RecyclerView还能这样滚动对齐?

前言

RecyclerView要想滚动到指定position，一般有scrollToPosition()和smoothScrollToPosition()两种方式。滚动到指定position后，通常还会要求itemView对齐RecyclerView起始点、中心点或结束点

熟悉RecyclerView的人应该知道，使用自定义SmoothScroller可以实现平滑滚动到指定position的同时，让itemView和RecyclerView的对齐；而scrollToPosition()方法只能滚动到指定position。那有办法让scrollToPosition()也做到对齐吗？

拆解行为

分析对齐的行为后，可以分为几步

让目标itemView可见
计算itemView和目的位置的偏移量
将itemView移动到目的位置

第一步scrollToPosition()就已经可以实现了，最后一步就是调用scrollBy()，那其实只需要实现第二步计算偏移量，而这可以参考SmoothScroller的实现

平滑滚动

来看下SmoothScroller是怎么做的。通常做法都是自定义LinearSmoothScroller

java 复制代码

RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
int preference = LinearSmoothScroller.SNAP_TO_START;// 对齐方式
LinearSmoothScroller smoothScroller = new LinearSmoothScroller(context){
    @Override
    protected int getHorizontalSnapPreference() {
        return preference;
    }

    @Override
    protected int getVerticalSnapPreference() {
        return preference;
    }
};
smoothScroller.setTargetPosition(targetPosition);
layoutManager.startSmoothScroll(smoothScroller);

简单介绍下几种对齐方式

SNAP_TO_START：对齐RecyclerView起始位置
SNAP_TO_END：对齐RecyclerView结束位置
SNAP_TO_ANY：对齐RecyclerView任意位置，确保itemView在RecyclerView内

接下来看下getVerticalSnapPreference()或者getHorizontalSnapPreference()的返回值是怎么影响到itemView的对齐的。查看LinearSmoothScroller源码发现这两个方法会在onTargetFound()里调用

java 复制代码

protected void onTargetFound(View targetView, RecyclerView.State state, Action action) {
    final int dx = calculateDxToMakeVisible(targetView, getHorizontalSnapPreference());
    final int dy = calculateDyToMakeVisible(targetView, getVerticalSnapPreference());
    final int distance = (int) Math.sqrt(dx * dx + dy * dy);
    final int time = calculateTimeForDeceleration(distance);
    if (time > 0) {
        action.update(-dx, -dy, time, mDecelerateInterpolator);
    }
}

不难看出，该方法是计算targetView当前要滚动的偏移量和时长，并设置给action。而calculateDxToMakeVisible()和calculateDyToMakeVisible()正是我们要找的计算偏移量的方法

由于这两个方法只依赖LayoutManager，所以我们可以将这些代码逻辑复制出来，创建一个Rangefinder类，用于计算偏移量

java 复制代码

public class Rangefinder {
    private final RecyclerView.LayoutManager mLayoutManager;

    public Rangefinder(RecyclerView.LayoutManager layoutManager) {
        mLayoutManager = layoutManager;
    }

    @Nullable
    public RecyclerView.LayoutManager getLayoutManager() {
        return mLayoutManager;
    }

    // 计算view在RecyclerView中完全可见所需的垂直偏移量
    public int calculateDyToMakeVisible(View view, int snapPreference) {
        final RecyclerView.LayoutManager layoutManager = getLayoutManager();
        if (layoutManager == null || !layoutManager.canScrollVertically()) {
            return 0;
        }
        final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
                view.getLayoutParams();
        final int top = layoutManager.getDecoratedTop(view) - params.topMargin;
        final int bottom = layoutManager.getDecoratedBottom(view) + params.bottomMargin;
        final int start = layoutManager.getPaddingTop();
        final int end = layoutManager.getHeight() - layoutManager.getPaddingBottom();
        return calculateDtToFit(top, bottom, start, end, snapPreference);
    }

    // 计算view在RecyclerView中完全可见所需的水平偏移量
    public int calculateDxToMakeVisible(View view, int snapPreference) {
        final RecyclerView.LayoutManager layoutManager = getLayoutManager();
        if (layoutManager == null || !layoutManager.canScrollHorizontally()) {
            return 0;
        }
        final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams)
                view.getLayoutParams();
        final int left = layoutManager.getDecoratedLeft(view) - params.leftMargin;
        final int right = layoutManager.getDecoratedRight(view) + params.rightMargin;
        final int start = layoutManager.getPaddingLeft();
        final int end = layoutManager.getWidth() - layoutManager.getPaddingRight();
        return calculateDtToFit(left, right, start, end, snapPreference);
    }

    public int calculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd,
                                @SnapPreference int snapPreference) {
        switch (snapPreference) {
            case LinearSmoothScroller.SNAP_TO_START:
                return boxStart - viewStart;
            case LinearSmoothScroller.SNAP_TO_END:
                return boxEnd - viewEnd;
            case LinearSmoothScroller.SNAP_TO_ANY:
                final int dtStart = boxStart - viewStart;
                if (dtStart > 0) {
                    return dtStart;
                }
                final int dtEnd = boxEnd - viewEnd;
                if (dtEnd < 0) {
                    return dtEnd;
                }
                break;
        }
        return 0;
    }
}

有了计算偏移量的方法，接下来就是实现itemView的对齐了

即时滚动

根据上面的拆解步骤，再分析下每一步要做的事情

调用scrollToPosition()使目标itemView可见。因为该方法最终会requestLayout()，所以要在layout后，才能通过获取到itemView。那么可以post()后调用LayoutManager的findViewByPosition()方法获取itemView
参考LinearSmoothScroller的onTargetFound()方法，使用上面的Rangefinder计算itemView和目的位置的偏移量
调用scrollBy()将itemView移动到目的位置

java 复制代码

RecyclerView.LayoutManager layoutManager = recyclerView.getLayoutManager();
recyclerView.scrollToPosition(targetPosition);
recyclerView.post(new Runnable() {
    @Override
    public void run() {
        View targetView = layoutManager.findViewByPosition(targetPosition);
        if (targetView != null) {
            Rangefinder rangefinder = new Rangefinder(layoutManager);
            final int dx = rangefinder.calculateDxToMakeVisible(targetView, preference);
            final int dy = rangefinder.calculateDyToMakeVisible(targetView, preference);
            if (dx != 0 || dy != 0) {
                recyclerView.scrollBy(-dx, -dy);
            }
        }
    }
});

至此，我们就实现了即时滚动到position的同时，让itemView和RecyclerView对齐的功能。当然，这也只是测试代码，实际使用还会对上面的逻辑进行封装

测试代码 recyclerView-scroll-demo