typedef NS_OPTIONS(NSInteger, NSStringDrawingOptions) {  

            NSStringDrawingUsesLineFragmentOrigin =  << ,

            // 整个文本将以每行组成的矩形为单位计算整个文本的尺寸

            // The specified origin is the line fragment origin, not the base line origin  

            NSStringDrawingUsesFontLeading =  << ,

            // 使用字体的行间距来计算文本占用的范围，即每一行的底部到下一行的底部的距离计算

            // Uses the font leading for calculating line heights  

            NSStringDrawingUsesDeviceMetrics =  << ,

            // 将文字以图像符号计算文本占用范围，而不是以字符计算。也即是以每一个字体所占用的空间来计算文本范围

            // Uses image glyph bounds instead of typographic bounds  

            NSStringDrawingTruncatesLastVisibleLine

            // 当文本不能适合的放进指定的边界之内，则自动在最后一行添加省略符号。如果NSStringDrawingUsesLineFragmentOrigin没有设置，则该选项不生效

            // Truncates and adds the ellipsis character to the last visible line if the text doesn't fit into the bounds specified. Ignored if NSStringDrawingUsesLineFragmentOrigin is not also set.  

        }