谢谢大家的帮助，这是我尝试的代码，这是为我工作

   UILabel *instructions = [[UILabel alloc]initWithFrame:CGRectMake(10, 225, 300, 180)];
   NSString *text = @"First take clear picture and then try to zoom in to fit the ";
   instructions.text = text;
   instructions.textAlignment = UITextAlignmentCenter;
   instructions.lineBreakMode = NSLineBreakByWordWrapping;
   [instructions setTextColor:[UIColor grayColor]];

   CGSize expectedLabelSize = [text sizeWithFont:instructions.font 
                                constrainedToSize:instructions.frame.size
                                    lineBreakMode:UILineBreakModeWordWrap];

    CGRect newFrame = instructions.frame;
    newFrame.size.height = expectedLabelSize.height;
    instructions.frame = newFrame;
    instructions.numberOfLines = 0;
    [instructions sizeToFit];
    [self addSubview:instructions];

我计算UILabel动态高度的方法。

    let width = ... //< width of this label 
    let text = ... //< display content

    label.numberOfLines = 0
    label.lineBreakMode = .byWordWrapping
    label.preferredMaxLayoutWidth = width

    // Font of this label.
    //label.font = UIFont.systemFont(ofSize: 17.0)
    // Compute intrinsicContentSize based on font, and preferredMaxLayoutWidth
    label.invalidateIntrinsicContentSize() 
    // Destination height
    let height = label.intrinsicContentSize.height

包装功能:

func computeHeight(text: String, width: CGFloat) -> CGFloat {
    // A dummy label in order to compute dynamic height.
    let label = UILabel()

    label.numberOfLines = 0
    label.lineBreakMode = .byWordWrapping
    label.font = UIFont.systemFont(ofSize: 17.0)

    label.preferredMaxLayoutWidth = width
    label.text = text
    label.invalidateIntrinsicContentSize()

    let height = label.intrinsicContentSize.height
    return height
}

这是使用Objective-c获取UILabel Height的一行代码:

labelObj.numberOfLines = 0;
CGSize neededSize = [labelObj sizeThatFits:CGSizeMake(screenWidth, CGFLOAT_MAX)];

使用.height你会得到标签的高度，如下所示:

neededSize.height

谢谢大家的帮助，这是我尝试的代码，这是为我工作

   UILabel *instructions = [[UILabel alloc]initWithFrame:CGRectMake(10, 225, 300, 180)];
   NSString *text = @"First take clear picture and then try to zoom in to fit the ";
   instructions.text = text;
   instructions.textAlignment = UITextAlignmentCenter;
   instructions.lineBreakMode = NSLineBreakByWordWrapping;
   [instructions setTextColor:[UIColor grayColor]];

   CGSize expectedLabelSize = [text sizeWithFont:instructions.font 
                                constrainedToSize:instructions.frame.size
                                    lineBreakMode:UILineBreakModeWordWrap];

    CGRect newFrame = instructions.frame;
    newFrame.size.height = expectedLabelSize.height;
    instructions.frame = newFrame;
    instructions.numberOfLines = 0;
    [instructions sizeToFit];
    [self addSubview:instructions];

一行是克里斯的答案是错的。

newFrame.size.height = maximumLabelSize.height;

应该是

newFrame.size.height = expectedLabelSize.height;

除此之外，这是正确的解决方案。

由于sizeWithFont已弃用，我使用这个代替。

这个得到标签特定属性。

-(CGFloat)heightForLabel:(UILabel *)label withText:(NSString *)text{

    NSAttributedString *attributedText = [[NSAttributedString alloc] initWithString:text attributes:@{NSFontAttributeName:label.font}];
    CGRect rect = [attributedText boundingRectWithSize:(CGSize){label.frame.size.width, CGFLOAT_MAX}
                                           options:NSStringDrawingUsesLineFragmentOrigin
                                           context:nil];

    return ceil(rect.size.height);
}