Can't apply line spacing with UITextView?

You need to use an attributed string and assign a paragraph style. For example:

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

See this SO answer for more details on attributed string usage: How do I make an attributed string using Swift?

Xcode 9.2 Swift 4

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

Xcode 10.1 Swift 4.2

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