ios - 在Swift(iOS)中一个接一个地调用异步任务

对于iOS Weather应用程序，我试图快速使用调度异步一一运行多个异步任务时遇到问题。我希望我的“update()”函数能够:

获取用户的位置(并将纬度和经度存储在类变量中)

定位服务完成后，根据新填充的经纬度变量

调用天气API
apit(和随后的XML解析)完成后，

，更新UI(iOS表视图)

(请放心，我是一位自学成才的编码员，所以我假设那里经验丰富的人将能够指出各种错误!非常感谢您的帮助。)

    var latitude: String = ""
    var longitude: String = ""
    var locationManager: CLLocationManager!
    var forecastData: Weather = Weather() // the weather class has it's own asynchronous NSURLSession called retrieveForecast()
                                          // which calls the Open Weather Map API and parses the XML

    func refresh() {
        // Here's where I don't know what I'm doing:
        let group = dispatch_group_create()
        dispatch_group_enter(group)
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
            self.getLocation()
            dispatch_group_leave(group)
        }
        dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
            self.getWeather()
        }
        self.updateUI() // ...and I know that this is in totally the wrong place!
    }


    // function(s) to get phone's location:
    func getLocation() {
        locationManager = CLLocationManager()
        locationManager.delegate = self
        locationManager.desiredAccuracy = kCLLocationAccuracyHundredMeters
        locationManager.requestWhenInUseAuthorization()
        locationManager.distanceFilter = 100.0
        locationManager.startUpdatingLocation()
    }
    func locationManager(manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        self.locationManager.stopUpdatingLocation()
        manager.stopUpdatingLocation()
        for item: AnyObject in locations {
            if let location = item as? CLLocation {
                if location.horizontalAccuracy < 1000 {
                    manager.stopUpdatingLocation()
                    self.latitude = String(location.coordinate.latitude)
                    self.longitude = String(location.coordinate.longitude)
                }
            }
        }
    }

    // function to download and parse the weather data within forecastData object
    func getWeather() {
            let apiCall = "http://api.openweathermap.org/data/2.5/forecast?lat=" + self.latitude
            + "&lon=" + self.longitude + "&mode=xml&appid=" + self.openWeatherMapAPIKey
        NSLog("getWeather called with api request: \(apiCall)")
        self.forecastData.retrieveForecast(apiCall)
    }

最佳答案

对于任何异步操作，拥有完成回调是一种好方法。
在您的情况下，如果您已经实现了getLocation和getWeather的回调，则永远不需要dispatch_groups，您只需从getWeather的回调中调用getLocation，然后就可以从refreshUI的回调中调用getWeather。

    var latitude: String = ""
    var longitude: String = ""
    var locationManager: CLLocationManager!
    var forecastData: Weather = Weather() // the weather class has it's own asynchronous NSURLSession called retrieveForecast()
                                          // which calls the Open Weather Map API and parses the XML

    func refresh() {
        self.getLocation()
    }


    // function(s) to get phone's location:
    func getLocation() {
        locationManager = CLLocationManager()
        locationManager.delegate = self
        locationManager.desiredAccuracy = kCLLocationAccuracyHundredMeters
        locationManager.requestWhenInUseAuthorization()
        locationManager.distanceFilter = 100.0
        locationManager.startUpdatingLocation()
    }
    func locationManager(manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        self.locationManager.stopUpdatingLocation()
        manager.stopUpdatingLocation()
        for item: AnyObject in locations {
            if let location = item as? CLLocation {
                if location.horizontalAccuracy < 1000 {
                    manager.stopUpdatingLocation()
                    self.latitude = String(location.coordinate.latitude)
                    self.longitude = String(location.coordinate.longitude)
                    self.getWeather()
                }
            }
        }
    }

    // function to download and parse the weather data within forecastData object
    func getWeather() {
            let apiCall = "http://api.openweathermap.org/data/2.5/forecast?lat=" + self.latitude
            + "&lon=" + self.longitude + "&mode=xml&appid=" + self.openWeatherMapAPIKey
        NSLog("getWeather called with api request: \(apiCall)")
        self.forecastData.retrieveForecast(apiCall)
        // assuming that self.forecastData.retrieveForecast(apiCall) is completely synchronous, we can call updateUI below
        self.updateUI()
    }

这是代码，演示如何正确使用dispatch_group:

func refetchOrdersAndChats(remoteNotificationData: [NSObject : AnyObject], completion: ((Bool)->())?) {
    var overallSuccess: Bool = true
    let refetchGroup = dispatch_group_create();

    dispatch_group_enter(refetchGroup);
    CPChatController.sharedInstance.updateChat(remoteNotificationData, completion: { success in
        overallSuccess = success && overallSuccess
        dispatch_group_leave(refetchGroup);
    })

    dispatch_group_enter(refetchGroup);
    CPChatController.sharedInstance.fetchNewOrdersWithNotification(remoteNotificationData, completion: { success in
        overallSuccess = success && overallSuccess
        dispatch_group_leave(refetchGroup);
    })

    dispatch_group_notify(refetchGroup,dispatch_get_main_queue(), {
        completion?(overallSuccess)
    })
}