Recycleview滑动分析

发表于2021-06-22更新于2021-11-10

通过使用as的Profiler监听cpu的运行状况，我们可以更容易的了解到Recycleview滑动的执行过程。由下图可以看到，recycle view的滑动一次调用了：

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RecyclerView.onTouchEvent()
RecyclerView.scrollByInternal()
RecyclerView.scrollStep()
LinearLayoutManager.scrollVerticallyBy()
LinearLayoutManager.scrollBy()
OrientationHelper.offsetChildren()
LayoutManager.offsetChildrenVertical()
RecyclerView.offsetChildrenVertical()
View.offsetTopAndBottom()

img1

获取Recycleview滑了多少距离得一个好方式：

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public int getScollYDistance() {
    LinearLayoutManager layoutManager = (LinearLayoutManager) this.getLayoutManager();
    int position = layoutManager.findFirstVisibleItemPosition();
    View firstVisiableChildView = layoutManager.findViewByPosition(position);
    int itemHeight = firstVisiableChildView.getHeight();
    return (position) * itemHeight - firstVisiableChildView.getTop();
}
img1

scrollByInternal

scrollStep

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// Recycleview
void scrollStep(int dx, int dy, @Nullable int[] consumed) {
    // 设置mInterceptRequestLayoutDepth++
    startInterceptRequestLayout();
    // 设置mLayoutOrScrollCounter++
    onEnterLayoutOrScroll();

    TraceCompat.beginSection(TRACE_SCROLL_TAG);
    fillRemainingScrollValues(mState);

    int consumedX = 0;
    int consumedY = 0;
    if (dx != 0) {
        // consumedX代表这次recycleview滚动具体消耗调的滚动距离
        consumedX = mLayout.scrollHorizontallyBy(dx, mRecycler, mState);
    }
    if (dy != 0) {
        // consumedY代表这次recycleview滚动具体消耗调的滚动距离
        // 在自定义LayoutManager的时候scrollVerticallyBy这个方法一定是要我们重写的
        // 这是实现页面滚动的关键。这个方法的返回值也就是我们真正消耗掉的滚动距离
        // 在LinearlayoutManger里面，  
        consumedY = mLayout.scrollVerticallyBy(dy, mRecycler, mState);
    }

    TraceCompat.endSection();
    repositionShadowingViews();

    onExitLayoutOrScroll();
    stopInterceptRequestLayout(false);

    if (consumed != null) {
        consumed[0] = consumedX;
        consumed[1] = consumedY;
    }
}
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// LinearLayoutManager
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler,
            RecyclerView.State state) {
    if (mOrientation == HORIZONTAL) {
            return 0;
    }
    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;
    final int layoutDirection = delta > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START;
    final int absDelta = Math.abs(delta);
    updateLayoutState(layoutDirection, absDelta, true, state);
    // 这里执行了很关键的 f
    final int consumed = mLayoutState.mScrollingOffset
            + fill(recycler, mLayoutState, state, false);
    if (consumed < 0) {
    // consumed小于0代表我们的recycleview并没有消耗这次滑动
        return 0;
    }
    final int scrolled = absDelta > consumed ? layoutDirection * consumed : delta;
    // 这里会回调到recycleview里面执行offsetChildrenVertical方法。遍历全部的view执行offsetTopAndBottom方法。实现他们的位移变化。
    // 既然这里是做了位移变化，那上面的fill方法是在干嘛？
    mOrientationHelper.offsetChildren(-scrolled);
    mLayoutState.mLastScrollDelta = scrolled;
    return scrolled;
}