font style on string in swift 4 code example

Example: change font of substring swift

let longestWordRange = (longString as NSString).range(of: longestWord)

let attributedString = NSMutableAttributedString(string: longString, attributes: [NSAttributedStringKey.font : UIFont.systemFont(ofSize: 20)])

attributedString.setAttributes([NSAttributedStringKey.font : UIFont.boldSystemFont(ofSize: 20), NSAttributedStringKey.foregroundColor : UIColor.red], range: longestWordRange)