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

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

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

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

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

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

参考

• 
  
• Recyclerview 滑动相关功能总结
  
• RecyclerView 指定 Item 滚动居中显示