ViewPager2的滑动回弹动画
首先剖析回弹动画的成果，滑动小于肯定阈值时，松手后触发，页面回弹到原地位。
切入点
第一反馈就是onTouchEvent，发现ViewPager2没有重写，并且它是一个ViewGroup，咱们看它初始化的时候设置了什么，调用了 initialize，看到最终嵌套了一个自定义的RecyclerView，发现没有重写onTouchEvent，那么只有可能通过addOnScrollListener来拦挡触摸。
持续剖析initialize，咱们能够把指标锁定在ScrollEventAdapter和PagerSnapHelperImpl，通过查看类形容，咱们能够根本确定是在PagerSnapHelperImpl中实现的。
源码剖析
咱们首要指标是追踪addOnScrollListener
public void attachToRecyclerView(@Nullable RecyclerView recyclerView) throws IllegalStateException { ... mRecyclerView = recyclerView; if (mRecyclerView != null) { setupCallbacks(); ... }}private void setupCallbacks() throws IllegalStateException { ... mRecyclerView.addOnScrollListener(mScrollListener); ...}mScrollListener：
@Overridepublic void onScrollStateChanged(RecyclerView recyclerView, int newState) { super.onScrollStateChanged(recyclerView, newState); // 滚动进行后 if (newState == RecyclerView.SCROLL_STATE_IDLE && mScrolled) { mScrolled = false; snapToTargetExistingView(); }}@Overridepublic void onScrolled(RecyclerView recyclerView, int dx, int dy) { // 解决visible change的状况，具体看办法正文 if (dx != 0 || dy != 0) { mScrolled = true; }}snapToTargetExistingView：
void snapToTargetExistingView() { ... View snapView = findSnapView(layoutManager); if (snapView == null) { return; } int[] snapDistance = calculateDistanceToFinalSnap(layoutManager, snapView); if (snapDistance[0] != 0 || snapDistance[1] != 0) { mRecyclerView.smoothScrollBy(snapDistance[0], snapDistance[1]); }}这段源码一眼看去，就是找了个什么snapView，而后计算出了一个snapDistance，而后滚动到该地位，八九不离十了，外围就在这两个办法中。
咱们持续剖析下：
snapView是指什么？snapDistance是指什么？
findSnapView
public View findSnapView(RecyclerView.LayoutManager layoutManager) { if (layoutManager.canScrollVertically()) { return findCenterView(layoutManager, getVerticalHelper(layoutManager)); } ... return null;}findCenterView：
@Nullableprivate View findCenterView(RecyclerView.LayoutManager layoutManager, OrientationHelper helper) { // 拿到rv以后可见的几个子View int childCount = layoutManager.getChildCount(); if (childCount == 0) { return null; } View closestChild = null; // 计算出rv的中点 final int center = helper.getStartAfterPadding() + helper.getTotalSpace() / 2; int absClosest = Integer.MAX_VALUE; // 取间隔rv中心点最近的子View for (int i = 0; i < childCount; i++) { final View child = layoutManager.getChildAt(i); // 计算出子View的中点 int childCenter = helper.getDecoratedStart(child) + (helper.getDecoratedMeasurement(child) / 2); int absDistance = Math.abs(childCenter - center); /* if child center is closer than previous closest, set it as closest */ // 记录最靠近的子View和间隔 if (absDistance < absClosest) { absClosest = absDistance; closestChild = child; } } return closestChild;}可知snapView就是最靠近RecyclerView两头点的子View
calculateDistanceToFinalSnap
public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager, @NonNull View targetView) { int[] out = new int[2]; ... if (layoutManager.canScrollVertically()) { out[1] = distanceToCenter(targetView, getVerticalHelper(layoutManager)); } ... return out;}distanceToCenter：
private int distanceToCenter(@NonNull RecyclerView.LayoutManager layoutManager, @NonNull View targetView, OrientationHelper helper) { // 跟findCenterView一样 // 计算出targetView中心点间隔rv中心点的间隔 final int childCenter = helper.getDecoratedStart(targetView) + (helper.getDecoratedMeasurement(targetView) / 2); final int containerCenter = helper.getStartAfterPadding() + helper.getTotalSpace() / 2; return childCenter - containerCenter;}可知snapDistance就是snapView的两头点间隔RecyclerView的两头点的间隔
论断
到此源码就剖析完了，滑动回弹的实现形式是：滑动进行后，遍历以后可见的子View，找到最靠近中心点的View，计算出间隔核心的挪动量，最初挪动过来。