您可以通过代码或简单地在故事板上执行以下操作来解决此问题：

将UILabel的行数设置为0。将UILabel嵌入UIView中。设置UIView约束（顶部、右侧、左侧和底部），以占用UILabel应增长到的最大空间。然后将UILabel设置为UIView的顶部、右侧和左侧，并将约束设置为底部，距离>=0。

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

Swift 2.0版本：

在空Swift文件中生成常量枚举值。

//  AppRef.swift

import UIKit
import Foundation

enum UILabelTextPositions : String {

 case VERTICAL_ALIGNMENT_TOP = "VerticalAlignmentTop"
 case VERTICAL_ALIGNMENT_MIDDLE = "VerticalAlignmentMiddle"
 case VERTICAL_ALIGNMENT_BOTTOM = "VerticalAlignmentBottom"

}

使用UILabel扩展：

创建一个空的Swift类并命名它

//  AppExtensions.swift

import Foundation
import UIKit

extension UILabel{ 
 func makeLabelTextPosition (sampleLabel :UILabel?, positionIdentifier : String) -> UILabel
 {
  let rect = sampleLabel!.textRectForBounds(bounds, limitedToNumberOfLines: 0)

  switch positionIdentifier
  {
  case "VerticalAlignmentTop":
   sampleLabel!.frame = CGRectMake(bounds.origin.x+5, bounds.origin.y, rect.size.width, rect.size.height)
   break;

  case "VerticalAlignmentMiddle":
   sampleLabel!.frame = CGRectMake(bounds.origin.x+5,bounds.origin.y + (bounds.size.height - rect.size.height) / 2,
    rect.size.width, rect.size.height);
   break;

  case "VerticalAlignmentBottom":
   sampleLabel!.frame = CGRectMake(bounds.origin.x+5, bounds.origin.y + (bounds.size.height - rect.size.height),rect.size.width, rect.size.height);
   break;

  default:
   sampleLabel!.frame = bounds;
   break;
  }
  return sampleLabel!

 }
}

用法：

myMessageLabel.makeLabelTextPosition(messageLabel, positionIdentifier: UILabelTextPositions.VERTICAL_ALIGNMENT_TOP.rawValue)

Swift 4.0中有一个解决方案：

func viewDidLoad() {

    super.viewDidLoad()

    // 20 point top and left margin. Sized to leave 20 pt at right.
    let labelFrame = CGRect(x: 20, y: 20, width: 280, height: 150)
    let myLabel = UILabel(frame: labelFrame)
    myLabel.backgroundColor = UIColor.orange

    let labelText = "I am the very model of a modern Major-General, 
    I've information vegetable, animal, and mineral"
    myLabel.text = labelText

    // Tell the label to use an unlimited number of lines
    myLabel.numberOfLines = 0
    myLabel.sizeToFit()

    view.addSubview(myLabel) // add label
}

使用故事板最简单的方法：

在StackView中嵌入标签，并在属性检查器中设置StackView的以下两个属性：

1-轴到水平，

2-与顶部对齐

创建UILabel的子类。就像一个魅力：

// TopLeftLabel.h

#import <Foundation/Foundation.h>

@interface TopLeftLabel : UILabel 
{
}

@end

// TopLeftLabel.m

#import "TopLeftLabel.h"

@implementation TopLeftLabel

- (id)initWithFrame:(CGRect)frame 
{
    return [super initWithFrame:frame];
}

- (CGRect)textRectForBounds:(CGRect)bounds limitedToNumberOfLines:(NSInteger)numberOfLines 
{
    CGRect textRect = [super textRectForBounds:bounds limitedToNumberOfLines:numberOfLines];    
    textRect.origin.y = bounds.origin.y;
    return textRect;
}

-(void)drawTextInRect:(CGRect)requestedRect 
{
    CGRect actualRect = [self textRectForBounds:requestedRect limitedToNumberOfLines:self.numberOfLines];
    [super drawTextInRect:actualRect];
}

@end

正如这里所讨论的。