利用Swift如何计算文本的size示例详解

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
  }

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
  }
}

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
  }
}