让UILabel的文字顶部对齐

转载自http://devtang.com/blog/2011/11/20/set-uilabel-text-align-top/

方法一

在显示文字时，首先计算显示当前的文字需要多宽和多高，然后将对应的UILabel的大小改变成对应的宽度和高度。此方法的相示意图如下：

CGSize maximumSize = CGSizeMake(300, 9999);
NSString *dateString = @"The date today is January 1st, 1999";
UIFont *dateFont = [UIFont fontWithName:@"Helvetica" size:14];
CGSize dateStringSize = [dateString sizeWithFont:dateFont
    constrainedToSize:maximumSize
    lineBreakMode:self.dateLabel.lineBreakMode];
CGRect dateFrame = CGRectMake(10, 10, 300, dateStringSize.height);
self.dateLabel.frame = dateFrame;

方法二

此方法更加简单粗暴，但是很有效。其方法是在文本后面加多一些。需要注意的是，后还得加至少一个空格，否则多余的会被UILabel忽略。从这一点上看，UILabel似乎又过于“聪明”了。

该方法的示意图如下：

该方法的代码如下：

for(int i=0; i<newLinesToPad; i++)
    self.text = [self.text stringByAppendingString:@"
 "];

方法三

最正统的方法，利用objective-c的category特性，修改UILabel的绘制代码。示例代码如下：

// -- file: UILabel+VerticalAlign.h
#pragma mark VerticalAlign
@interface UILabel (VerticalAlign)
- (void)alignTop;
- (void)alignBottom;
@end

// -- file: UILabel+VerticalAlign.m
@implementation UILabel (VerticalAlign)
- (void)alignTop {
    CGSize fontSize = [self.text sizeWithFont:self.font];
    double finalHeight = fontSize.height * self.numberOfLines;
    double finalWidth = self.frame.size.width;    //expected width of label
    CGSize theStringSize = [self.text sizeWithFont:self.font constrainedToSize:CGSizeMake(finalWidth, finalHeight) lineBreakMode:self.lineBreakMode];
    int newLinesToPad = (finalHeight  - theStringSize.height) / fontSize.height;
    for(int i=0; i<newLinesToPad; i++)
        self.text = [self.text stringByAppendingString:@"
 "];
}

- (void)alignBottom {
    CGSize fontSize = [self.text sizeWithFont:self.font];
    double finalHeight = fontSize.height * self.numberOfLines;
    double finalWidth = self.frame.size.width;    //expected width of label
    CGSize theStringSize = [self.text sizeWithFont:self.font constrainedToSize:CGSizeMake(finalWidth, finalHeight) lineBreakMode:self.lineBreakMode];
    int newLinesToPad = (finalHeight  - theStringSize.height) / fontSize.height;
    for(int i=0; i<newLinesToPad; i++)
        self.text = [NSString stringWithFormat:@" 
%@",self.text];
}
@end