如何在进入后台后杀死NSTimer并在应用恢复活动后创建新的计时器？

Question

所以我正在使用Swift构建一个简单的iOS应用程序.我需要NSTimer在应用程序进入后台后杀死我,并在应用程序再次激活后创建一个新应用程序.

最初我的解决方案是在主控制器文件中创建NSTimerclass in 的计时器ViewDidLoad().这导致我的应用程序中的错误.

我想我需要applicationDidEnterBackground()在AppDelegate.swift中使用来杀死计时器.但我不确定如何做到这一点.我应该在AppDelegate.swift或主控制器中创建计时器类吗？我不知道Swift文件如何共享类.

我在网上搜索了解决方案,但这些帖子都太旧了,解决方案是用Objective-C编写的.

我绝对是初学者.所以我希望有人可以在Swift中解释它.

这是我的主控制器TodoTableViewController.swift中的代码:

import UIKit
import CoreData
class TodoTableViewController: UITableViewController, NSFetchedResultsControllerDelegate {....
...
...
override func viewDidLoad() {
    super.viewDidLoad()
    var fetchRequest = NSFetchRequest(entityName: "Todo")
    let sortDescriptor = NSSortDescriptor(key: "dayleft", ascending: true)
    fetchRequest.sortDescriptors = [sortDescriptor]
    if let managedObjectContext = (UIApplication.sharedApplication().delegate as AppDelegate).managedObjectContext {
        fetchResultController = NSFetchedResultsController(fetchRequest:fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)
        fetchResultController.delegate = self
        var e: NSError?
        var result = fetchResultController.performFetch(&e)
        todos = fetchResultController.fetchedObjects as [Todo]
        if result != true {
        println(e?.localizedDescription)
        } }
    var timer = NSTimer.scheduledTimerWithTimeInterval(10, target: self, selector:"update", userInfo:nil, repeats: true)
}
...
}

如果我要在AppDelegate.swift 中使计时器无效,我怎样才能将计时器引用到我在TodoTableViewController.swift中创建的计时器？或许我应该把所有与计时器相关的代码放在AppDelegate.swift中？

---

更新

我试过使用NSNotificationCenter,这是我的代码.

import UIKit
import CoreData

class TodoTableViewController: UITableViewController, NSFetchedResultsControllerDelegate {
...

var timer = NSTimer.scheduledTimerWithTimeInterval(10, target: self, selector:"viewDidLoad", userInfo:nil, repeats: true)

func update(){
    .....
}

override func viewDidLoad() {
    super.viewDidLoad()
    ...

    let notificationCenter = NSNotificationCenter.defaultCenter()

    notificationCenter.addObserver(self, selector: "didEnterBackground", name: "UIApplicationDidEnterBackgroundNotification", object: UIApplication.sharedApplication())

    notificationCenter.addObserver(self, selector: "didBecomeActive", name: "UIApplicationWillEnterForegroundNotification", object: UIApplication.sharedApplication())   
}


func didEnterBackground() {
    timer.invalidate()
}
func didBecomeActive() {
    var timer = NSTimer.scheduledTimerWithTimeInterval(10, target: self, selector:"update", userInfo:nil, repeats: true)
}

...
}

由于声明了计时器didBecomeActive(),didEnterBackground()因此存在错误:没有名为"timer"的成员.如果我像上面发布的代码一样在timer外面声明didBecomeActive(),那么在调用中会出现"额外参数'选择器'的错误".我已经将函数解析update()为选择器.我不知道这个错误来自哪里.

Answer 1

更新了您的代码并添加了一些注释:

class TodoTableViewController: UITableViewController, NSFetchedResultsControllerDelegate 

   //better to instantiate timer inside viewDidLoad
   var timer: NSTimer!

   func update(){ }

   override func viewDidLoad() {
      super.viewDidLoad()
      startTimer()

      let notificationCenter = NSNotificationCenter.defaultCenter()

      //UIApplicationDidEnterBackgroundNotification & UIApplicationWillEnterForegroundNotification shouldn't be quoted
      notificationCenter.addObserver(self, selector: "didEnterBackground", name: UIApplicationDidEnterBackgroundNotification, object: nil)
      notificationCenter.addObserver(self, selector: "didBecomeActive", name: UIApplicationWillEnterForegroundNotification, object: nil)   
   }

   func didEnterBackground() {
      self.timer.invalidate()
   }

   func didBecomeActive() {
      startTimer()
   }

   func startTimer() {
      self.timer = NSTimer.scheduledTimerWithTimeInterval(10, target: self, selector:"update", userInfo:nil, repeats: true)
   }
}

还要注意NSTimer强烈保持它target.所以当关闭viewcontroller时 - 定时器应该是无效的.或者会发生内存泄漏,TodoTableViewController永远不会被破坏.