在调用 UI 之前不等待函数完成的快速信号量

Question

我试图延迟一个 segue，直到我得到一个reverseGeocodeLocation调用的响应。但是，当使用断点检查值实际更改的时间时，它仍然在 UI 转换发生后发生。我尝试让函数为空，并返回当前String。

编辑的代码：

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调度到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

我认为这不是

信号量的问题，而是您的getReversedGeocodeLocation函数的问题。 CLGeocoder().reverseGeocodeLocation是一个异步函数，因此 return 语句将在执行 CLGeocoder().reverseGeocodeLocation 的完成块之前执行，这就是self.newAddress被分配一个空字符串的原因。

若要解决此问题，可以使用getReversedGeocodeLocation内的另一个信号量来阻止，直到完成处理程序完成，或者使用委托通知某些内容完成处理程序已完成（这通常是正确的方法）。