如何在应用即将进入背景时更改UIImage - swift

Question

我希望在应用输入或即将进入背景时更改图像。

文件名AppDelegate.swift

func applicationWillResignActive(application: UIApplication) { 
    ViewController().changeImg();   
} 

文件名ViewController.swift

func changeImg(){ 
    backgroundImg.image = UIImage(named: "blue") as UIImage; 
} 

但我代一个错误

fatal error: unexpectedly found nil while unwrapping an Optional value 

Answer 1

这一行

ViewController().changeImg() 

您正在实例化一个新的空的ViewController，它没有changeImg方法。您需要对您的自定义viewController的引用。

如果是的viewController你RootViewController的，你可以通过

// create a property in appDelegate for your viewController 
var viewController: ViewController? 


func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: NSDictionary?) -> Bool { 

    let navigationController = application.windows[0].rootViewController as UINavigationController 
    viewController = navigationController.viewControllers[0] as ViewController 
} 

在你的appDelegate找到它，那么你可以调用

viewController?.changeImg()