这可以用于OS X

let appDelegate = NSApplication.sharedApplication().delegate as AppDelegate
var managedObjectContext = appDelegate.managedObjectContext?

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

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
    

从iOS 12.2和Swift 5.0开始，AppDelegate不能被识别。UIApplicationDelegate。因此，任何涉及AppDelegate的答案都不再正确。下面的答案是正确的，并避免了强制展开，一些开发人员认为这是一种代码气味:

import UIKit

extension UIViewController {
  var appDelegate: UIApplicationDelegate {
    guard let appDelegate = UIApplication.shared.delegate else {
      fatalError("Could not determine appDelegate.")
    }
    return appDelegate
  }
}

在Swift 3.0中，你可以通过

let appDelegate = UIApplication.shared.delegate as! AppDelegate