Syncronize async functions

Question

I try to create my first project with swift 3.

I try to get data from my API. This works pretty good if I manually start the function. I need to synchronize the async request.

I need to trigger my function 3 times and wait for the others to complete.

makeGetCall(URLstring: "api1")

wait to complete

makeGetCall(URLstring: "api2")

wait to complete

makeGetCall(URLstring: "api3")

Set this to background and trigger every 5 seconds.

func makeGetCall(URLstring: String, update: Bool) {

    let completeURL = "http://myapi/" + URLstring


    // Set up the URL request
    guard let url = URL(string: completeURL) else {
        print("Error: cannot create URL")
        return
    }
    let urlRequest = URLRequest(url: url)

    // set up the session
    let config = URLSessionConfiguration.default
    let session = URLSession(configuration: config)

    // make the request
    let task = session.dataTask(with: urlRequest) {
        (data, response, error) in
        // check for any errors
        guard error == nil else {
            print("error calling GET on /todos/1")
            print(error as Any)
            return
        }
        // make sure we got data
        guard let responseData = data else {
            print("Error: did not receive data")
            return
        }

            // parse the result as XML
        if URLstring == "devicelist.cgi" {
            self.readDevice(XMLData: responseData)
        }

        if URLstring == "statelist.cgi" {
            self.readDeviceData(XMLData: responseData, update: update)
        }

        if URLstring == "functionlist.cgi" {
            self.readGewerke(XMLData: responseData)
        }
    }

    task.resume()

}

Can somebody help please.

Hagen

This is what I tried with completion handler:

    override func viewDidLoad() {
    super.viewDidLoad()

    makeGetCall(input: "statelist.cgi") {
        (result: Bool) in
        print("finished statelist")

    }
    makeGetCall(input: "devicelist.cgi") {
        (result: Bool) in
        print("finished devicelist")
    }


    makeGetCall(input: "functionlist.cgi") {
        (result: Bool) in
        print("finished functionlist")
    }

}

func makeGetCall(input: String, completion: @escaping (_ result: Bool) -> Void) {



    let completeURL = "http://192.168.0.25/addons/xmlapi/" + input


    // Set up the URL request
    guard let url = URL(string: completeURL) else {
        print("Error: cannot create URL")
        return
    }
    let urlRequest = URLRequest(url: url)

    // set up the session
    let config = URLSessionConfiguration.default
    let session = URLSession(configuration: config)

    // make the request
    let task = session.dataTask(with: urlRequest) {
        (data, response, error) in
        // check for any errors
        guard error == nil else {
            print("error calling GET on /todos/1")
            print(error as Any)
            return
        }
        // make sure we got data
        guard data != nil else {
            print("Error: did not receive data")
            return
        }
         completion(true)

    }

    task.resume()


}

If I put the 3 calls together it worked as it should. I thing it should also work with GCD but most of examples for swift 2.

    makeGetCall(input: "devicelist.cgi") {
        (result: Bool) in
        print("finished devicelist")
        self.makeGetCall(input: "functionlist.cgi") {
            (result: Bool) in
            print("finished functionlist")
           self.makeGetCall(input: "statelist.cgi") {
                (result: Bool) in
                print("finished statelist")

            }
        }
    }

Maybe now somebody can help.

Thanks Hagen

Add a completion handler to makeGetCall() and use a DispatchGroup to wait or get notified. — shallowThought
– shallowThought, Commented Feb 9, 2017 at 20:38
I tried to solve my problem but didn't´t get working. Can somebody give me advice. — Hagen
– Hagen, Commented Feb 10, 2017 at 15:41

shallowThought · Accepted Answer · 2017-02-20 10:29:32Z

1

Use a DispatchGroup to get notified when something happens.

override func viewDidLoad() {
    super.viewDidLoad()

    let stateListGroup = DispatchGroup()

    stateListGroup.enter()
    makeGetCall(input: "statelist.cgi") {
        (result: Bool) in
        print("finished statelist")
        stateListGroup.leave()
    }

    let deviceListGroup = DispatchGroup()
    deviceListGroup.enter()

    // the notify closure is called when the (stateList-) groups enter and leave counts are balanced.
    stateListGroup.notify(queue: DispatchQueue.main)  {
        self.makeGetCall(input: "devicelist.cgi") {
            (result: Bool) in
            print("finished devicelist")
            deviceListGroup.leave()
        }
    }

    let functionListGroup = DispatchGroup()
    functionListGroup.enter()

    deviceListGroup.notify(queue: DispatchQueue.main)  {
        self.makeGetCall(input: "functionList") {
            (result: Bool) in
            print("finished functionlist")
            functionListGroup.leave()
        }
    }

    functionListGroup.notify(queue: DispatchQueue.main)  {
        print("update ui here")
    }
}

Prints:

statelist.cgi
finished statelist
devicelist.cgi
finished devicelist
functionList
finished functionlist
update ui here

Also keep in mind that the completion handler of session.dataTask() is called on a background queue, so I recommend to dispatch completion(true) on the main queue to avoid unexpected behaviour:

DispatchQueue.main.async {
    completion(true)
}

edited Feb 20, 2017 at 10:29

answered Feb 13, 2017 at 19:45

shallowThought

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2 Comments

Hagen Over a year ago

Thanks a lot shallowThought. I modified it to group.notify(queue: DispatchQueue.main) and now it work as it should.

shallowThought Over a year ago

Oh, sorry. Written in the subway ... Fixed that in answer update. Here at SO we thank by up-voting/accepting the answer btw.

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