height为0, 可以自动计算weith; weith为0, 可以自动计算height, 两者不可以同时为0
NSDictionary *dic = @{NSFontAttributeName: [UIFont systemFontOfSize:17]};
CGRect rect = [joke.content boundingRectWithSize:CGSizeMake(375 - 20, 0) options:(NSStringDrawingUsesFontLeading | NSStringDrawingUsesLineFragmentOrigin) attributes:dic context:nil];