UILabel的文字默认是垂直居中显示的，而且UILabel并没有为我们提供设置垂直对齐方式的选项，所以要自己想办法了。
我的办法是给UILabel添加一个Category，来修改UILabel的绘制。话不多说上代码：

//  UILabel+VerticalAlignment.h
#import <UIKit/UIKit.h>

@interface UILabel (VerticalAlignment)

// 文字顶部对齐
- (void)textAlignmentTop;

// 文字底部对齐
- (void)textAlignmentBottom;

@end

//  UILabel+VerticalAlignment.m
#import "UILabel+VerticalAlignment.h"

@implementation UILabel (VerticalAlignment)

- (void)textAlignmentTop {
    self.numberOfLines = 0;
    CGSize fontSize = [self.text sizeWithAttributes:@{NSFontAttributeName:self.font}];
    double finalWidth = self.frame.size.width;
    CGSize maximumSize = CGSizeMake(finalWidth, CGFLOAT_MAX);
    CGRect stringSize = [self.text boundingRectWithSize:maximumSize options:NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:@{NSFontAttributeName:self.font} context:nil];
    int lines = (self.frame.size.height - stringSize.size.height) / fontSize.height;
    for (int i = 0; i < lines; i++) {
        self.text = [self.text stringByAppendingString:@"\n"];
    }
}

- (void)textAlignmentBottom {
    self.numberOfLines = 0;
    CGSize fontSize = [self.text sizeWithAttributes:@{NSFontAttributeName:self.font}];
    double finalWidth = self.frame.size.width;
    CGSize maximumSize = CGSizeMake(finalWidth, CGFLOAT_MAX);
    CGRect stringSize = [self.text boundingRectWithSize:maximumSize options:NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:@{NSFontAttributeName:self.font} context:nil];
    int lines = (self.frame.size.height - stringSize.size.height) / fontSize.height;
    for (int i = 0; i < lines; i++) {
        self.text = [NSString stringWithFormat:@" \n%@",self.text];
    }
}

@end