RecycleView设置顶部分割线（记录一个坑）

public class MyDividerItemDecoration extends RecyclerView.ItemDecoration {

    private Drawable mDivider;

    /**
     * Custom divider will be used
     */
    public MyDividerItemDecoration(Context context, int resId) {
        mDivider = ContextCompat.getDrawable(context, resId);
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, 
                               RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        //因为绘制了顶部分割线，因此需要将第一行的item下移相应的距离
        //这里要注意，判断该RecycleView是什么布局，如果是竖直方向上的线性布局（水平方向上的这里没有考虑）
        //，那么需要让第一个Item下移，移动的距离是分割线的高度，因为分割线会占据Item的空间
        //如果是网格布局，那么需要把第一行的所有Item都下移相应的高度

        if (layoutManager instanceof LinearLayoutManager) {
            if (parent.getChildAdapterPosition(view) == 0) {
                outRect.set(0, mDivider.getIntrinsicHeight(), 0, 0);
            }
        }
        if (layoutManager instanceof GridLayoutManager) {
            if (parent.getChildAdapterPosition(view) >= 0 && 
                parent.getChildAdapterPosition(view) < getSpanCount(parent)) {
                outRect.set(0, mDivider.getIntrinsicHeight(), 0, 0);
            }
        }
    }


    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {

        //绘制分割线
        int left = 0;
        int right = parent.getWidth();
        View child = parent.getChildAt(0);

        RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();

        //不可以直接设置top = 0；因为这样的话分隔线就不会跟着移动，因为top = 0,是绝对位置，
        //所以应该设置为子view的相对位置
        //这样才可以跟着滑动。
        //child的顶部坐标，减去设置的margin_top的值，再减去child为了给分割线腾出空间所下滑的高度，
        //这样分割线才会在顶部

        int top = child.getTop() - params.topMargin - mDivider.getIntrinsicHeight();
        int bottom;
        bottom = top + mDivider.getIntrinsicHeight();
        mDivider.setBounds(left, top, right, bottom);
        mDivider.draw(c);
    }

    private int getSpanCount(RecyclerView parent) {
        // 列数
        int spanCount = -1;
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) {
            spanCount = ((GridLayoutManager) layoutManager).getSpanCount();
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            spanCount = ((StaggeredGridLayoutManager) layoutManager)
                    .getSpanCount();
        } else if (layoutManager instanceof LinearLayoutManager) {
            spanCount = layoutManager.getItemCount();
        }
        return spanCount;
    }

<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <size android:height="10dp" />
    <solid android:color="@color/comic_gray_bg" />
</shape>