一篇文章全面 & 解析RecyclerView.ItemDeco

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