首先在Xcode中新建.h文件，将以下代码复制进去

//
//  myUILabel.h
//  
//
//  Created by yexiaozi_007 on 3/4/13.
//  Copyright (c) 2013 yexiaozi_007. All rights reserved.
//

#import <UIKit/UIKit.h>
typedef enum
{
    VerticalAlignmentTop = 0,// default
    VerticalAlignmentMiddle,VerticalAlignmentBottom,} VerticalAlignment;
@interface myUILabel : UILabel
{
@private
VerticalAlignment _verticalAlignment;
}

@property (nonatomic) VerticalAlignment verticalAlignment;

@end

//
//  myUILabel.m
//  
//
//  Created by yexiaozi_007 on 3/4/13.
//  Copyright (c) 2013 yexiaozi_007. All rights reserved.
//

#import "myUILabel.h"

@implementation myUILabel
@synthesize verticalAlignment = verticalAlignment_;

- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        self.verticalAlignment = VerticalAlignmentMiddle;
    }
    return self;
}

- (void)setVerticalAlignment:(VerticalAlignment)verticalAlignment {
    verticalAlignment_ = verticalAlignment;
    [self setNeedsDisplay];
}

- (CGRect)textRectForBounds:(CGRect)bounds limitedToNumberOfLines:(NSInteger)numberOfLines {
    CGRect textRect = [super textRectForBounds:bounds limitedToNumberOfLines:numberOfLines];
    switch (self.verticalAlignment) {
        case VerticalAlignmentTop:
            textRect.origin.y = bounds.origin.y;
            break;
        case VerticalAlignmentBottom:
            textRect.origin.y = bounds.origin.y + bounds.size.height - textRect.size.height;
            break;
        case VerticalAlignmentMiddle:
            // Fall through.
        default:
            textRect.origin.y = bounds.origin.y + (bounds.size.height - textRect.size.height) / 2.0;
    }
    return textRect;
}

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


@end

在新创建的Bridging-Header文件里拷入下方代码

#import "myUILabel.h"

然后右键拖动Label或者按住Control键左键拖动连线到Label所在的父View的Class中生成Outlet，如果之前已经连线好，则改完Custom Class后，将连线生成代码中的UILabel改为myUILabel，示意图如下

然后就可以调用该label的类方法

label.verticalAlignment = VerticalAlignmentBottom