public class SectionDecoration extends RecyclerView.ItemDecoration {
    private List<NameBean> beanList;
    private TextPaint textPaint;
    private Paint paint;
    private int topGap;
    private int alignBottom;
    private DecorationCallback callback;
    private Paint.FontMetrics fontMetrics
 
;
    interface DecorationCallback{
        String getGroupId(int position);
String getGroupFirstLine(int position);
}
    private boolean isFirstInGroup(int pos){
        if (pos==0){
            return  true;
}else {
             //因為是根據字串的內容的相同與否來判斷是不是同意組的，所以此處的標記id要是String型別
            //如果只做聯絡人列表懸浮框裡面顯示的是字母則標記id直接用int型別就行了
 
String prevGroupId=callback.getGroupId(pos-1);
String groupId=callback.getGroupId(pos);
//判斷前一個字串與當前字串是否相同
if(prevGroupId.equals(groupId)){
                return false;
}else {
                return true;
}
        }

    }
    public SectionDecoration(List<NameBean> beanList, Context context,
 
DecorationCallback callback ){
        Resources resources=context.getResources();
        this.beanList=beanList;
        this.callback=callback;
//設定懸浮欄的畫筆
paint=new Paint();
paint.setColor(ContextCompat.getColor(context,R.color.colorPrimary));
//設定懸浮欄中文字的畫筆
textPaint=new TextPaint();
textPaint.setAntiAlias(true);
textPaint.setTextSize(DensityUtil.dip2px(context,12));
textPaint.setTextAlign(Paint.Align.LEFT);
textPaint.setColor(Color.WHITE);
fontMetrics=new Paint.FontMetrics();
//決定懸浮欄的高度等
topGap=resources.getDimensionPixelSize(R.dimen.sectioned_top);
//決定文字的顯示位置等
alignBottom=resources.getDimensionPixelSize(R.dimen.sectioned_alignBottom);
}

    @Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);
        int pos=parent.getChildAdapterPosition(view);
String groupId=callback.getGroupId(pos);
        if (groupId.equals("-1"))return;
//只有是同一組的第一個才顯示懸浮欄
if(pos==0 || isFirstInGroup(pos)){
            outRect.top=topGap;
            if (beanList.get(pos).getName()==""){
                outRect.top=0;
}
        }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);
String groupId=callback.getGroupId(position);
            if (groupId.equals("-1"))return;
String textLine=callback.getGroupFirstLine(position).toUpperCase();
            if (textLine==""){
                float top=view.getTop();
                float bottom=view.getTop();
c.drawRect(left,top,right,bottom,paint);
                return;
}else {
                if (position==0|| isFirstInGroup(position)){
                    float top=view.getTop()-topGap;
                    float bottom=view.getTop();
//繪製懸浮欄
c.drawRect(left,top-topGap,right,bottom,paint);
//繪製文字
c.drawText(textLine,left,bottom,textPaint);
}

            }
        }
    }

    @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();
String preGroupId="";
String 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.equals("-1") || groupId.equals(preGroupId))continue;
String textLine=callback.getGroupFirstLine(position).toUpperCase();
            if (TextUtils.isEmpty(textLine))continue;
            int viewBottom=view.getBottom();
            float textY=Math.max(topGap,view.getTop());
//下一個和當前不一樣移動當前
if (position+1<itemCount){
                String nextGroupId=callback.getGroupId(position+1);
//組內最後一個view進入了header
if (nextGroupId!=groupId&&viewBottom<textY){
                    textY=viewBottom;
}
            }
            //textY-toGap決定了懸浮欄繪製的高度和位置
c.drawRect(left,textY-topGap,right,textY,paint);
//left+2*alignbottom決定了文字往右偏移的多少
c.drawText(textLine,left+2*alignBottom,textY-alignBottom,textPaint);
}
    }


}