每次用户打开应用程序时如何调用函数

Question

每当用户点击我的应用程序的图标并打开它时,我想调用名为update的函数.我该怎么办？每当应用程序启动时,无论是从后台杀死还是用户按下主页按钮,我都可以覆盖哪种方法.

 import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var label: UILabel!

    @IBOutlet weak var button: UIButton!

    @IBOutlet weak var labeltwo: UILabel!

    var NSDateDefalt = NSUserDefaults.standardUserDefaults()
    var date : NSDate?

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        date = NSUserDefaults.standardUserDefaults().objectForKey("yourKey") as? NSDate
        label.text = "\(date)"
        update()
    }


    @IBAction func buttona(sender: AnyObject) {
        date = NSDate()
        label.text = "\(date)"

        NSUserDefaults.standardUserDefaults().setObject(NSDate(), forKey:"yourKey")
    }



    func update(){
        let now = NSDate()
        let seconds = now.timeIntervalSinceDate(date!)
        labeltwo.text = "\(seconds))"
    }

}

更新:我在我的App Delegate中实现了

 func applicationWillEnterForeground(application: UIApplication) {
        ViewController().update()     
    }

Answer 1

您需要在AppDelegate中实现您的逻辑

class AppDelegate: UIResponder, UIApplicationDelegate {

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

        // first launch
        // this method is called only on first launch when app was closed / killed
        return true
    }

    func applicationWillEnterForeground(application: UIApplication) {

        // app will enter in foreground
        // this method is called on first launch when app was closed / killed and every time app is reopened or change status from background to foreground (ex. mobile call)
    }

    func applicationDidBecomeActive(application: UIApplication) {

        // app becomes active
        // this method is called on first launch when app was closed / killed and every time app is reopened or change status from background to foreground (ex. mobile call)
    }
}

更新

正如Paulw11所建议的那样,考虑使用applicationWillEnterForeground而不是applicationDidBecomeActive

Answer 2

在ViewController.swift以viewDidLoad()相应的方式写入其中任何一个:

NotificationCenter.default.addObserver(self, selector: #selector(self.update), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil)
NotificationCenter.default.addObserver(self, selector: #selector(self.update), name: NSNotification.Name.UIApplicationDidBecomeActive, object: nil)

(编辑:Swift 3的更新)

(编辑:删除了这篇文章中不正确的部分.iOS会自动向任何添加的观察者触发这些通知,无需添加代码以在App Delegate中触发它们)

Answer 3

在你的AppDelegate.swift中,有didFinishLaunchingWithOptions和applicationWillEnterForeground.

applicationWillEnterForeground类似于viewWillAppear您的viewControllers而不是您的应用程序,而didFinishLaunchingWithOptions类似于viewDidLoad您的应用程序.有关更多信息,请查看UIApplicationDelegate