原文：RecyclerView之ItemDecoration由浅入深

一个RecyclerView使用ItemDecoration实现滑动悬停的效果

image.png

public class SectionDecoration extends RecyclerView.ItemDecoration {

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


    public SectionDecoration(Context context, DecorationCallback callback) {
        this.callback = callback;

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

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

        topGap = 100;

    }

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

        int index = parent.getChildAdapterPosition(view);
        long groupId = callback.getGroupId(index);
        if (groupId < 0) {
            return;
        }
        if (groupId == 0 || isFirstInGroup(index)) {
            //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 count = parent.getChildCount();
//        for (int i = 0; i < count; i++) {
//            View view = parent.getChildAt(i);
//            int index = parent.getChildAdapterPosition(view);
//            long groupId = callback.getGroupId(index);
//            if (groupId < 0) {
//                return;
//            }
//
//            String textLine = callback.getGroupFirstLine(index).toUpperCase();
//            if (index == 0 || isFirstInGroup(index)) {
//                int top = view.getTop() - topGap;
//                int bottom = view.getTop();
//                c.drawRect(left, top, right, bottom, paint);
//                c.drawText(textLine, left, bottom, textPaint);
//            }
//        }
    }

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

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

        int itemCount = state.getItemCount();
        int childCount = parent.getChildCount();
        float left = parent.getPaddingLeft();
        float right = parent.getWidth() - parent.getPaddingRight();
        float lineHeight = textPaint.getTextSize() + fontMetrics.descent;

        long lastGroupId, groupId = -1;

        for (int i = 0; i < childCount; i++) {
            View view = parent.getChildAt(i);
            int position = parent.getChildAdapterPosition(view);

            lastGroupId = groupId;
            groupId = callback.getGroupId(position);

            if (groupId < 0 || lastGroupId == groupId) {
                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 - 15, textPaint);

        }

    }
}

调用

   LinearLayoutManager manager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(manager);
        list = getList();
        adapter = new RecyclerViewAdapter(this, list);
        recyclerView.addItemDecoration(new SectionDecoration(this, new DecorationCallback() {
            @Override
            public long getGroupId(int position) {
                return Character.toUpperCase(list.get(position).getA().charAt(0));
            }

            @Override
            public String getGroupFirstLine(int position) {
                return list.get(position).getA().substring(0, 1).toUpperCase();
            }
        }));
        recyclerView.setAdapter(adapter);