NSStringDrawingOptions 讲解

typedef NS_OPTIONS(NSInteger, NSStringDrawingOptions) {  
            NSStringDrawingUsesLineFragmentOrigin = 1 << 0,  
            // 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 = 1 << 1,  
            // 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 = 1 << 3,  
            // 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.  
        }