Swift 信号量在调用 UI 之前不等待函数完成

Question

我正在尝试延迟转场，直到我收到来自 reverseGeocodeLocation 电话的回复。但是，当使用断点检查值何时实际更改时，它仍然会在 UI 转换发生后发生。我尝试过让函数为 void 并使用当前的 String 返回。

编辑代码：

func getReversedGeocodeLocation(completionHandler: (String, NSError?) ->()) {

    let semaphore = dispatch_semaphore_create(0)

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) { () -> Void in

        CLGeocoder().reverseGeocodeLocation(self.newMeetupLocation, completionHandler: {(placemarks, error) -> Void in

            if error != nil {
                print("Reverse geocoder failed with error" + error!.localizedDescription)
                return
            }
            else if placemarks?.count > 0 {

            }
            else {
                print("Problem with the data received from geocoder")
            }

            completionHandler(placemarks!.first!.name! ?? "", error)
        })

        dispatch_semaphore_signal(semaphore)

    }
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
}

旧代码：

let semaphore = dispatch_semaphore_create(1) //even with 0, it's not working

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) { () -> Void in

            self.newAddress = self.getReversedGeocodeLocation()

            dispatch_semaphore_signal(semaphore)
        }

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)

        //dispatch_semaphore_signal(semaphore)

        print(self.newAddress + ".")

        self.performSegueWithIdentifier("mainToNewAddress", sender: self)

func getReversedGeocodeLocation() -> String{

    var address = ""
    CLGeocoder().reverseGeocodeLocation(self.newAddressLocation, completionHandler: {(placemarks, error) -> Void in

        if error != nil {
            print("Reverse geocoder failed with error" + error!.localizedDescription)
            return
        }
        else if placemarks?.count > 0 {
            let pm = placemarks?.first!
            address = pm!.name!
        }
        else {
            print("Problem with the data received from geocoder")
        }
    })

    return address
}

Answer 1

使用信号量并将调用分派到getReversedGeocodeLocation 是不必要的复杂。 CLGeocoder().reverseGeocodeLocation 已经是异步的。如果您只是将完成处理程序闭包传递给getReversedGeocodeLocation，那么您可以使用它来调用segue；

self.getReversedGeocodeLocation(self.newAddressLocation, completionHandler: { (address,error) in 
    guard error == nil else {
        print("Reverse geocoder failed with error" + error!.localizedDescription)
        return
    }

    guard let address = address else {
        print("No address returned")
        return
    }

    self.newAddress = address
    dispatch_async(dispatch_get_main_queue(), {
        self.performSegueWithIdentifier("mainToNewAddress", sender: self)
    })
})

func getReversedGeocodeLocation(addressLocation: String, completionHandler:((address: String?, error: NSError?) -> Void))) {
    CLGeocoder().reverseGeocodeLocation(self.newAddressLocation, completionHandler: {(placemarks, error) -> Void in

        var address = nil

        if placeMarks.count > 0 {
            if let pm = placeMarks!.first {
                address  = pm.name
            }
        } else {
            print("Problem with the data received from geocoder")
        }

        completionHandler(address,error)
    })
}

Answer 2

我不认为信号量有问题，它与您的getReversedGeocodeLocation 函数有关。 CLGeocoder().reverseGeocodeLocation 是一个异步函数，因此 return 语句将在 CLGeocoder().reverseGeocodeLocation 的完成块执行之前执行，这就是为什么 self.newAddress 被分配一个空字符串。

要解决此问题，您可以使用 getReversedGeocodeLocation 中的另一个信号量来阻止直到完成处理程序完成，或者使用委托通知完成处理程序完成的某些内容（这通常是正确的方法）。