为什么 .geocodeAddressString 不会更改外部变量？

Question

我有一个外部变量chooseCoordinates，它的类型是CLLocationCoordinate2D。这个 var 需要从 geocodeAddressString 闭包中保存坐标，但显然它不会改变。

我想请教一下，如何让这个真正的闭包存储数据，这样我就可以将它解析到另一个viewController

var chooseCoordinates = CLLocationCoordinate2D()
////////////

let geocoder = CLGeocoder()

    geocoder.geocodeAddressString(sender.text!, completionHandler: { (placemarks, error) -> Void in

        if(error != nil) {
            print("\(error)")
        }

        if let placemark = placemarks?.first {
            let coordinates: CLLocationCoordinate2D = placemark.location!.coordinate
            self.chooseCoordinates = coordinates
        }
    })

Answer 1

geocodeAddressString 异步运行（即，即使该方法立即返回，它的completionHandler 闭包也可能在稍后被调用）。那么，您确定它没有改变，或者只是在您尝试使用chooseCoordinates 时没有改变？您应该启动 UI 的任何更新或从闭包（或 chooseCoordinates 的 didSet）中进行的任何更新，而不是立即这样做。我们看不到您如何使用chooseCoordinates，因此很难说得更具体。

例如：

geocoder.geocodeAddressString(sender.text!) { placemarks, error in
    if error != nil {
        print(error!)
    }

    if let placemark = placemarks?.first {
        let coordinates: CLLocationCoordinate2D = placemark.location!.coordinate
        self.chooseCoordinates = coordinates
        // call the method that uses `chooseCoordinates` here
    }
}

// don't try to use `chooseCoordinates` here, as it hasn't been set yet.

或者，您可以自己使用completionHandler 模式：

@IBAction func didEndEditingTextField(sender: UITextField) {
    geocodeAddressString(sender.text!) { coordinate, error in
        self.chooseCoordinates = coordinate
        // trigger whatever the next step is here, or in the `didSet` of `chooseCoordinates`
    }

    // don't try to use `chooseCooordinates` here
}

var chooseCoordinates: CLLocationCoordinate2D?  // you probably should make this optional

let geocoder = CLGeocoder()

/// Geocode string
///
/// - parameter string: The string to geocode.
/// - parameter completionHandler: The closure that is called asynchronously (i.e. later) when the geocoding is done.

func geocodeAddressString(string: String, completionHandler: (CLLocationCoordinate2D?, NSError?) -> ()) {
    geocoder.geocodeAddressString(string) { placemarks, error in
        if error != nil {
            print(error!)
        }

        if let placemark = placemarks?.first {
            completionHandler(placemark.location!.coordinate, error)
        } else {
            completionHandler(nil, error)
        }
    }
}