How To Access AppDelegate Methods In Other Class?

Ans : 

let appDelegate: AppDelegate? = UIApplication.shared.delegate as? AppDelegate
appDelegate?.application(UIApplication.shared, didFinishLaunchingWithOptions: nil)

You can make your custom method any called from viewcontroller like :

let appDelegate = UIApplication.shared.delegate as! AppDelegate appDelegate.anyAppDelegateInstaceMethod()

But above is only possible in iOS 10.0 or newer version.