Swift 计算文本的size

iOS 11之前限制宽高计算字符串的size用的是UILabel的textRect(forBounds bounds: CGRect, limitedToNumberOfLines numberOfLines: Int) -> CGRect方法，当时也没考虑线程安全问题（low爆了），Xcode也没提示，用了好几个版本，所幸一直都没问题。

贴下方法（当时为什么选这个方法就不解释了）：

func textSize(font: UIFont, constrainedSize: CGSize, lineSpacing: CGFloat?, lines: Int) -> CGSize {
        if self.isEmpty || lines < 0 {
            return CGSize.zero
        }
        
        let attributedString = NSMutableAttributedString(string: self)
        let range = NSRange(location: 0, length: attributedString.length)
        attributedString.addAttributes([NSFontAttributeName: font], range: range)
        if lineSpacing != nil {
            let paragraphStyle = NSMutableParagraphStyle()
            paragraphStyle.lineBreakMode = .byTruncatingTail
            paragraphStyle.lineSpacing = lineSpacing!
            attributedString.addAttribute(NSParagraphStyleAttributeName, value: paragraphStyle, range: range)
        }
        
        let calculatedLabel = UILabel()
        calculatedLabel.font = font
        calculatedLabel.attributedText = attributedString
        calculatedLabel.numberOfLines = lines
        let rect = calculatedLabel.textRect(forBounds: CGRect(x: 0, y: 0, width: constrainedSize.width, height: constrainedSize.height), limitedToNumberOfLines: lines)
        
        return rect.size
    }

最近升级了Xcode 9，运行时警告我let calculatedLabel = UILabel()要在主线程执行，这时才意识到问题的严重性，马上进行了修改：

extension String {

   func boundingRect(with constrainedSize: CGSize, font: UIFont, lineSpacing: CGFloat? = nil) -> CGSize {
        let attritube = NSMutableAttributedString(string: self)
        let range = NSRange(location: 0, length: attritube.length)
        attritube.addAttributes([NSAttributedStringKey.font: font], range: range)
        if lineSpacing != nil {
            let paragraphStyle = NSMutableParagraphStyle()
            paragraphStyle.lineSpacing = lineSpacing!
            attritube.addAttribute(NSAttributedStringKey.paragraphStyle, value: paragraphStyle, range: range)
        }
        
        let rect = attritube.boundingRect(with: constrainedSize, options: [.usesLineFragmentOrigin, .usesFontLeading], context: nil)
        var size = rect.size
        
        if let currentLineSpacing = lineSpacing {
            // 文本的高度减去字体高度小于等于行间距，判断为当前只有1行
            let spacing = size.height - font.lineHeight
            if spacing <= currentLineSpacing && spacing > 0 {
                size = CGSize(width: size.width, height: font.lineHeight)
            }
        }
        
        return size
    }
    
    func boundingRect(with constrainedSize: CGSize, font: UIFont, lineSpacing: CGFloat? = nil, lines: Int) -> CGSize {
        if lines < 0 {
            return .zero
        }
        
        let size = boundingRect(with: constrainedSize, font: font, lineSpacing: lineSpacing)
        if lines == 0 {
            return size
        }

        let currentLineSpacing = (lineSpacing == nil) ? (font.lineHeight - font.pointSize) : lineSpacing!
        let maximumHeight = font.lineHeight*CGFloat(lines) + currentLineSpacing*CGFloat(lines - 1)
        if size.height >= maximumHeight {
            return CGSize(width: size.width, height: maximumHeight)
        }
        
        return size
    }
}

参数解释
constrainedSize：限制的size
font:字号
lineSpacing：默认为nil，使用系统默认的行间距
lines：限制的行数
注：代码版本为Swift 4.0

上面的两个方法分别取代名：方法1和方法2。
方法1：限制宽高，可设置行间距，计算准确
方法2：比方法1多了限制行数功能。

配合使用UILabel的扩展方法：

extension UILabel {
    
    // 设置`numberOfLines = 0`的原因：
    // 配合方法`func boundingRect(with constrainedSize: CGSize, font: UIFont, lineSpacing: CGFloat? = nil, lines: Int) -> CGSize`使用，可以很好的解决不能正常显示限制行数的问题；
    // 如果为label设置了限制行数（大于0的前提），使用上面的计算方法（带行间距），同时字符串的实际行数大于限制行数，这时候的高度会使label不能正常显示。
    func setText(with normalString: String, lineSpacing: CGFloat?, frame: CGRect) {
        self.frame = frame
        self.numberOfLines = 0
                
        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.lineBreakMode = .byTruncatingTail
        if lineSpacing != nil {
            if (frame.height - font.lineHeight) <= lineSpacing! {
                paragraphStyle.lineSpacing = 0
            } else {
                paragraphStyle.lineSpacing = lineSpacing!
            }
        }
        let attributedString = NSMutableAttributedString(string: normalString)
        let range = NSRange(location: 0, length: attributedString.length)
        attributedString.addAttributes([NSAttributedStringKey.font: font], range: range)
        attributedString.addAttribute(NSAttributedStringKey.paragraphStyle, value: paragraphStyle, range: range)

        self.attributedText = attributedString
    }
}