code: private Path gotLeftTopArrowPath() {
          Path path = new Path();
          int diameter = cSize * 2;
          // 左上角圆角起点
          path.moveTo(aWidth, cSize);
          RectF rectF = new RectF(aWidth, 0, aWidth + diameter, diameter);
          path.arcTo(rectF, 180, 90);
          // 右上角圆角起点
          path.lineTo(mWidth - cSize, 0);
          rectF = new RectF(mWidth - diameter, 0, mWidth, diameter);
          path.arcTo(rectF, -90, 90);
          // 右下角圆角起点
          path.lineTo(mWidth, mHeight - cSize);
          rectF = new RectF(mWidth - diameter, mHeight - diameter, mWidth, mHeight);
          path.arcTo(rectF, 0, 90);
          // 左下角圆角起点
          path.lineTo(aWidth + cSize, mHeight);
          rectF = new RectF(aWidth, mHeight - diameter, aWidth + diameter, mHeight);
          path.arcTo(rectF, 90, 90);
          // 箭头起点
          path.lineTo(aWidth, aMargin + aHeight);
          // 箭头顶点
          path.lineTo(0, aMargin + aHeight / 2);
          // 箭头结束点
          path.lineTo(aWidth, aMargin);
          path.close();
          return path;
      }
docs: 箭头在左侧上边