private Paint mLabelTextPaint;
private Paint mFloatCirclePaint;
private Paint mLabelAreaPaint;
private Paint mFloatCircleTextPaint;

private Drawable mItemDivider;
private Drawable mItemGap;

public RecyclerViewDecoration(Context context, ArrayList<ItemInfo> itemInfos) {
    this.mItemInfos = itemInfos;

    mItemMargin = context.getResources().getDimensionPixelSize(R.dimen.item_margin);
    mItemLabelHeight = context.getResources().getDimensionPixelSize(R.dimen.item_label_height);
    mItemDividerHeight = context.getResources().getDimensionPixelSize(R.dimen.item_divider_height);
    mItemLabelTextTopPadding = context.getResources().getDimensionPixelSize(R.dimen.item_label_text_top_padding);
    mItemGapHeight = context.getResources().getDimensionPixelSize(R.dimen.item_gap_height);

    mItemLabelTextSize = context.getResources().getDimensionPixelSize(R.dimen.item_label_text_size);
    mLabelTextColor = context.getResources().getColor(R.color.labelTextColor);
    mFloatCircleColor = context.getResources().getColor(R.color.itemGapColor);

    mFloatCircleRadius = 3 * mItemLabelTextSize / 2;

    mLabelTextPaint = new Paint();
    mLabelTextPaint.setColor(mLabelTextColor);
    mLabelTextPaint.setTypeface(Typeface.DEFAULT_BOLD);
    mLabelTextPaint.setTextSize(mItemLabelTextSize);

    mLabelAreaPaint = new Paint();
    mLabelAreaPaint.setColor(Color.WHITE);

    mFloatCirclePaint = new Paint();
    mFloatCirclePaint.setColor(mFloatCircleColor);


    mFloatCircleTextPaint = new Paint();
    mFloatCircleTextPaint.setColor(Color.WHITE);
    mFloatCircleTextPaint.setTypeface(Typeface.DEFAULT_BOLD);
    mFloatCircleTextPaint.setTextSize(2 * mItemLabelTextSize);

    if (context instanceof Activity) {
        Activity activity = (Activity) context;
        mItemDivider = activity.getResources().getDrawable(R.drawable.recycler_view_divider, null);
        mItemGap = activity.getResources().getDrawable(R.drawable.recycler_view_gap, null);
    }
}

private void drawGroupViewReverse(Canvas c, RecyclerView parent) {
    int childCount = parent.getChildCount();
    int itemCount = mItemInfos.size();
    //Log.d("lk", "drawGroupView childCount =  " + childCount);
    for (int i = 0; i < childCount; i++) {
        View itemView = parent.getChildAt(i);
        int j = parent.getChildAdapterPosition(itemView);
        ItemInfo itemInfo = mItemInfos.get(j);
        //第0个位置不需要绘制
        if (j < itemCount - 1) {
            ItemInfo nextItemInfo = mItemInfos.get(j + 1);
            if (nextItemInfo.getAppNameInPinyin().charAt(0) != itemInfo.getAppNameInPinyin().charAt(0)) {
                //需要绘item上方item gap、label title及divider分隔线
                if ((nextItemInfo.getAppNameInPinyin().charAt(0) >= 'a' && nextItemInfo.getAppNameInPinyin().charAt(0) <= 'z')
                        || (itemInfo.getAppNameInPinyin().charAt(0) >= 'a' && itemInfo.getAppNameInPinyin().charAt(0) <= 'z')) {
                    mItemGap.setBounds(0, itemView.getBottom() + mItemLabelHeight + mItemDividerHeight,
                            parent.getWidth(), itemView.getBottom() + mItemLabelHeight + mItemDividerHeight + mItemGapHeight);
                    mItemGap.draw(c);
                    mLabelTextPaint.setAntiAlias(true);
                    char labelTitle = Character.toUpperCase(itemInfo.getAppNameInPinyin().charAt(0));
                    if (!Character.isLetter(labelTitle)) {
                        labelTitle = '#';
                    }
                    c.drawText("" + labelTitle, mItemMargin,
                            itemView.getBottom() + mItemDividerHeight + mItemLabelTextTopPadding, mLabelTextPaint);
                    mLabelTextPaint.setAntiAlias(false);
                    mItemDivider.setBounds(mItemMargin, itemView.getBottom(), itemView.getWidth() + mItemMargin, itemView.getBottom() + mItemDividerHeight);
                    mItemDivider.draw(c);
                }
            } else {
                //仅绘item下方的divider分隔线
                mItemDivider.setBounds(mItemMargin, itemView.getBottom(), itemView.getWidth() + mItemMargin, itemView.getBottom() + mItemDividerHeight);
                mItemDivider.draw(c);
            }
        } else if (j == itemCount - 1) {
            mItemGap.setBounds(0, itemView.getBottom() + mItemLabelHeight + mItemDividerHeight,
                    parent.getWidth(), itemView.getBottom() + mItemLabelHeight + mItemDividerHeight + mItemGapHeight);
            mItemGap.draw(c);
            mLabelTextPaint.setAntiAlias(true);
            char labelTitle = Character.toUpperCase(itemInfo.getAppNameInPinyin().charAt(0));
            if (!Character.isLetter(labelTitle)) {
                labelTitle = '#';
            }
            c.drawText("" + labelTitle, mItemMargin,
                    itemView.getBottom() + mItemDividerHeight + mItemLabelTextTopPadding, mLabelTextPaint);
            mLabelTextPaint.setAntiAlias(false);
            mItemDivider.setBounds(mItemMargin, itemView.getBottom(), itemView.getWidth() + mItemMargin, itemView.getBottom() + mItemDividerHeight);
            mItemDivider.draw(c);
        }
    }
}

