How to call 2 different api using dispatch_async and update UI in Swift

Question

I'm trying to make background api call and update UI when it returns the result, I've check GCD documentation.
Is there a problem with logic below?

    // get data from server
    let priority = DISPATCH_QUEUE_PRIORITY_BACKGROUND
    dispatch_async(dispatch_get_global_queue(priority, 0)) {
       self.getUserData() // update self.data1 global variable
       self.getCompanyData() // update self.data2 global variable

        dispatch_async(dispatch_get_main_queue()) {
                self.updateUI() //update UI using self.data1 and self.data2 global variable
        }

    }


func getUserData(){
... api1 call
... print("1")

}

func getCompanyData(){
... api2 call
... print("2")
}

func updateUI(){
... print("UI")
}

When it executes, output is basically; UI
1
2

I would like to call updateUI function after api both of api calls are finished.

one of the api call function below;

let fullUrl = self.apiDomain + "/user_details/" + String(self.user_id)
    let url:NSURL = NSURL(string: fullUrl)!
    let session = NSURLSession.sharedSession()
    let request = NSMutableURLRequest(URL: url)
    request.HTTPMethod = "GET"
    request.cachePolicy = NSURLRequestCachePolicy.ReloadIgnoringCacheData

    let task = session.dataTaskWithRequest(request) {
        (
        let data, let response, let error) in
        guard let _:NSData = data, let _:NSURLResponse = response  where error == nil else {
            print(error)
            return
        }

            var apiCallResult: [String:AnyObject]?
            let result = NSString(data: data!, encoding: NSUTF8StringEncoding)
            do {
                apiCallResult = try NSJSONSerialization.JSONObjectWithData(result!.dataUsingEncoding(NSUTF8StringEncoding)!, options: []) as? Dictionary<String, AnyObject>
            } catch let error as NSError {
                print(error.localizedDescription)
                return
            }

            let aa = apiCallResult!["data"]! as! NSDictionary
            print(aa)

    }

    task.resume()

There is no problem if both getUserData() and getCompanyData() do not work asynchronously. — vadian
– vadian, Commented Jan 6, 2016 at 14:44
Like @vadian hinted, the problem is because "getUserData" and "getCompanyData" are also asynchronous API calls. — Eric Aya
– Eric Aya, Commented Jan 6, 2016 at 14:55
@EricD. any advice? How can I wait both of them finish and update the UI? — code_ada
– code_ada, Commented Jan 6, 2016 at 14:59
I guess just like this right? self.getUserData() -> self.getCompanyData() -> self.updateUI() .... 3 of them should be in same dispatch_async — code_ada
– code_ada, Commented Jan 6, 2016 at 15:01

Kiet Nguyen · Accepted Answer · 2016-01-06 17:49:25Z

4

You can use dispatch groups. Here is the code example for waiting on synchronous tasks:

let group = dispatch_group_create()

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) { () -> Void in
    print("Block1")
    NSThread.sleepForTimeInterval(1.0)
    print("Block1 End")
}

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) { () -> Void in
    print("Block2")
    NSThread.sleepForTimeInterval(1.0)
    print("Block2 End")
}

dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) { () -> Void in
    print("Block3")
}

and could produce output like this:

Block1
Block2
Block1 End
Block2 End
Block3

If you use dispatch groups to wait on multiple asynchronous tasks, you need to use dispatch_group_enter, dispatch_group_leave and dispatch_group_wait. Here is the code example.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) { () -> Void in
    let group = dispatch_group_create()

    print("Block1")
    dispatch_group_enter(group)
    foo1.sendAsynchronousRequest() {
        print("Block1 End")
        dispatch_group_leave(group)
    }

    print("Block2")
    dispatch_group_enter(group)
    foo2.sendAsynchronousRequest() {
        print("Block1 End")
        dispatch_group_leave(group)
    }

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER)
    dispatch_async(dispatch_get_main_queue(), { () -> Void in
        print("Block3")
    })
}

Read here for more details http://www.raywenderlich.com/79150/grand-central-dispatch-tutorial-swift-part-2

edited Jan 6, 2016 at 17:49

answered Jan 6, 2016 at 15:47

Kiet Nguyen

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7 Comments

code_ada Over a year ago

When I try with prints, it works as you mentioned. But when I call function instead of print, which calls api to get results, it prints block3 and then api call result.

Kiet Nguyen Over a year ago

I think getUserData() and getCompanyData() needs to work synchronous because dispatch_group_async already supports asynchronous.

code_ada Over a year ago

can you explain a bit more please? Btw, thank you for "dispatch groups", it's very useful.

Kiet Nguyen Over a year ago

I think you call the functions that work asynchronous inside the dispatch_group_async for block 1 and block 2 so It immediately marks the block 1 and block 2 finished and notify to block 3.

code_ada Over a year ago

What is foo1 and foo2?

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