计算文字的高度和宽度--以微博会话界面中用户名（userName）为例

// NOTE: All of the following methods will default to drawing on a baseline, limiting drawing to a single line.
// To correctly draw and size multi-line text, pass NSStringDrawingUsesLineFragmentOrigin in the options parameter.
@interface NSString (NSExtendedStringDrawing)
- (void)drawWithRect:(CGRect)rect options:(NSStringDrawingOptions)options attributes:(NSDictionary *)attributes context:(NSStringDrawingContext *)context NS_AVAILABLE_IOS(7_0);
- (CGRect)boundingRectWithSize:(CGSize)size options:(NSStringDrawingOptions)options attributes:(NSDictionary *)attributes context:(NSStringDrawingContext *)context NS_AVAILABLE_IOS(7_0);
@end