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

private void drawFlaotView(Canvas c, RecyclerView parent) {
    int lastChildIndex = parent.getChildCount() - 1;
    View lastChild = parent.getChildAt(lastChildIndex);
    ItemInfo lastItemInfo = mItemInfos.get(parent.getChildAdapterPosition(lastChild));
    char floatLabelTitle = Character.toUpperCase(lastItemInfo.getAppNameInPinyin().charAt(0));
    int floatLabelTranslationY = 0;
    int screenHeight = Utilities.getDisplayMetrics(parent.getContext()).heightPixels;
    char floatCircleTitle = lastItemInfo.getAppNameInDefault().charAt(0);
    int childCount = parent.getChildCount();
    //screenHeight = screenHeight - 75;
    for (int i = 0; i < parent.getChildCount(); i++) {
        View childView = parent.getChildAt(i);
        int adapterPosition = parent.getChildAdapterPosition(childView);
        if (adapterPosition == mItemInfos.size() - 1) {
            continue;
        }
        //找到当前recyclerView中第一个带有Label的Item
        if (parent.findChildViewUnder(mItemMargin + childView.getWidth() / 2,
                childView.getBottom() + mItemDividerHeight + mItemLabelHeight / 2) == null) {
            //第二个Label将第一个Label顶上去的情况
            if ((childView.getBottom() > screenHeight - 75 - 2 * mItemLabelHeight - mItemDividerHeight)
                    && childView.getBottom() < screenHeight - 75) {
                floatLabelTitle = Character.toUpperCase(mItemInfos.get(adapterPosition).getAppNameInPinyin().charAt(0));
                floatLabelTranslationY = 2 * mItemLabelHeight - (screenHeight - childView.getBottom());
                Log.d("lk", "floatLabelTranslationY = " + floatLabelTranslationY +
                        ",bottom = " + childView.getBottom() + ",title = " + mItemInfos.get(adapterPosition).getAppNameInDefault());
                break;
            }
        }
    }
    c.drawRect(new Rect(0, screenHeight - mItemLabelHeight - 75 + floatLabelTranslationY,
            parent.getWidth(), screenHeight - 75 + floatLabelTranslationY), mLabelAreaPaint);
    mItemDivider.setBounds(0, screenHeight - mItemLabelHeight - mItemDividerHeight - 75 + floatLabelTranslationY,
            parent.getWidth(), screenHeight - mItemLabelHeight - 75);
    mItemDivider.draw(c);
    mLabelTextPaint.setAntiAlias(true);
    if (!Character.isLetter(floatLabelTitle)) {
        floatLabelTitle = '#';
    }
    c.drawText("" + floatLabelTitle, mItemMargin, mItemLabelTextTopPadding + screenHeight - mItemLabelHeight - 75 + floatLabelTranslationY, mLabelTextPaint);
    mLabelTextPaint.setAntiAlias(false);

    /*if (parent.getScrollState() != RecyclerView.SCROLL_STATE_IDLE) {
        mFloatCirclePaint.setAntiAlias(true);
        c.drawCircle(Utilities.getDisplayMetrics(parent.getContext()).widthPixels / 2,
                Utilities.getDisplayMetrics(parent.getContext()).heightPixels / 2, mFloatCircleRadius, mFloatCirclePaint);
        mFloatCirclePaint.setAntiAlias(false);
        mFloatCircleTextPaint.setAntiAlias(true);
        //设置文本水平左右居中
        mFloatCircleTextPaint.setTextAlign(Paint.Align.CENTER);
        Paint.FontMetrics fm = mFloatCircleTextPaint.getFontMetrics();
        float y = Utilities.getDisplayMetrics(parent.getContext()).heightPixels / 2 - fm.descent + (fm.bottom - fm.top) / 2;
        c.drawText("" + floatCircleTitle, Utilities.getDisplayMetrics(parent.getContext()).widthPixels / 2,
                y, mFloatCircleTextPaint);
        mFloatCircleTextPaint.setAntiAlias(false);
    }*/
}

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

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    super.getItemOffsets(outRect, view, parent, state);
    int count = mItemInfos.size();
    int childPosition = parent.getChildAdapterPosition(view);
    ItemInfo itemInfo = mItemInfos.get(childPosition);
    if (childPosition == count - 1) {
        outRect.set(0, 0, 0, mItemLabelHeight + mItemDividerHeight);
    } else {
        ItemInfo nextItemInfo = mItemInfos.get(childPosition + 1);

        if (nextItemInfo.getAppNameInPinyin().charAt(0) != itemInfo.getAppNameInPinyin().charAt(0)) {
            if ((nextItemInfo.getAppNameInPinyin().charAt(0) >= 'a' && nextItemInfo.getAppNameInPinyin().charAt(0) <= 'z')
                    || (itemInfo.getAppNameInPinyin().charAt(0) >= 'a' && itemInfo.getAppNameInPinyin().charAt(0) <= 'z')) {
                outRect.set(0, 0, 0, mItemGapHeight + mItemLabelHeight + mItemDividerHeight);
            }
        } else {
            outRect.set(0, 0, 0, mItemDividerHeight);
        }
    }
}