这个解决方案将有助于在运行时计算高度和宽度。

    let messageText = "Your Text String"
    let size = CGSize.init(width: 250, height: 1000)
    let options = NSStringDrawingOptions.usesFontLeading.union(.usesLineFragmentOrigin)
    let estimateFrame = NSString(string: messageText).boundingRect(with:  size, options: options, attributes: [NSAttributedString.Key.font: UIFont(name: "HelveticaNeue", size: 17)!], context: nil)

在这里，你可以计算你的字符串的估计高度，并将其传递给UILabel框架。

estimateFrame.Width
estimateFrame.Height 

我创建了以下扩展来根据宽度计算字符串的高度。

   extension String {
    func heightWithConstrainedWidth(width: CGFloat, font: UIFont) -> CGFloat {
        let textView = UITextView()
        textView.text = self
        textView.font = font
        var newFrame = textView.frame
        newFrame.size = CGSize(width: width, height: .greatestFiniteMagnitude)
        textView.frame = newFrame
        return textView.contentSize.height
    }

我无法让@KaanDedeoglu的解决方案在Swift 5中为多行标签和文本视图工作-无论出于什么原因-所以我只是写了一个“手工”解决方案，保持与@KaanDedeoglu的答案中看到的相同的函数签名。在我的程序中就像有魅力一样。

宽度

extension String {

    func width(withConstrainedHeight height: CGFloat, font: UIFont) -> CGFloat {
        
        var wordBoxes = [CGSize]()
        var calculatedHeight = CGFloat.zero
        var calculatedWidth = CGFloat.zero
        
        for word in self.wordsWithWordSeparators() {
            
            let box = word.boundingRect(with: CGSize.zero, attributes: [.font: font], context: nil)
            let boxSize = CGSize(width: box.width, height: box.height)
            wordBoxes.append(boxSize)
            calculatedHeight += boxSize.height
            calculatedWidth = calculatedWidth < boxSize.width ? boxSize.width : calculatedWidth
        }
        
        while calculatedHeight > height && wordBoxes.count > 1 {
            
            var bestLineToRelocate = wordBoxes.count - 1
            
            for i in 1..<wordBoxes.count {
                
                let bestPotentialWidth = wordBoxes[bestLineToRelocate - 1].width + wordBoxes[bestLineToRelocate].width
                let thisPotentialWidth = wordBoxes[i - 1].width + wordBoxes[i].width
                
                if bestPotentialWidth > thisPotentialWidth {
                    bestLineToRelocate = i
                }
            }
            
            calculatedHeight -= wordBoxes[bestLineToRelocate].height
            wordBoxes[bestLineToRelocate - 1].width += wordBoxes[bestLineToRelocate].width
            wordBoxes.remove(at: bestLineToRelocate)
            calculatedWidth = max(wordBoxes[bestLineToRelocate - 1].width, calculatedWidth)
        }
        
        return ceil(calculatedWidth)
    }
}

高度

extension String {
    
    func height(withConstrainedWidth width: CGFloat, font: UIFont) -> CGFloat {
        
        var wordBoxes = [CGSize]()
        var calculatedHeight = CGFloat.zero
        var currentLine = 0
        
        for word in self.wordsWithWordSeparators() {
            
            let box = word.boundingRect(with: CGSize.zero, attributes: [.font: font], context: nil)
            let boxSize = CGSize(width: box.width, height: box.height)
            
            if wordBoxes.isEmpty == true {
                wordBoxes.append(boxSize)
            }
            else if wordBoxes[currentLine].width + boxSize.width > width {
                wordBoxes.append(boxSize)
                currentLine += 1
            }
            else {
                wordBoxes[currentLine].width += boxSize.width
                wordBoxes[currentLine].height = max(wordBoxes[currentLine].height, boxSize.height)
            }
        }
        
        for wordBox in wordBoxes {
            calculatedHeight += wordBox.height
        }
        
        return calculatedHeight
    }
}

使用的辅助方法

extension String {
    
    // Should work with any language supported by Apple
    func wordsWithWordSeparators () -> [String] {
        
        let range = self.startIndex..<self.endIndex
        var words = [String]()
        
        self.enumerateSubstrings(in: range, options: .byWords) { (substr, substrRange, enclosingRange, stop) in
            let wordWithWordSeparators = String(self[enclosingRange])
            words.append(wordWithWordSeparators)
        }
        
        return words
    }
}

注意:这些高度和宽度计算假设给定的标签或文本视图在执行换行时不会拆分或连字符。如果这不是你的情况，你应该只用文字来代替字符。此外，如果您处于运行时敏感的环境中，可能需要考虑限制这些函数调用或缓存结果，因为根据字符串包含的单词数量，它们的开销可能会有点大。

这里有一个简单的解决方案，对我来说很有用。类似于其他发布的一些，但它不需要sizeToFit

注意这是用Swift 5写的

let lbl = UILabel()
lbl.numberOfLines = 0
lbl.font = UIFont.systemFont(ofSize: 12) // make sure you set this correctly 
lbl.text = "My text that may or may not wrap lines..."

let width = 100.0 // the width of the view you are constraint to, keep in mind any applied margins here
 
let height = lbl.systemLayoutSizeFitting(CGSize(width: width, height: UIView.layoutFittingCompressedSize.height), withHorizontalFittingPriority: .required, verticalFittingPriority: .fittingSizeLevel).height

它处理行换行等。不是最优雅的代码，但它完成了工作。

我发现公认的答案适用于固定的宽度，但不适用于固定的高度。对于固定的高度，它只会增加宽度以适应一行中的所有内容，除非文本中有换行符。

width函数多次调用height函数，但这是一个快速计算，在UITable的行中使用该函数时，我没有注意到性能问题。

extension String {

    public func height(withConstrainedWidth width: CGFloat, font: UIFont) -> CGFloat {
        let constraintRect = CGSize(width: width, height: .greatestFiniteMagnitude)
        let boundingBox = self.boundingRect(with: constraintRect, options: .usesLineFragmentOrigin, attributes: [.font : font], context: nil)

        return ceil(boundingBox.height)
    }

    public func width(withConstrainedHeight height: CGFloat, font: UIFont, minimumTextWrapWidth:CGFloat) -> CGFloat {

        var textWidth:CGFloat = minimumTextWrapWidth
        let incrementWidth:CGFloat = minimumTextWrapWidth * 0.1
        var textHeight:CGFloat = self.height(withConstrainedWidth: textWidth, font: font)

        //Increase width by 10% of minimumTextWrapWidth until minimum width found that makes the text fit within the specified height
        while textHeight > height {
            textWidth += incrementWidth
            textHeight = self.height(withConstrainedWidth: textWidth, font: font)
        }
        return ceil(textWidth)
    }
}

在Swift 5中:

label.textRect(forBounds: label.bounds, limitedToNumberOfLines: 1)

顺便说一下，limitedToNumberOfLines的值取决于你想要的标签文本行。