最后一个位置+方向
int targetPos = anchorPos;
for (int i = 0; i < mInitialPrefetchItemCount; i++) {
if (targetPos >= 0 && targetPos < adapterItemCount) {
layoutPrefetchRegistry.addPosition(targetPos, 0);
} else {
break; // no more to prefetch
}
targetPos += direction;
}
}
LayoutPrefetchRegistryImpl.addPosition
@Override
public void addPosition(int layoutPosition, int pixelDistance) {
if (layoutPosition < 0) {
throw new IllegalArgumentException("Layout positions must be non-negative");
}
if (pixelDistance < 0) {
throw new IllegalArgumentException("Pixel distance must be non-negative");
}
// allocate or expand array as needed, doubling when needed
final int storagePosition = mCount * 2;
if (mPrefetchArray == null) {
mPrefetchArray = new int[4];
Arrays.fill(mPrefetchArray, -1);
} else if (storagePosition >= mPrefetchArray.length) {
final int[] oldArray = mPrefetchArray;
mPrefetchArray = new int[storagePosition * 2];
System.arraycopy(oldArray, 0, mPrefetchArray, 0, oldArray.length);
}
// add position
// 数组元素2个为一组,第一保存预加载位置,第二个保存pixelDistance
mPrefetchArray[storagePosition] = layoutPosition;
mPrefetchArray[storagePosition + 1] = pixelDistance;
mCount++;
}
GapWorker.run
@Override
public void run() {
if (latestFrameVsyncMs == 0) {
// abort - either no views visible, or couldn't get last vsync for estimating next
return;
}
long nextFrameNs = TimeUnit.MILLISECONDS.toNanos(latestFrameVsyncMs) + mFrameIntervalNs;
prefetch(nextFrameNs);
// TODO: consider rescheduling self, if there's more work to do
} finally {
mPostTimeNs = 0;
TraceCompat.endSection();
}
}
Recycler.tryGetViewHolderForPositionByDeadline
if (deadlineNs != FOREVER_NS
&& !mRecyclerPool.willCreateInTime(type, start, deadlineNs)) {
// abort - we have a deadline we can't meet
return null;
}
RecyclerView.willCreateInTime
boolean willCreateInTime(int viewType, long approxCurrentNs, long deadlineNs) {
long expectedDurationNs = getScrapDataForType(viewType).mCreateRunningAverageNs;
return expectedDurationNs == 0 || (approxCurrentNs + expectedDurationNs < deadlineNs);
}