swift中void函数中的dispatch_async和意外的非void返回值 [英] dispatch_async and unexpected non-void return value in void function in Swift

问题描述

我的 appDelegate 中有一个函数可以返回用户的当前位置。

I have a function in my appDelegate that returns user's current location.

现在我想调用它在其他地方异步，我做了：

Now I want to call it asynchronously at some other place and I did:

func handleLocation() -> CLLocation {
  let priority = DISPATCH_QUEUE_PRIORITY_DEFAULT
  dispatch_async(dispatch_get_global_queue(priority, 0)) {
    let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
    appDelegate.startGPS()
    while (!appDelegate.isLocationFixed()) {
      sleep(1)
    }
    dispatch_async(dispatch_get_main_queue()) {
      return appDelegate.getLocation()
    }
  }
}

但现在这行返回appDelegate.getLocation（）给我带来错误：

but now this line return appDelegate.getLocation() brings me the error:

void函数中意外的非void返回值

unexpected non-void return value in void function

我不太了解中的线程Swift 但是，你能帮我解决这个问题吗？

I don't know too much about threads in Swift yet, could you help me with fixing this issue?

推荐答案

问题是

dispatch_async(dispatch_get_global_queue(priority, 0)) {

和

dispatch_async(dispatch_get_main_queue()) {

实际上是创建闭包/函数，因此其中的任何代码都与该函数有关，而不是

are actually creating closures/functions, so any code within them will relate to that function, not

func handleLocation() -> CLLocation {

如果你在函数中进行异步操作，你真的不能拥有异步操作完成后的return语句。相反，您应该在函数中使用完成处理程序。例如：

If you're doing an asynchronous operation within a function, you can't really have a return statement after the asynchronous operation is completed. Instead, you should use a completion handler in your function. e.g.:

func aSyncFunction(completionHandler: (AnyObject) -> ()) {

    //do async opporation

    completionHandler("finished") // call the completion block when finished
}

以下是我将如何为您的用例实现它：

Here's how I would implement it for your use case:

func handleLocation(completionBlock: (CLLocation?) -> ()) {

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {

        guard let appDelegate = UIApplication.sharedApplication().delegate as? AppDelegate else {
            dispatch_async(dispatch_get_main_queue()) {
                completionBlock(nil)
            }
            return
        }
        appDelegate.startGPS()
        while (!appDelegate.isLocationFixed()) {
            sleep(1)
        }
        dispatch_async(dispatch_get_main_queue()) {
            completionBlock(appDelegate.getLocation())
        }
    }
}

示例用法：

handleLocation { (location: CLLocation?) in
    if let location = location {
         //use valid location
    }
}

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