包装时,UILabel未完全对准中心

如图所示: http://img25.imageshack.us/img25/6996/90754687.png
灰色背景表示UILabel框架的大小。

出于某种原因,第一行包装文本似乎并不总是居中,即使我使用的是UITextAlignmentCenter。

以下是我用来设置标签的代码:

    self.titleLabel = [[UILabel alloc] initWithFrame:CGRectZero];
    titleLabel.font = [UIFont boldSystemFontOfSize:fontHeight];
    titleLabel.backgroundColor = [UIColor grayColor];
    titleLabel.numberOfLines = 2;
    titleLabel.lineBreakMode = UILineBreakModeMiddleTruncation;

    NSString * title = file.name;
    CGSize maximumSize = CGSizeMake(thumbnailWidth+4,fontHeight * 3);
    UIFont * font = [UIFont boldSystemFontOfSize:12];
    CGSize stringSize = [title sizeWithFont:font constrainedToSize:maximumSize lineBreakMode:titleLabel.lineBreakMode];

    CGRect stringFrame = CGRectMake(0, thumbnailHeight + thumbnailPadding, thumbnailWidth + 4, stringSize.height);
    titleLabel.text = title;
    titleLabel.frame = stringFrame;
    titleLabel.textAlignment = UITextAlignmentCenter;