在Swift（iOS）中一个接一个地调用异步任务 [英] Calling asynchronous tasks one after the other in Swift (iOS)

问题描述

对于iOS天气应用程序，尝试在swift中使用dispatch async逐个运行多个异步任务时遇到问题。我想要'update（）'函数：

I'm having problems with trying to run several asynchronous tasks one by one using dispatch async in swift, for an iOS weather app. I want my 'update()' function to:

• 获取用户的位置（并将纬度和经度存储在类变量中）


• 当位置服务完成后，根据新填充的lat& amp;来调用weather API。 API调用（以及后续的XML解析）完成后，更新UI（iOS表视图）

（对我来说很容易，我是一名最近自学成才的编码员，所以我假设有更多经验的人可以指出各种错误！任何帮助都将受到大力赞赏。）

(Go easy on me, I'm a recently self-taught coder, so I'm assuming the more experienced of you out there will be able to point out various errors! Any help would be massively appreciated.)

    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)
    }

推荐答案

对于任何异步操作，最好有一个完成回调。
在你的情况下，如果你实现了 getLocation 和 getWeather 的回调，你将永远不需要 dispatch_groups ，你只需从 getLocation 的回调中调用 getWeather ，然后你只需从 getWeather 的回调中调用 refreshUI 。

For any asynchronous operation it is a good manner to have a finish callback. In your case, if you have implemented callbacks for getLocation and getWeather you'll never need dispatch_groups, you just call getWeather from getLocation's callback, and then you just call refreshUI from getWeather's callback.

    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()
    }

这里我代码，演示如何正确使用 dispatch_group ：

Here is the code, demonstrating how dispatch_group can be used right way:

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)
    })
}

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