UILabel的文字默認是垂直居中顯示的，而且UILabel并沒有為我們提供設置垂直對齊方式的選項，所以要自己想辦法了。
我的辦法是給UILabel添加一個Category，來修改UILabel的繪制。話不多說上代碼：

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

@interface UILabel (VerticalAlignment)

// 文字頂部對齊
- (void)textAlignmentTop;

// 文字底部對齊
- (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