我发现这个问题的答案现在有点过时了，所以为汽车布局爱好者添加这个。

自动布局使这个问题变得非常微不足道。假设我们要将标签添加到UIView*视图中，以下代码将完成此操作：

UILabel *label = [[UILabel alloc] initWithFrame:CGRectZero];
[label setText:@"Some text here"];
[label setTranslatesAutoresizingMaskIntoConstraints:NO];
[view addSubview:label];

[view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[label]|" options:0 metrics:nil views:@{@"label": label}]];
[view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[label]" options:0 metrics:nil views:@{@"label": label}]];

标签的高度将自动计算（使用其intrinsicContentSize），标签将在视图顶部水平放置。

我发现这个问题的答案现在有点过时了，所以为汽车布局爱好者添加这个。

自动布局使这个问题变得非常微不足道。假设我们要将标签添加到UIView*视图中，以下代码将完成此操作：

UILabel *label = [[UILabel alloc] initWithFrame:CGRectZero];
[label setText:@"Some text here"];
[label setTranslatesAutoresizingMaskIntoConstraints:NO];
[view addSubview:label];

[view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[label]|" options:0 metrics:nil views:@{@"label": label}]];
[view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[label]" options:0 metrics:nil views:@{@"label": label}]];

标签的高度将自动计算（使用其intrinsicContentSize），标签将在视图顶部水平放置。

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)
  }
}

参考扩展解决方案：

for(int i=1; i< newLinesToPad; i++) 
    self.text = [self.text stringByAppendingString:@"\n"];

应替换为

for(int i=0; i<newLinesToPad; i++)
    self.text = [self.text stringByAppendingString:@"\n "];

每增加一行换行符都需要额外的空间，因为iPhone UILabels的尾部回车符似乎被忽略了：(

同样，alignBottom也应该用@“\n@%”代替“\n@”进行更新（对于周期初始化，也必须用“for（int i=0…”替换）。

以下扩展适用于我：

// -- file: UILabel+VerticalAlign.h
#pragma mark VerticalAlign
@interface UILabel (VerticalAlign)
- (void)alignTop;
- (void)alignBottom;
@end

// -- file: UILabel+VerticalAlign.m
@implementation UILabel (VerticalAlign)
- (void)alignTop {
    CGSize fontSize = [self.text sizeWithFont:self.font];
    double finalHeight = fontSize.height * self.numberOfLines;
    double finalWidth = self.frame.size.width;    //expected width of label
    CGSize theStringSize = [self.text sizeWithFont:self.font constrainedToSize:CGSizeMake(finalWidth, finalHeight) lineBreakMode:self.lineBreakMode];
    int newLinesToPad = (finalHeight  - theStringSize.height) / fontSize.height;
    for(int i=0; i<newLinesToPad; i++)
        self.text = [self.text stringByAppendingString:@"\n "];
}

- (void)alignBottom {
    CGSize fontSize = [self.text sizeWithFont:self.font];
    double finalHeight = fontSize.height * self.numberOfLines;
    double finalWidth = self.frame.size.width;    //expected width of label
    CGSize theStringSize = [self.text sizeWithFont:self.font constrainedToSize:CGSizeMake(finalWidth, finalHeight) lineBreakMode:self.lineBreakMode];
    int newLinesToPad = (finalHeight  - theStringSize.height) / fontSize.height;
    for(int i=0; i<newLinesToPad; i++)
        self.text = [NSString stringWithFormat:@" \n%@",self.text];
}
@end

然后调用[yourLabel alignTop]；或[yourLabel alignBottom]；在每次标签文本分配后。

这是一个旧的解决方案，在iOS上使用自动布局>=6

我的解决方案：

自行拆分线条（忽略标签换行设置）自己绘制线（忽略标签对齐）

@interface UITopAlignedLabel : UILabel

@end

@implementation UITopAlignedLabel

#pragma mark Instance methods

- (NSArray*)splitTextToLines:(NSUInteger)maxLines {
    float width = self.frame.size.width;

    NSArray* words = [self.text componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
    NSMutableArray* lines = [NSMutableArray array];

    NSMutableString* buffer = [NSMutableString string];    
    NSMutableString* currentLine = [NSMutableString string];

    for (NSString* word in words) {
        if ([buffer length] > 0) {
            [buffer appendString:@" "];
        }

        [buffer appendString:word];

        if (maxLines > 0 && [lines count] == maxLines - 1) {
            [currentLine setString:buffer];
            continue;
        }

        float bufferWidth = [buffer sizeWithFont:self.font].width;

        if (bufferWidth < width) {
            [currentLine setString:buffer];
        }
        else {
            [lines addObject:[NSString stringWithString:currentLine]];

            [buffer setString:word];
            [currentLine setString:buffer];
        }
    }

    if ([currentLine length] > 0) {
        [lines addObject:[NSString stringWithString:currentLine]];
    }

    return lines;
}

- (void)drawRect:(CGRect)rect {
    if ([self.text length] == 0) {
        return;
    }

    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSetFillColorWithColor(context, self.textColor.CGColor);
    CGContextSetShadowWithColor(context, self.shadowOffset, 0.0f, self.shadowColor.CGColor);

    NSArray* lines = [self splitTextToLines:self.numberOfLines];
    NSUInteger numLines = [lines count];

    CGSize size = self.frame.size;
    CGPoint origin = CGPointMake(0.0f, 0.0f);

    for (NSUInteger i = 0; i < numLines; i++) {
        NSString* line = [lines objectAtIndex:i];

        if (i == numLines - 1) {
            [line drawAtPoint:origin forWidth:size.width withFont:self.font lineBreakMode:UILineBreakModeTailTruncation];            
        }
        else {
            [line drawAtPoint:origin forWidth:size.width withFont:self.font lineBreakMode:UILineBreakModeClip];
        }

        origin.y += self.font.lineHeight;

        if (origin.y >= size.height) {
            return;
        }
    }
}

@end

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
}