Swift < 3

在AppDelegate类中为ex创建一个方法

func sharedInstance() -> AppDelegate{
        return UIApplication.sharedApplication().delegate as! AppDelegate
    }

叫它"前任"吧

let appDelegate : AppDelegate = AppDelegate().sharedInstance()

Swift >= 3.0

func sharedInstance() -> AppDelegate{
    return UIApplication.shared.delegate as! AppDelegate
}

以下是Swift 5的版本:

let delegate = UIApplication.shared.delegate as? AppDelegate

并访问管理对象上下文:

    if let delegate = UIApplication.shared.delegate as? AppDelegate {

        let moc = delegate.managedObjectContext
        
        // your code here
        
    }

或者，用guard:

    guard let delegate = UIApplication.shared.delegate as? AppDelegate  else {
        return
    }

    let moc = delegate.managedObjectContext
        
    // your code here
    

试试简单的方法:

斯威夫特4

// Call the method
(UIApplication.shared.delegate as? AppDelegate)?.whateverWillOccur()

在你的AppDelegate中:

// MARK: - Whatever
func whateverWillOccur() {

    // Your code here.

}

在Swift 3.0中，你可以通过

let appDelegate = UIApplication.shared.delegate as! AppDelegate

在Xcode 6.2中，这也是可行的

let appDelegate = UIApplication.sharedApplication().delegate! as AppDelegate

let aVariable = appDelegate.someVariable

下面是一个UIApplicationDelegate的扩展，它避免了硬编码AppDelegate类名:

extension UIApplicationDelegate {

    static var shared: Self {    
        return UIApplication.shared.delegate! as! Self
    }
}

// use like this:
let appDelegate = MyAppDelegate.shared // will be of type MyAppDelegate