Async Download of Information from server

Question

I want to download data from my server to be displayed on a map. Therefore I use async methods to get the data. The goal is to have an array of annotation objects to be displayed.

Therefore I first download Information A and then Information B. As both are async methods, I guess I need to wait for the completionHandler to return true so I know the data is loaded. This is easy for one method. But how do I handle to wait for both methods before the completionHandler of getInformationFromServer returns true and triggers therefore the addition of annotations?

override func viewWillAppear(animated: Bool) {
    self.customizeInterface()
    self.getInformationFromServer { (completed) -> Void in
        if(completed) {
            self.mapView.addAnnotations(self.annotationArray)
        }
    }        
}

func getInformationFromServer(completionHandler: (completed: Bool) -> Void) {

    getInformationFromServerA { (downloadCompleted) -> Void in
        completionHandler(completed: downloadCompleted)
    }

// HOW DO I MANAGE TO ONLY RETURN THE COMPLETION HANDLER TRUE WHEN
// BOTH FUNCTIONS RETURNED TRUE? 

}

func getInformationFromServerA(completionHandler: (downloadCompleted: Bool) -> Void) {
    Server().getJsonInformationFromServer(url: "aeds", completionHandler: { (response) -> Void in
        self.parseAEDInformationToAnnotation(response["data"])
        completionHandler(downloadCompleted: true)
    })
}

func getInformationFromServerB(completionHandler: (downloadCompleted: Bool) -> Void) {
    Server().getJsonInformationFromServer(url: "aeds", completionHandler: { (response) -> Void in
        self.parseAEDInformationToAnnotation(response["data"])
        completionHandler(downloadCompleted: true)
    })
}

iOSX · Accepted Answer · 2015-10-21 22:01:35Z

You may use a dispatch group to wait until both downloads finish.

func getInformationFromServer(completionHandler: (completed: Bool) -> Void) {
    let dispatchGroup = dispatch_group_create()

    var downloadCompletedA: Bool = false
    dispatch_group_enter(dispatchGroup)
    getInformationFromServerA { (downloadCompleted) -> Void in
        downloadCompletedA = downloadCompleted
        dispatch_group_leave(dispatchGroup)
    }

    var downloadCompletedB: Bool = false
    dispatch_group_enter(dispatchGroup)
    getInformationFromServerB { (downloadCompleted) -> Void in
        downloadCompletedB = downloadCompleted
        dispatch_group_leave(dispatchGroup)
    }

    // wait until both downloads are finished
    dispatch_group_wait(dispatchGroup, DISPATCH_TIME_FOREVER)

    completionHandler(downloadCompletedA && downloadCompletedB)
}

See Apple's Concurrency Programming Guide:

Dispatch groups are a way to block a thread until one or more tasks finish executing. You can use this behavior in places where you cannot make progress until all of the specified tasks are complete.

Iurie Manea · Accepted Answer · 2015-10-21 22:12:30Z

Another solution which I can recommend for you and it's not so "complex" as solution with dispatch_group_wait:

func getInformationFromServer(completionHandler: (completed: Bool) -> Void) {

    getInformationFromServerA { [weak self] (downloadCompleted: Bool) -> Void in
        if downloadCompleted {
            self?.getInformationFromServerB({ (downloadCompleted: Bool) -> Void in
                completionHandler(completed: downloadCompleted)
            })
        }
        else {
            completionHandler(completed: downloadCompleted)
        }
    }
}

I was somehow hesitant to nest the methods for accessing the server. Hm

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