UILabel实现两边对齐

在Storyboard中要设置UILabel的两边对齐会出现无效果的情况，需要通过代码来实现，关键操作为NSMutableParagraphStyle的设置，其中必须设置NSUnderlineStyleAttributeName属性，否则没有效果，具体如下：

let infoString = NSMutableAttributedString(string:"Test")
let line = NSMutableParagraphStyle()
line.lineHeightMultiple = 1.5
line.paragraphSpacingBefore = 10
line.alignment = NSTextAlignment.Justified
infoString.addAttribute(NSParagraphStyleAttributeName, value: line, range: NSMakeRange(0, infoString.length))
infoString.addAttribute(NSUnderlineStyleAttributeName, value: NSUnderlineStyle.StyleNone.rawValue, range: NSMakeRange(0, infoString.length))