RecyclerView中ItemDecoration的精妙用法，实现自定义分隔线、边距和背景效果

ItemDecoration 是 RecyclerView 组件的一个非常有用的功能，用于添加自定义的装饰项（如分隔线、边距、背景等）到 RecyclerView 的每个 item 之间或周围。

recyclerView.addItemDecoration()

ItemDecoration主要的三个方法：

图片

• 图1：代表了getItemOffsets()，可以实现类似padding的效果。
• 图2：代表了onDraw()，可以实现类似绘制背景的效果，内容在上面。
• 图3：代表了onDrawOver()，可以绘制在内容的上面，覆盖内容。

分割线

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);
        }
    }
}

图片

标签

标签都是覆盖在内容之上的，可以用onDrawOver()来实现，这里简单实现一个颜色标签。

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);
            }
        }
    }
}

图片

ItemDecoration组合

ItemDecoration是可以叠加的，可以将多个效果通过addItemDecoration方法叠加，将上面两种效果叠加。

recyclerView.addItemDecoration(new LeftAndRightTagDecoration(this));
recyclerView.addItemDecoration(new SimpleDividerDecoration(this));

图片

Section分组

定义接口用来进行数据分组和获取首字母，重写getItemOffsets()和onDraw()方法，并根据数据进行分组处理。

public interface DecorationCallback {

        long getGroupId(int position);

        String getGroupFirstLine(int position);
    }

public class SectionDecoration extends RecyclerView.ItemDecoration {
    private static final String TAG = "SectionDecoration";

    private DecorationCallback callback;
    private TextPaint textPaint;
    private Paint paint;
    private int topGap;
    private Paint.FontMetrics fontMetrics;


    public SectionDecoration(Context context, DecorationCallback decorationCallback) {
        Resources res = context.getResources();
        this.callback = decorationCallback;

        paint = new Paint();
        paint.setColor(res.getColor(R.color.colorAccent));

        textPaint = new TextPaint();
        textPaint.setTypeface(Typeface.DEFAULT_BOLD);
        textPaint.setAntiAlias(true);
        textPaint.setTextSize(80);
        textPaint.setColor(Color.BLACK);
        textPaint.getFontMetrics(fontMetrics);
        textPaint.setTextAlign(Paint.Align.LEFT);
        fontMetrics = new Paint.FontMetrics();
        topGap = res.getDimensionPixelSize(R.dimen.sectioned_top);//32dp


    }


    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        int pos = parent.getChildAdapterPosition(view);
        Log.i(TAG, "getItemOffsets：" + pos);
        long groupId = callback.getGroupId(pos);
        if (groupId < 0) return;
        if (pos == 0 || isFirstInGroup(pos)) {//同组的第一个才添加padding
            outRect.top = topGap;
        } else {
            outRect.top = 0;
        }
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDraw(c, parent, state);
        int left = parent.getPaddingLeft();
        int right = parent.getWidth() - parent.getPaddingRight();
        int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            View view = parent.getChildAt(i);
            int position = parent.getChildAdapterPosition(view);
            long groupId = callback.getGroupId(position);
            if (groupId < 0) return;
            String textLine = callback.getGroupFirstLine(position).toUpperCase();
            if (position == 0 || isFirstInGroup(position)) {
                float top = view.getTop() - topGap;
                float bottom = view.getTop();
                c.drawRect(left, top, right, bottom, paint);//绘制红色矩形
                c.drawText(textLine, left, bottom, textPaint);//绘制文本
            }
        }
    }

    
    private boolean isFirstInGroup(int pos) {
        if (pos == 0) {
            return true;
        } else {
            long prevGroupId = callback.getGroupId(pos - 1);
            long groupId = callback.getGroupId(pos);
            return prevGroupId != groupId;
        }
    }

    public interface DecorationCallback {

        long getGroupId(int position);

        String getGroupFirstLine(int position);
    }
}

recyclerView.addItemDecoration(new SectionDecoration(this, new SectionDecoration.DecorationCallback() {
    @Override
    public long getGroupId(int position) {
        return Character.toUpperCase(dataList.get(position).getName().charAt(0));
    }

    @Override
    public String getGroupFirstLine(int position) {
        return dataList.get(position).getName().substring(0, 1).toUpperCase();
    }
}));

StickyHeader

头部吸顶效果，header不动肯定是要绘制item内容之上，需要重写onDrawOver()方法，其和Section实现一样。

public class PinnedSectionDecoration extends RecyclerView.ItemDecoration {
    private static final String TAG = "PinnedSectionDecoration";

    private DecorationCallback callback;
    private TextPaint textPaint;
    private Paint paint;
    private int topGap;
    private Paint.FontMetrics fontMetrics;


    public PinnedSectionDecoration(Context context, DecorationCallback decorationCallback) {
        Resources res = context.getResources();
        this.callback = decorationCallback;

        paint = new Paint();
        paint.setColor(res.getColor(R.color.colorAccent));

        textPaint = new TextPaint();
        textPaint.setTypeface(Typeface.DEFAULT_BOLD);
        textPaint.setAntiAlias(true);
        textPaint.setTextSize(80);
        textPaint.setColor(Color.BLACK);
        textPaint.getFontMetrics(fontMetrics);
        textPaint.setTextAlign(Paint.Align.LEFT);
        fontMetrics = new Paint.FontMetrics();
        topGap = res.getDimensionPixelSize(R.dimen.sectioned_top);


    }


    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        int pos = parent.getChildAdapterPosition(view);
        long groupId = callback.getGroupId(pos);
        if (groupId < 0) return;
        if (pos == 0 || isFirstInGroup(pos)) {
            outRect.top = topGap;
        } else {
            outRect.top = 0;
        }
    }


    @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);
        }
    }
}

图片