iOS中UILabel设置居上对齐、居中对齐、居下对齐及文字置顶显示

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

lbl_mylabel = [[myUILabel alloc] initWithFrame:CGRectMake(20, 50, 150, 600)]; 
UIColor *color = [UIColor colorWithPatternImage:[UIImage imageNamed:@"halfTransparent.png"]];//使用半透明图片作为label的背景色 
lbl_mylabel.backgroundColor = color; 
lbl_mylabel.textAlignment = UITextAlignmentLeft; 
lbl_mylabel.textColor = UIColor.whiteColor; 
lbl_mylabel.lineBreakMode = UILineBreakModeWordWrap; 
lbl_mylabel.numberOfLines = 0; 
[lbl_mylabel setVerticalAlignment:VerticalAlignmentTop]; 
[self addSubview:lbl_mylabel]; 

- (void)viewDidLoad {
 [super viewDidLoad];
 UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake((self.view.bounds.size.width - 200)/2, 100, 200, 150)];
 label.backgroundColor = [UIColor yellowColor];
 NSString *labelText = @"我不知道如何置顶，谁来告诉我？";
 [label setText:labelText];
 [label setNumberOfLines:0];  
 //让内容置顶
 [label sizeToFit];
 [self.view addSubview:label];
}