您需要使用属性化字符串并指定段落样式。例如：

let style = NSMutableParagraphStyle()
style.lineSpacing = 20
let attributes = [NSParagraphStyleAttributeName : style]
textView.attributedText = NSAttributedString(string: txtString, attributes: attributes)

有关属性化字符串用法的更多详细信息，请参见此SO答案：How do I make an attributed string using Swift?
注意，从2022年开始，你不需要使用属性化字符串。如果愿意，你可以使用.typingAttributes。

Xcode 9.2快速移动4

let style = NSMutableParagraphStyle()
style.lineSpacing = 0
let attributes = [NSAttributedStringKey.paragraphStyle : style]
txtViewAbout.attributedText = NSAttributedString(string: txtViewAbout.text, attributes: attributes)

Xcode 10.1快速移动4.2

let style = NSMutableParagraphStyle()
style.lineSpacing = 19
let attributes = [NSAttributedString.Key.paragraphStyle: style]
textView.attributedText = NSAttributedString(string: model.text, attributes: attributes)

使用typingAttributes将确保属性应用于用户输入的新文本。

let style = NSMutableParagraphStyle()
style.lineSpacing = 10
let attributes = [NSAttributedString.Key.paragraphStyle : style]
textView.typingAttributes = attributes

let contentTextView: UITextView = {
    let textView = UITextView()

    textView.backgroundColor = .clear

    // Custom style
    let style = NSMutableParagraphStyle()
    style.lineSpacing = 10
    let attributes = [
        NSAttributedString.Key.paragraphStyle: style,
        NSAttributedString.Key.foregroundColor: UIColor.white,
        NSAttributedString.Key.font: UIFont.systemFont(ofSize: 16)
    ]
    textView.typingAttributes = attributes

    return textView
}()

此字典包含应用于新键入文本的属性键（和相应值）。当文本视图的选择更改时，字典的内容将自动清除。
https://developer.apple.com/documentation/uikit/uitextview/1618629-typingattributes

• 雨燕5.7，iOS 16.0*

下面是提供textView.lineSpacing getter & setter的扩展：

extension UITextView {
    
    /// Gets & Sets line spacing via `typingAttributes`.
    /// Preserves defined `style` and `textColor`.
    var lineSpacing: CGFloat {
        get {
            if let style = typingAttributes[NSAttributedString.Key.paragraphStyle] {
                return (style as! NSMutableParagraphStyle).lineSpacing
            }
            return 0
        }
        set {
            let style = NSMutableParagraphStyle()
            style.lineSpacing = newValue
            let attributes = [
                NSAttributedString.Key.paragraphStyle: style,
                NSAttributedString.Key.foregroundColor: textColor,
                NSAttributedString.Key.font: font
            ]
            typingAttributes = attributes as [NSAttributedString.Key : Any]
        }
    }
}

它可以在初始化UITextView时使用，如下所示：

textView.font = UIFont.systemFont(ofSize: 18)
textView.textColor = UIColor.purple
textView.lineSpacing = 3.4