dispatch_async 和 Swift 中 void 函数中意外的非空值 return

dispatch_async and unexpected non-void return value in void function in Swift

我的 appDelegate 中有一个函数可以 returns 用户的当前位置。

现在我想在其他地方异步调用它,我做到了:

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()
    }
  }
}

但是现在这一行 return appDelegate.getLocation() 给我带来了错误:

unexpected non-void return value in void function

我还不太了解 Swift 中的线程,你能帮我解决这个问题吗?

问题是

dispatch_async(dispatch_get_global_queue(priority, 0)) {

dispatch_async(dispatch_get_main_queue()) {

实际上正在创建 closures/functions,因此其中的任何代码都将与该函数相关,而不是

func handleLocation() -> CLLocation {

如果你在一个函数内做异步操作,你不可能在异步操作完成后真正有一个return语句。相反,您应该在函数中使用完成处理程序。例如:

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施它:

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())
        }
    }
}

用法示例:

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