给RecyclerView添加分割线的步骤

【吐槽】RecyclerView没有提供分割线的方法，想要加个线还要自己画，点击事件的监听都要自己实现，不过真的好用。

给RecyclerView添加分割线的步骤

1, 新建类继承于RecyclerView.ItemDecoration，此为是抽象类:

    public static abstract class ItemDecoration {

    public void onDraw(Canvas c, RecyclerView parent, State state) {
        onDraw(c, parent);
    }

    public void onDrawOver(Canvas c, RecyclerView parent, State state) {
        onDrawOver(c, parent);
    }

    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
        getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
                parent);
    }

}

“onDraw和onDrawOver,显然，这两个方法是用于绘制的，那么绘制分割线的逻辑可以放在这里面，它们二者的具体区别是：onDraw是在item view绘制之前调用，而onDrawOver是在item view绘制之后调用，因此我们一般选择重写其中一个方法即可。getItemOffsets，这个方法是告诉RecyclerView在绘制完一个item view的时候，应该留下多少空位，以便于绘制分割线。”

好像逻辑也不难，但是真的好麻烦，比如我要给我的瀑布流布局加一条系统自带的分割线，系统自带的就行，那我要新建一个实现类：

public class DividerItemDecoration extends RecyclerView.ItemDecoration {
        //使用系统自带的listDivider
        private static final int[] ATTRS = new int[]{
                android.R.attr.listDivider
        };

        private Drawable mDivider;

        public DividerItemDecoration(Context context) {
            super();
            // 从TypedArray中得到一个Drawable对象
            final TypedArray typedArray = context.obtainStyledAttributes(ATTRS);
            mDivider = typedArray.getDrawable(0);

            typedArray.recycle();
        }

        @Override
        public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
            super.onDraw(c, parent, state);
            drawDivider(c, parent);
        }

        /**
         * 遍历childView，为每一个childView描绘divider
         * @param c 画布对象
         * @param parent 父控件即RecyclerView
         */
        private void drawDivider(Canvas c, RecyclerView parent) {
            //获取分割线的上边距，即RecyclerView的padding值
            final int top = parent.getPaddingTop();

            //分割线下边距
            final int bottom = parent.getHeight() - parent.getPaddingBottom();

            final int childCount = parent.getChildCount();

            //遍历所有item view，为它们的右边方绘制分割线，就是计算出上下左右四个值画一个矩形
            for (int i = 0; i < childCount; i++) {
                final View child = parent.getChildAt(i);
                final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();
                final int left = child.getRight() + params.rightMargin;
                final int right = left + mDivider.getIntrinsicWidth();
                //画右边的divider
                mDivider.setBounds(left, top, right, bottom);
                mDivider.draw(c);

                //画完右边画底边
                mDivider.setBounds(child.getLeft() + child.getPaddingLeft(), child.getBottom() + params.bottomMargin,
                        child.getRight() - child.getPaddingRight(), child.getBottom() + mDivider.getIntrinsicHeight());
                mDivider.draw(c);
            }
        }

        @Override
        public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
            super.getItemOffsets(outRect, view, parent, state);
            outRect.set(0, 0, mDivider.getIntrinsicHeight(), mDivider.getIntrinsicHeight());
        }
    }

2, 为RecyclerView添加写好的ItemDecoration即可

mRecyclerView.addItemDecoration(new DividerItemDecoration(this));

3, 没有3。

其实从 RecyclerView.ItemDecoration 的名字就看得出来，它是RecyclerView的item的装饰品，也就是说，除了画分割线，想画什么都是可以