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

我设置了

var arrayLength:Int = 0

在代码的开头。稍后在viewDidLoad（）内部我将其更改为3.当我调用它时，在任务块之外它再次等于0.这里的问题是什么，我做错了什么？

Answer 1

是的，您正在更改关闭内的arrayLength var。您正面临的问题只是线程工作的正常方式。

当viewDidLoad启动时，您将进入主线程
行let task = session.dataTaskWithURL(url, completionHandler: { ...定义了一个尚未执行的新任务
task.resume()在另一个线程上启动此任务。这需要一些时间才能完成。完成后，arrayLength将被更改
立即在主线程上询问您的arrayLength值

下面：

     println(arrayLength) // is equal to 0 again

不是arrayLength再次等于0 ，你没有给你的任务足够的时间来执行和改变那个值。

编辑：如果你需要在关闭完成后检查arrayLength ......

只需创建一个函数并在闭包内调用它。像这样：

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
}

变量不会在闭包之外变化

1 个答案: