Check out example codes for "change font of substring swift". It will help you in understanding the concepts better.

Code Example 1

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)

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