Swift：for循环中的异步调用

Question

我正在尝试快速执行以下操作-尝试反向解码数组中的地址列表并打印其纬度/经度坐标。我的代码如下。

    let addressArray = ["Address 1", "Address 2"]
    var coordinatesArray = [CLLocationCoordinate2D]()

    override func viewDidLoad() {
        super.viewDidLoad()
        createAddressList()
        printAddressList()
    }

    func printAddressList() {
        for i in 0 ..< addressArray.count  {
            print("Address = \(addressArray[i]) Coordinates = \(coordinatesArray[i].latitude),\(coordinatesArray[i].latitude)")
    }

    func createAddressList() {
        for i in 0 ..< addressArray.count {
            let address = addressArray[i]
            geocoder.geocodeAddressString(address, completionHandler: {(placemarks, error) -> Void in
                print("Address = \(address)");
                if let placemark = placemarks?.first {
                    let coordinate = placemark.location?.coordinate
                    self.coordinatesArray.append(coordinate!)
                }
            })
        }
    }
}

该代码仅显示已解码的第一个地址，然后没有任何反应。

我确实有如下修复方法，如下所示，它是将printAddressList调用从viewDidLoad方法中移出，如下所示

func createAddressList() {
    if count < self.addressArray.count {
        let address = addressArray[count]
        geocoder.geocodeAddressString(address, completionHandler: {(placemarks, error) -> Void in
            print("Address = \(address)");
            if let placemark = placemarks?.first {
                let coordinate = placemark.location?.coordinate
                self.coordinatesArray.append(coordinate!)
            }
            print("Count = \(self.count)")
            self.count += 1
            self.createAddressList()
        })
    } else {
        printAddressList()
    }
}

尽管后一种解决方案可行，但我发现它并不干净，我想知道在使代码可读和干净的同时执行此操作的正确方法。

Answer 1

如何使用这种结构？

let workGroup = dispatch_group_create()

for i in 0..<addressArray.count {

    dispatch_group_enter(workGroup)

    performGeoCoding({  successCallback : 

            dispatch_group_leave(workGroup)

    })
}

dispatch_group_notify(workGroup, dispatch_get_main_queue()){ 
    successCallback()   
    printAddressList()                    
}

有一个关于dispatch_group很不错的教程在这里。