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

It depends on how your `checkSpeed()` is invoking the download task. You need to show some more details of the method. — OOPer, Dec 17 '16 at 21:19
If you are talking about **one** file you need an asynchronous completion handler rather than a group. — vadian, Dec 17 '16 at 21:21

OOPer · Answer 1 · 2016-12-17T22:56:14.823

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)")
}

OOPer: apologize GCD has already confused me from the beginning, but without going into too much details, `DispatchGroup` group is mainly used for..?...groups of asynchronous tasks? — Pangu, Dec 17 '16 at 22:02
_`DispatchGroup` is mainly used for groups of asynchronous tasks?_ Right. `DispatchGroup` is intended to use with multiple asynchronous tasks, to expect to be notified when **all** the tasks in the group has finished. When all the tasks entered in the group have called `DispatchGroup.leave()`, the closure specified with `notify(queue:execute:)` is invoked. — OOPer, Dec 17 '16 at 22:11

score 3 · Accepted Answer · edited May 23 '17 at 12:16

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.

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

2 Answers2