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

FINALLY figured this out. Cleanest way I could find. (Swift 4) Solution doesn't require you to set font sizes and won't override all font sizes.

  UILabel.appearance().font = UIFont.preferredFont(forTextStyle: UIFontTextStyle(rawValue: "Roboto"))

For those looking for where to place this code. I put it in my AppDelegate.swift file inside of

 func application(_ application: UIApplication, 
 didFinishLaunchingWithOptions...

Swift 4.2

 UILabel.appearance().font = UIFont.preferredFont(forTextStyle: UIFont.TextStyle(rawValue: "Roboto"))

I found out a way to do same in Swift 4 with iOS 11

Only need to add @objc keyword before variable name. So variable declaration will be like this

@objc public var substituteFontName : String {
    get {}
    set {}
}

Hope this helps others who are facing this issue.

The answer is a combination of the answers offered already, but I had to try a combination of things to get it working in iOS 13:

This is all placed in a Table View Controller, which is made to conform to the UIFontPickerViewControllerDelegate in order to offer a view for the user to pick the font. This will update all the UILabels with the new font, but keep the other attributes.

override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    let cell = tableView.cellForRow(at: indexPath)

    if cell === fontCell {
        if #available(iOS 13, *) {
            let configuration = UIFontPickerViewController.Configuration()
            configuration.includeFaces = true

            let fontVC = UIFontPickerViewController(configuration: configuration)
            fontVC.delegate = self

            present(fontVC, animated: true)
        } else {
            let ac = UIAlertController(title: "iOS 13 is required", message: "Custom fonts are only supported by devices running iOS 13 or above.", preferredStyle: .alert)
            ac.addAction(UIAlertAction(title: "OK", style: .default))
            present(ac, animated: true)
        }
    }
}

}

@available(iOS 13.0, *)
extension AppearanceTableViewController: UIFontPickerViewControllerDelegate {
    func fontPickerViewControllerDidPickFont(_ viewController: UIFontPickerViewController) {
        // attempt to read the selected font descriptor, but exit quietly if that fails
        guard let descriptor = viewController.selectedFontDescriptor else { return }

        let font = UIFont(descriptor: descriptor, size: 20)
        UILabel.appearance().substituteFontName = font.fontName
    }
}

extension UILabel {
    @objc public 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)
        }
    }
}