RecyclerView滚动指定位置到屏幕中间

背景

接到个产品需求，需要使RecyclerView点击item后滚动到RecyclerView的中间位置

思路

通过自定义LayoutManager

1. 自定义SmoothScroller继承LinearSmoothScroller
2. 继承LinearLayoutManager重写smoothScrollToPosition调用自定义SmoothScroller
3. 搞清楚calculateDtToFit方法中int viewStart, int viewEnd, int boxStart, int boxEnd的含义计算滚动位置
   下面是摘取的部分LinearSmoothScroller的源码

//LinearSmoothScroller源码中调用calculateDtToFit方法的入参
//top = item view的顶部
final int top = layoutManager.getDecoratedTop(view) - params.topMargin;
//bottom = item view的底部
final int bottom = layoutManager.getDecoratedBottom(view) + params.bottomMargin;
//start = RecyclerView 的顶部
final int start = layoutManager.getPaddingTop();
//end = RecyclerView 的顶部
final int end = layoutManager.getHeight() - layoutManager.getPaddingBottom();
return calculateDtToFit(top, bottom, start, end, snapPreference);

实现

import android.content.Context;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.LinearSmoothScroller;
import android.support.v7.widget.RecyclerView;
import android.util.AttributeSet;
import android.util.DisplayMetrics;

public class CenterLinearLayoutManager extends LinearLayoutManager {

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

    public CenterLinearLayoutManager(Context context, int orientation, boolean reverseLayout) {
        super(context, orientation, reverseLayout);
    }

    public CenterLinearLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
        RecyclerView.SmoothScroller smoothScroller = new CenterSmoothScroller(recyclerView.getContext());
        smoothScroller.setTargetPosition(position);
        startSmoothScroll(smoothScroller);
    }

    private static class CenterSmoothScroller extends LinearSmoothScroller {

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

		//计算位置
        @Override
        public int calculateDtToFit(int viewStart, int viewEnd, int boxStart, int boxEnd, int snapPreference) {
            return (boxStart + (boxEnd - boxStart) / 2) - (viewStart + (viewEnd - viewStart) / 2);
        }

		//滚动速度
        @Override
        protected float calculateSpeedPerPixel(DisplayMetrics displayMetrics) {
            return 100f / displayMetrics.densityDpi;
        }

    }

}

使用

只需要切换RecyclerView的LayoutManager即可

mListView.setLayoutManager(new CenterLinearLayoutManager(getContext(), LinearLayoutManager.HORIZONTAL, false));