How to subclass UIApplication using UIApplicationMain

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

Subclassing UIApplication allows you to override functionality such as opening URLs or changing your icon, but it’s a non-trivial task in Swift because of the @UIApplicationMain attribute.

If you look in your AppDelegate.swift file you’ll see @UIApplicationMain appears directly before class AppDelegate – this is a special attribute that tells the Swift compiler to generate code to launch your application using default settings.

You need to delete @UIApplicationMain from that file. We’re going to replace it with custom code.

With that line deleted, create a new Cocoa Touch Class called “MyApplication”, making it subclass from UIApplication. We’re going to make our class override the open() method so that no part of our application can open URLs outside of https://www.hackingwithswift.com:

import UIKit

class MyApplication: UIApplication {
    override func open(_ url: URL, options: [UIApplication.OpenExternalURLOptionsKey : Any] = [:], completionHandler completion: ((Bool) -> Void)? = nil) {
        if let host = url.host, host.contains("hackingwithswift.com") {
            super.open(url, options: options, completionHandler: completion)
        } else {
            completion?(false)
        }
    }
}

The third and final step is to create a file called main.swift that is responsible for launching our application. The name is important: it must be exactly “main.swift”, because that’s the only file that can contain top-level code – i.e., code that isn’t wrapped inside a function, class, struct, or enum.

So, create a new Swift file called main.swift, and give it this code:

import UIKit

UIApplicationMain(
    CommandLine.argc,
    CommandLine.unsafeArgv, 
    NSStringFromClass(MyApplication.self),
    NSStringFromClass(AppDelegate.self)
)

You have now subclassed UIApplication and can add any more customizations you need. It should go without saying that dumping code into a UIApplication subclass is no better than dumping code into AppDelegate.