使用textRect（对于边界：limitedToNumberOfLines:）。

class TopAlignedLabel: UILabel {
  override func drawText(in rect: CGRect) {
    let textRect = super.textRect(forBounds: bounds, limitedToNumberOfLines: numberOfLines)
    super.drawText(in: textRect)
  }
}

设置新文本：myLabel.text=@“某些文本”将最大行数设置为0（自动）：myLabel.numberOfLines=0将标签的框架设置为最大大小：myLabel.frame=CGRectMake（20,20200800）调用sizeToFit以减小框架大小，使内容刚好适合：[myLabel sizeToFit]

标签框架现在刚好足够高和宽，以适合您的文本。左上角应保持不变。我只测试了左上对齐的文本。对于其他路线，您可能需要随后修改框架。

此外，我的标签启用了换行。

此代码帮助您使文本与顶部对齐，并使标签高度与文本内容固定。

使用以下代码的说明

默认情况下，isHeightToChange会自动将标签的高度设置为与文本内容相同。isHeightToChange如果为false，则使文本对齐到顶部，而不降低标签的高度。

#import "DynamicHeightLable.h"

@implementation DynamicHeightLable
@synthesize isHeightToChange;
- (id)initWithFrame:(CGRect)frame 
{
    self = [super initWithFrame:frame];
    if (self)
    {
        // Initialization code.
        self.isHeightToChange=FALSE;//default

    }
    return self;
}
- (void)drawTextInRect:(CGRect)rect
{
    if(!isHeightToChange){
    CGSize maximumLabelSize = CGSizeMake(self.frame.size.width,2500);

    CGFloat height = [self.text sizeWithFont:self.font
                           constrainedToSize:maximumLabelSize
                               lineBreakMode:self.lineBreakMode].height;
    if (self.numberOfLines != 0) {
        height = MIN(height, self.font.lineHeight * self.numberOfLines);
    }
    rect.size.height = MIN(rect.size.height, height);
    }
    [super drawTextInRect:rect];

}
- (void) layoutSubviews
{
    [super layoutSubviews];
       if(isHeightToChange){
     CGRect ltempFrame = self.frame;
    CGSize maximumLabelSize = CGSizeMake(self.frame.size.width,2500);

    CGFloat height = [self.text sizeWithFont:self.font
                           constrainedToSize:maximumLabelSize
                               lineBreakMode:self.lineBreakMode].height;
    if (self.numberOfLines != 0) {
        height = MIN(height, self.font.lineHeight * self.numberOfLines);
    }
    ltempFrame.size.height = MIN(ltempFrame.size.height, height);

    ltempFrame.size.height=ltempFrame.size.height;
    self.frame=ltempFrame;
       }
}
@end

对于Swift 3。。。

@IBDesignable class TopAlignedLabel: UILabel {
    override func drawText(in rect: CGRect) {
        if let stringText = text {
            let stringTextAsNSString = stringText as NSString
            let labelStringSize = stringTextAsNSString.boundingRect(with: CGSize(width: self.frame.width,height: CGFloat.greatestFiniteMagnitude),
                                                                            options: NSStringDrawingOptions.usesLineFragmentOrigin,
                                                                            attributes: [NSFontAttributeName: font],
                                                                            context: nil).size
            super.drawText(in: CGRect(x:0,y: 0,width: self.frame.width, height:ceil(labelStringSize.height)))
        } else {
            super.drawText(in: rect)
        }
    }
    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()
        layer.borderWidth = 1
        layer.borderColor = UIColor.black.cgColor
    }
}

一种更快（更脏）的方法是将UILabel的换行模式设置为“Clip”并添加固定数量的换行符。

myLabel.lineBreakMode = UILineBreakModeClip;
myLabel.text = [displayString stringByAppendingString:"\n\n\n\n"];

这种解决方案不适用于所有人——特别是，如果您仍然希望在字符串末尾显示“…”，如果它超过了您要显示的行数，则需要使用较长的代码段之一——但在很多情况下，这将满足您的需要。

在发布的所有其他解决方案的基础上，我创建了一个简单的小UILabel子类，在设置其对齐属性时将为您处理垂直对齐。这还将在方向更改时更新标签，将高度约束到文本，并将标签的宽度保持在其原始大小。

.h

@interface VAlignLabel : UILabel
@property (nonatomic, assign) WBZVerticalAlignment alignment;
@end


.m

 -(void)setAlignment:(WBZVerticalAlignment)alignment{

    _alignment = alignment;

    CGSize s = [self.text sizeWithFont:self.font constrainedToSize:CGSizeMake(self.frame.size.width, 9999) lineBreakMode:NSLineBreakByWordWrapping];

    switch (_alignment)
    {
        case wbzLabelAlignmentVerticallyTop:
            self.frame = CGRectMake(self.frame.origin.x, self.frame.origin.y, self.frame.size.width, s.height);
            break;
        case wbzLabelAlignmentVerticallyMiddle:
            self.frame = CGRectMake(self.frame.origin.x, self.frame.origin.y + (self.frame.size.height - s.height)/2, self.frame.size.width, s.height);
            break;
        case wbzLabelAlignmentVerticallyBottom:
            self.frame = CGRectMake(self.frame.origin.x, self.frame.origin.y + (self.frame.size.height - s.height), self.frame.size.width, s.height);
            break;
        default:
            break;
    }
}

-(void)layoutSubviews{
    [self setAlignment:self.alignment];
}