RecyclerView没有像之前ListView提供divider属性，而是提供了方法

recyclerView.addItemDecoration()

这时候我们需要去定制自己的ItemDecoration

ItemDecoration类主要是三个方法：

public void onDraw(Canvas c, RecyclerView parent, State state)
public void onDrawOver(Canvas c, RecyclerView parent, State state)
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)

• 重写getItemOffsets()，可以实现类似padding的效果
• 重写onDraw，可以实现类似绘制背景的效果，用来绘制分隔线
• 重写onDrawOver，可以在View上覆盖内容

padding

直接传递dp

class SpaceItemDecoration(
    private var left: Int = 0,
    private var top: Int = 0,
    private var right: Int = 0,
    private var bottom: Int = 0
) : RecyclerView.ItemDecoration() {

    override fun getItemOffsets(
        outRect: Rect,
        view: View,
        parent: RecyclerView,
        state: RecyclerView.State
    ) {
        super.getItemOffsets(outRect, view, parent, state)
        outRect.left = ScreenUtils.dpToPx(left.toFloat())
        outRect.right = ScreenUtils.dpToPx(right.toFloat())
        outRect.top = ScreenUtils.dpToPx(top.toFloat())
        outRect.bottom = ScreenUtils.dpToPx(bottom.toFloat())
    }
}

grid列表间隔设置

class GridSpaceItemDecoration(@Px val space: Int) : RecyclerView.ItemDecoration() {

    override fun getItemOffsets(
        outRect: Rect,
        view: View,
        parent: RecyclerView,
        state: RecyclerView.State
    ) {
        super.getItemOffsets(outRect, view, parent, state)
        if (parent.layoutManager is GridLayoutManager) {
            val spanCount = (parent.layoutManager as GridLayoutManager?)?.spanCount ?: return
            val layoutParams = view.layoutParams as GridLayoutManager.LayoutParams
            outRect.top = space / 2
            outRect.bottom = space / 2
            when (layoutParams.spanIndex) {
                0 -> {
                    //第一列左边全部 右边一半
                    outRect.left = space
                    outRect.right = space / 2
                }
                spanCount - 1 -> {
                    //最后一列左边一半右边全部
                    outRect.left = space / 2
                    outRect.right = space
                }
                else -> {
                    //中间列 左右各一半
                    outRect.right = space / 2
                    outRect.left = space / 2
                }
            }
        }
    }

}

分割线

要实现分割线效果需要 getItemOffsets()和 onDraw()2个方法，首先用 getItemOffsets给item下方空出一定高度的空间（例子中是1dp），然后用onDraw绘制这个空间

public class SimpleDividerDecoration extends RecyclerView.ItemDecoration {

    private int dividerHeight;
    private Paint dividerPaint;

    public SimpleDividerDecoration(Context context) {
        dividerPaint = new Paint();
        dividerPaint.setColor(context.getResources().getColor(R.color.colorAccent));
        dividerHeight = context.getResources().getDimensionPixelSize(R.dimen.divider_height);
    }


    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        outRect.bottom = dividerHeight;
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        int childCount = parent.getChildCount();
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();

        for (int i = 0; i < childCount - 1; i++) {
            View view = parent.getChildAt(i);
            float top = view.getBottom();
            float bottom = view.getBottom() + dividerHeight;
            c.drawRect(left, top, right, bottom, dividerPaint);
        }
    }
}

标签

使用onDrawOver()来实现“推荐”，”热卖“，“秒杀“

public class LeftAndRightTagDecoration extends RecyclerView.ItemDecoration {
    private int tagWidth;
    private Paint leftPaint;
    private Paint rightPaint;

    public LeftAndRightTagDecoration(Context context) {
        leftPaint = new Paint();
        leftPaint.setColor(context.getResources().getColor(R.color.colorAccent));
        rightPaint = new Paint();
        rightPaint.setColor(context.getResources().getColor(R.color.colorPrimary));
        tagWidth = context.getResources().getDimensionPixelSize(R.dimen.tag_width);
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);
        int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            View child = parent.getChildAt(i);
            int pos = parent.getChildAdapterPosition(child);
            boolean isLeft = pos % 2 == 0;
            if (isLeft) {
                float left = child.getLeft();
                float right = left + tagWidth;
                float top = child.getTop();
                float bottom = child.getBottom();
                c.drawRect(left, top, right, bottom, leftPaint);
            } else {
                float right = child.getRight();
                float left = right - tagWidth;
                float top = child.getTop();
                float bottom = child.getBottom();
                c.drawRect(left, top, right, bottom, rightPaint);

            }
        }
    }
}

分组、StickyHeader

   @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);
        int itemCount = state.getItemCount();
        int childCount = parent.getChildCount();
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();
        float lineHeight = textPaint.getTextSize() + fontMetrics.descent;

        long preGroupId, groupId = -1;
        for (int i = 0; i < childCount; i++) {
            View view = parent.getChildAt(i);
            int position = parent.getChildAdapterPosition(view);

            preGroupId = groupId;
            groupId = callback.getGroupId(position);
            if (groupId < 0 || groupId == preGroupId) continue;

            String textLine = callback.getGroupFirstLine(position).toUpperCase();
            if (TextUtils.isEmpty(textLine)) continue;

            int viewBottom = view.getBottom();
            float textY = Math.max(topGap, view.getTop());
            if (position + 1 < itemCount) { //下一个和当前不一样移动当前
                long nextGroupId = callback.getGroupId(position + 1);
                if (nextGroupId != groupId && viewBottom < textY ) {//组内最后一个view进入了header
                    textY = viewBottom;
                }
            }
            c.drawRect(left, textY - topGap, right, textY, paint);
            c.drawText(textLine, left, textY, textPaint);
        }

    }

自定义的功能就结束了，详细可了解RecyclerView之ItemDecoration由浅入深