有时我们需要对label进行自适应，就要实时的算出label的长度，即文字所占的长度：
+ (CGFloat)getWidthWithString:(NSString *)string font:(UIFont *)font{
NSDictionary *attrs = @{NSFontAttributeName:font};
CGFloat width = [string boundingRectWithSize:CGSizeZero options:NSStringDrawingUsesLineFragmentOrigin attributes:attrs context:nil].size.width;
return width;
}
