UILabel顶端对齐

苹果并未提供Label顶端对齐的设置，但是我们经常会有这种需求，现提供一个方法。

//.h文件
typedef enum VerticalAlignment {
    VerticalAlignmentTop,
    VerticalAlignmentMiddle,
    VerticalAlignmentBottom,
} VerticalAlignment;

@interface VerticallyAlignedLabel : UILabel {
@private
    VerticalAlignment verticalAlignment_;
}

@property (nonatomic, assign) VerticalAlignment verticalAlignment;

//.m文件
@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];
}

除了可以顶端对齐还可以底端对齐，但估计这种需求不多。
调用时只需继承这个label，然后调用

[label setContentMode:UIViewContentModeTop];

是不是很简单呢？