Using DispatchGroup() in Swift 3 to perform a task?

Question

With Swift 3, using GCD has changed to DispatchGroup(), and I'm trying to learn to use it in my code.

Currently I have a function in another class that attempts to download a file and output its speed. I like to have that function finish first because I assign its speed to a var that I will be using in the first class to perform other tasks that is dependent upon that var.

It goes something like this:

Second class:

func checkSpeed()
{
    // call other functions and perform task to download file from link

    // print out speed of download

    nMbps = speedOfDownload
}

First Class:

let myGroup = DispatchGroup()
let check: SecondClass = SecondClass()

myGroup.enter()

check.checkSpeed()

myGroup.leave()

myGroup.notify(queue: DispatchQueue.main, execute: {

    print("Finished all requests.")

    print("speed = \(check.nMbps)")
})

The problem is Finish all requests gets output first, thus returning nil for speed, then afterwards checkSpeed finishes and outputs the correct download speed.

I believe I'm doing this wrong, but I'm not sure?

How can I ensure that speed obtains the correct value after the completion of checkSpeed in my first class?

The details of checkSpeed is exactly the same as connectedToNetwork from GitHub: connectedness.swift

Answer 1

You need to call DispatchGroup.leave() when the entered task has completed. So, in your code, myGroup.leave() needs be placed at the end of the completion handler inside your checkSpeed() method.

You may need to modify your code like this:

func checkSpeed(in myGroup: DispatchGroup) {
    //...
    ...downLoadTask... {...its completion handler... in
        //...

        // print out speed of download

        nMbps = speedOfDownload

        myGroup.leave() //<- This needs to be placed at the end of the completion handler
    }
    //You should not place any code after invoking asynchronous task.
}

And use it as:

myGroup.enter()

check.checkSpeed(in: myGroup)

myGroup.notify(queue: DispatchQueue.main, execute: {

    print("Finished all requests.")

    print("speed = \(check.nMbps)")
})

But, as noted in vadian's comment or Pangu's answer, you usually do not use DispatchGroup for a single asynchronous task.

---

ADDITION

I need to say, I strongly recommend completion handler pattern shown in Pangu's answer. It's a more general way to handle asynchronous tasks.

If you have modified your checkSpeed() to checkSpeed(completion:) as suggested, you can easily experiment DispatchGroup like this:

let myGroup = DispatchGroup()
let check: SecondClass = SecondClass()
let anotherTask: ThirdClass = ThirdClass()

myGroup.enter() //for `checkSpeed`
myGroup.enter() //for `doAnotherAsync`

check.checkSpeed {
    myGroup.leave()
}
anotherTask.doAnotherAsync {
    myGroup.leave()
}

myGroup.notify(queue: DispatchQueue.main) {

    print("Finished all requests.")

    print("speed = \(check.nMbps)")
}

Answer 2

With hint provided in the comment and solution found here: from @vadian, since I am only performing one task, I used a async completion handler:

Second Class:

func checkSpeed(completion: @escaping () -> ())
{
    // call other functions and perform task to download file from link

    // print out speed of download

    nMbps = speedOfDownload
    completion()

}

First Class:

let check: SecondClass = SecondClass()

check.checkSpeed {
print("speed = \(check.nMbps)")

}

Now checkSpeed will complete first and speed is assigned the appropriate value.