UILabel+Extension.h

@interface UILabel (Calculate)

//根据宽度求高度  content 计算的内容  width 计算的宽度 font字体大小
+ (CGFloat)getLabelHeightWithText:(NSString *)text
                            width:(CGFloat)width
                             font: (CGFloat)font;

//根据高度度求宽度  text 计算的内容  Height 计算的高度 font字体大小
+ (CGFloat)getLabelWidthWithText:(NSString *)text
                          height:(CGFloat)height
                            font:(CGFloat)font;

@end

UILabel+Extension.m文件

@implementation UILabel (Calculate)

//根据宽度求高度  content 计算的内容  width 计算的宽度 font字体大小
+ (CGFloat)getLabelHeightWithText:(NSString *)text
                            width:(CGFloat)width
                             font:(CGFloat)font
{
    CGRect rect = [text boundingRectWithSize:CGSizeMake(width, MAXFLOAT)
                                     options:NSStringDrawingUsesLineFragmentOrigin
                                  attributes:@{NSFontAttributeName:[UIFont systemFontOfSize:font]}
                                     context:nil];
    
    return rect.size.height;
}
//根据高度度求宽度  text 计算的内容  Height 计算的高度 font字体大小
+ (CGFloat)getLabelWidthWithText:(NSString *)text
                          height:(CGFloat)height
                            font:(CGFloat)font
{
    CGRect rect = [text boundingRectWithSize:CGSizeMake(MAXFLOAT, height)
                                     options:NSStringDrawingUsesLineFragmentOrigin
                                  attributes:@{NSFontAttributeName:[UIFont systemFontOfSize:font]}
                                     context:nil];
    return rect.size.width;
}

@end

使用UILabel+Extension.h

    UIView *bgView = [[UIView alloc] init];
    [bgView mas_makeConstraints:^(MASConstraintMaker *make) {
        //make.width.mas_equalTo(200);
        make.height.mas_equalTo(20);
        make.top.mas_equalTo(bottomLine.mas_bottom).offset(40);
    }];
    
    [bgView layoutIfNeeded];
    
    CGFloat readLabWidth = [UILabel getLabelWidthWithText:readLab.text height:20 font:14];
    CGFloat protocolBtnWidth = [UILabel getLabelWidthWithText:[protocolBtn titleForState:UIControlStateNormal] height:20 font:14.0];
    CGFloat bgViewWidth = readLabWidth + protocolBtnWidth;
    [bgView mas_updateConstraints:^(MASConstraintMaker *make) {
        make.width.mas_equalTo(bgViewWidth);
        make.centerX.mas_equalTo(bottomLine);
    }];