iOS - swift 3 - DispatchGroup

Question

I created this basic architecture to handle my networking stuff,

i wanted to keep it modular and structured:

public class NetworkManager {

    public private(set) var queue: DispatchQueue = DispatchQueue(label: "com.example.app.dispatchgroups", attributes: .concurrent, target: .main)
    public private(set) var dispatchGroup: DispatchGroup = DispatchGroup()

    private static var sharedNetworkManager: NetworkManager = {
        let networkManager = NetworkManager()
        return networkManager
    }()

    private init() {}

    class func shared() -> NetworkManager {
        return sharedNetworkManager
    }

    public func getData() {
        dispatchGroup.enter()

        queue.async(group: dispatchGroup) {
            Alamofire.request(Content.url).responseJSON { response in
                switch response.result {
                case .success(let value):
                    let json = JSON(value)
                    // do some stuff and save to Content struct
                    Content.annotations += [Station(...)]

                case .failure(let error):
                    print("error: ",error)
                }
            }

            self.dispatchGroup.leave()
        }
    }

}

struct Content {

    static var url = "url"

    static var annotations = [Station]()

}

So when i call this in my seperate class:

class MainViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // some stuff ...

        NetworkManager.shared().getData()

        NetworkManager.shared().dispatchGroup.notify(queue: DispatchQueue.main) {
            self.mapView.removeAnnotations(Content.annotations)
            self.mapView.addAnnotations(Content.annotations)
        }
    }

}

Buuut, it seems like DispatchGroup().notify() is executed before all requests finished... because no annotations are added to mapview.

I already checked and annotations are loaded.

Anybody could help me with this architecture?

Thanks and Greetings!

Answer 1

I think you need to put self.dispatchGroup.leave() inside the Alamofire response handler. As written, you leave as soon as you queue the request.

    queue.async(group: dispatchGroup) {
        Alamofire.request(Content.url).responseJSON { response in
            switch response.result {
            case .success(let value):
                let json = JSON(value)
                // do some stuff and save to Content struct
                Content.annotations += [Station(...)]

            case .failure(let error):
                print("error: ",error)
            }
            self.dispatchGroup.leave()
        }
    }

Answer 2

Change your code as shown below.

public func getData() {
    dispatchGroup.enter()
    queue.async(group: dispatchGroup) {
        Alamofire.request(Content.url).responseJSON { response in
            switch response.result {
            case .success(let value):
                let json = JSON(value)
                // do some stuff and save to Content struct
                Content.annotations += [Station(...)]

            case .failure(let error):
                print("error: ",error)
            }
            self.dispatchGroup.leave() // This statement has been moved
        }
    }
}

The mistake was that you were leaving the DispatchGroup soon after you entered it. If you have to wait for your network operation to complete, you should leave from within the response handler.