變量不發生變化外封

Question

var arrayLength:Int = 0 // is equal to 0 

override func viewDidLoad() { 
    super.viewDidLoad() 
    // Do any additional setup after loading the view, typically from a nib. 

    let urlPath = "http://example.com/json" 

    let url = NSURL(string: urlPath)! 
    let session = NSURLSession.sharedSession() 

    let task = session.dataTaskWithURL(url, completionHandler: { 
     data, response, error in 

     if (error? != nil) { 
      println(error) 
     } else { 
      var jsonResponse: AnyObject? = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: nil) 

      let json = JSON(jsonResponse!) 
      self.arrayLength = json["dump"].array?.count as Int! 
      println("Second: \(self.arrayLength)") // is equal to 3 

      for postIndex in 0...self.arrayLength-1 { 
       println(json["dump"][postIndex]["title"]) 
      } 
     } 

    }) 
    task.resume() 

    println(arrayLength) // is equal to 0 again 

的我的代碼開始設置

var arrayLength:Int = 0 

。稍後在viewDidLoad（）內部，我將其更改爲3.當我調用它時，在任務塊之外再次將它等於0.此處存在什麼問題以及我做錯了什麼？

Answer 1

是的，你正在更改你的閉包中的arrayLength var。您正在面對的問題這只是線程工作的常規方式。

1. 當viewDidLoad開始你在主線程
2. 行let task = session.dataTaskWithURL(url, completionHandler: { ...定義了一個新的任務，那就是尚未執行
3. task.resume()啓動另一個線程此任務。這需要一段時間才能完成。當它完成後您的arrayLength將立即改變
4. 你問主線程爲您arrayLength值

這裏：

 println(arrayLength) // is equal to 0 again 

這並不是說arrayLength等於0 再次 ，你沒有給你足夠的時間來執行和改變這個值。

編輯：如果你需要檢查arrayLength關閉後結束...

只需創建一個函數，並調用它最後一件事，你的封內。像這樣：

override func viewDidLoad() { 
    ... 

    let task = session.dataTaskWithURL(url, completionHandler: { 
     data, response, error in 

     if (error? != nil) { 
      println(error) 
     } else { 
      var jsonResponse: AnyObject? = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: nil) 

      let json = JSON(jsonResponse!) 
      self.arrayLength = json["dump"].array?.count as Int! 
      println("Second: \(self.arrayLength)") // is equal to 3 

      for postIndex in 0...self.arrayLength-1 { 
       println(json["dump"][postIndex]["title"]) 
      } 

      functionToDoSomethingAtTheEndOfTheClosure(); 
     } 

    }) 
    task.resume() 
} 

func functionToDoSomethingAtTheEndOfTheClosure() { 
    // this is going to be executed at the end of the closure's code 
}