RecyclerViewBackedScrollView.java 文件源码

@Override
protected void onScrollChanged(int l, int t, int oldl, int oldt) {
    super.onScrollChanged(l, t, oldl, oldt);

    if (mOnScrollDispatchHelper.onScrollChanged(l, t)) {
        getReactContext().getNativeModule(UIManagerModule.class).getEventDispatcher()
                .dispatchEvent(ScrollEvent.obtain(
                        getId(),
                        ScrollEventType.SCROLL,
                        0, /* offsetX = 0, horizontal scrolling only */
                        computeVerticalScrollOffset(),
                        mOnScrollDispatchHelper.getXFlingVelocity(),
                        mOnScrollDispatchHelper.getYFlingVelocity(),
                        getWidth(),
                        computeVerticalScrollRange(),
                        getWidth(),
                        getHeight()));
    }

    final int firstIndex = ((LinearLayoutManager) getLayoutManager()).findFirstVisibleItemPosition();
    final int lastIndex = ((LinearLayoutManager) getLayoutManager()).findLastVisibleItemPosition();

    if (firstIndex != mFirstVisibleIndex || lastIndex != mLastVisibleIndex) {
        getReactContext().getNativeModule(UIManagerModule.class).getEventDispatcher()
                .dispatchEvent(new VisibleItemsChangeEvent(
                        getId(),
                        SystemClock.nanoTime(),
                        firstIndex,
                        lastIndex));

        mFirstVisibleIndex = firstIndex;
        mLastVisibleIndex = lastIndex;
    }
}