RecyclerView滑动指定item到屏幕最顶部

一开始调用RecyclerView提供的方法

public void smoothScrollToPosition(int position) {
        if (mLayoutFrozen) {
            return;
        }
        if (mLayout == null) {
            Log.e(TAG, "Cannot smooth scroll without a LayoutManager set. "
                    + "Call setLayoutManager with a non-null argument.");
            return;
        }
        mLayout.smoothScrollToPosition(this, mState, position);
    }

发现当指定item滑动到屏幕可见时就停止了，即item可能位于屏幕顶部、中部或者底部。

由代码可以看出，RecyclerView的滑动方法是调用LayoutManager的smoothScrollToPosition方法

public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state,
            int position) {
        LinearSmoothScroller linearSmoothScroller =
                new LinearSmoothScroller(recyclerView.getContext());
        linearSmoothScroller.setTargetPosition(position);
        startSmoothScroll(linearSmoothScroller);
    }

其中LinearSmoothScroller提供了滑动策略

    public static final int SNAP_TO_START = -1;

    /**
     * Align child view's right or bottom with parent view's right or bottom
     *
     * @see #calculateDtToFit(int, int, int, int, int)
     * @see #calculateDxToMakeVisible(android.view.View, int)
     * @see #calculateDyToMakeVisible(android.view.View, int)
     */
    public static final int SNAP_TO_END = 1;

    /**
     * <p>Decides if the child should be snapped from start or end, depending on where it
     * currently is in relation to its parent.</p>
     * <p>For instance, if the view is virtually on the left of RecyclerView, using
     * {@code SNAP_TO_ANY} is the same as using {@code SNAP_TO_START}</p>
     *
     * @see #calculateDtToFit(int, int, int, int, int)
     * @see #calculateDxToMakeVisible(android.view.View, int)
     * @see #calculateDyToMakeVisible(android.view.View, int)
     */
    public static final int SNAP_TO_ANY = 0;

重写LinearSmoothScroller的getVerticalSnapPreference方法

class LinearTopSmoothScroller extends LinearSmoothScroller{

        public LinearTopSmoothScroller(Context context) {
            super(context);
        }

        @Override
        protected int getVerticalSnapPreference() {
            return SNAP_TO_START;
        }
    }

然后滑动的代码可以这样写

    void scrollItemToTop(int position){
        LinearSmoothScroller smoothScroller = new LinearTopSmoothScroller(this);
        smoothScroller.setTargetPosition(position);
        linearLayoutManager.startSmoothScroll(smoothScroller);
    }

完美解决。