根据字体计算CGRect

    UILabel *label = [[UILabel alloc]init];
    label.numberOfLines = 0;//多行显示
    label.backgroundColor = [UIColor yellowColor];
    label.font = [UIFont systemFontOfSize:20];
    NSString *string = @"我爱中国我爱中国我爱中国我爱中国我爱中国我爱中国我爱中国我爱中国我爱中国我爱中国我爱中国我爱中国";
    UIFont *font = [UIFont systemFontOfSize:20];
    NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc] init];
    paragraphStyle.lineSpacing = 20;//行间距
    NSDictionary *attributes = @{
                                 NSFontAttributeName:font,
                                 NSParagraphStyleAttributeName:paragraphStyle
                                 };//其他属性可以在UIKit的第一个头文件中查看，颜色。。
    CGRect size = [string boundingRectWithSize:CGSizeMake(200, 10000)
                                       options:NSStringDrawingUsesLineFragmentOrigin
                                    attributes:attributes
                                       context:nil];//iOS 7.0有效
    NSAttributedString *attributeString =  [[NSAttributedString alloc]initWithString:string attributes:attributes];//设置属性字体

    label.frame = size;
    label.attributedText = attributeString;
    label.center = self.view.center;
    [self.view addSubview:label];

封装的方法：

- (CGSize)textSizeWithFont:(UIFont *)font constrainedToSize:(CGSize)size lineBreakMode:(NSLineBreakMode)lineBreakMode
{
    CGSize textSize;
    if ([XWCHelper isIOS7orHigher]) {

        if (CGSizeEqualToSize(size, CGSizeZero)) {

            textSize =  [self sizeWithAttributes:@{NSFontAttributeName:font}];
        }
        else{

            NSStringDrawingOptions option = NSStringDrawingUsesLineFragmentOrigin;
            //NSStringDrawingTruncatesLastVisibleLine如果文本内容超出指定的矩形限制，文本将被截去并在最后一个字符后加上省略号。 如果指定了NSStringDrawingUsesLineFragmentOrigin选项，则该选项被忽略 NSStringDrawingUsesFontLeading计算行高时使用行间距。（译者注：字体大小+行间距=行高）
            NSDictionary *attributes = [NSDictionary dictionaryWithObject:font forKey:NSFontAttributeName];
            CGRect rect = [self boundingRectWithSize:size
                                             options:option
                                          attributes:attributes
                                             context:nil];
            textSize = rect.size;
        }

    }
    else{

        if (CGSizeEqualToSize(size, CGSizeZero)) {

            textSize =[self sizeWithFont:font];
        }
        else{

            textSize =[self  sizeWithFont:font constrainedToSize:size lineBreakMode:lineBreakMode];
        }
    }
    return textSize;
}