Using custom font for entire iOS app swift

As a more detailed answer and with these benefits I recommend using extensions:

• No size override (whatever you set in designer will be used)
• No style override (Bold, Light, Medium, UltraLight is implemented in my code but you can customize it as you need)

import UIKit

extension UILabel {
    @objc var substituteFontName : String {
        get {
            return self.font.fontName;
        }
        set {
            let fontNameToTest = self.font.fontName.lowercased();
            var fontName = newValue;
            if fontNameToTest.range(of: "bold") != nil {
                fontName += "-Bold";
            } else if fontNameToTest.range(of: "medium") != nil {
                fontName += "-Medium";
            } else if fontNameToTest.range(of: "light") != nil {
                fontName += "-Light";
            } else if fontNameToTest.range(of: "ultralight") != nil {
                fontName += "-UltraLight";
            }
            self.font = UIFont(name: fontName, size: self.font.pointSize)
        }
    }
}

extension UITextView {
    @objc var substituteFontName : String {
        get {
            return self.font?.fontName ?? "";
        }
        set {
            let fontNameToTest = self.font?.fontName.lowercased() ?? "";
            var fontName = newValue;
            if fontNameToTest.range(of: "bold") != nil {
                fontName += "-Bold";
            } else if fontNameToTest.range(of: "medium") != nil {
                fontName += "-Medium";
            } else if fontNameToTest.range(of: "light") != nil {
                fontName += "-Light";
            } else if fontNameToTest.range(of: "ultralight") != nil {
                fontName += "-UltraLight";
            }
            self.font = UIFont(name: fontName, size: self.font?.pointSize ?? 17)
        }
    }
}

extension UITextField {
    @objc var substituteFontName : String {
        get {
            return self.font?.fontName ?? "";
        }
        set {
            let fontNameToTest = self.font?.fontName.lowercased() ?? "";
            var fontName = newValue;
            if fontNameToTest.range(of: "bold") != nil {
                fontName += "-Bold";
            } else if fontNameToTest.range(of: "medium") != nil {
                fontName += "-Medium";
            } else if fontNameToTest.range(of: "light") != nil {
                fontName += "-Light";
            } else if fontNameToTest.range(of: "ultralight") != nil {
                fontName += "-UltraLight";
            }
            self.font = UIFont(name: fontName, size: self.font?.pointSize ?? 17)
        }
    }
}

Samples for using Extensions:

e.g. put these lines in your starting controller viewDidLoad

UILabel.appearance().substituteFontName = "IRANSans"; // USE YOUR FONT NAME INSTEAD
UITextView.appearance().substituteFontName = "IRANSans"; // USE YOUR FONT NAME INSTEAD
UITextField.appearance().substituteFontName = "IRANSans"; // USE YOUR FONT NAME INSTEAD

P.S. as @Christian mentioned, you can write your own extensions for almost AnyUIView

You can set the appearance of the UILabel and other UIViews:

UILabel.appearance().font = UIFont(name: "yourFont", size: yourSize)

More General:

AnyUIView.appearance().font = UIFont(name: "yourFont", size: yourSize)