Waiting until the task finishes

Question

How could I make my code wait until the task in DispatchQueue finishes? Does it need any CompletionHandler or something?

func myFunction() {
    var a: Int?

    DispatchQueue.main.async {
        var b: Int = 3
        a = b
    }

    // wait until the task finishes, then print 

    print(a) // - this will contain nil, of course, because it
             // will execute before the code above

}

I'm using Xcode 8.2 and writing in Swift 3.

Answer 1

Use DispatchGroups to achieve this. You can either get notified when the group's enter() and leave() calls are balanced:

func myFunction() {
    var a: Int?

    let group = DispatchGroup()
    group.enter()

    DispatchQueue.main.async {
        a = 1
        group.leave()
    }

    // does not wait. But the code in notify() gets run 
    // after enter() and leave() calls are balanced

    group.notify(queue: .main) {
        print(a)
    }
}

or you can wait:

func myFunction() {
    var a: Int?

    let group = DispatchGroup()
    group.enter()

    // avoid deadlocks by not using .main queue here
    DispatchQueue.global(attributes: .qosDefault).async {
        a = 1
        group.leave()
    }

    // wait ...
    group.wait()

    print(a) // you could also `return a` here
}

Note: group.wait() blocks the current queue (probably the main queue in your case), so you have to dispatch.async on another queue (like in the above sample code) to avoid a deadlock.

Answer 2

In Swift 3, there is no need for completion handler when DispatchQueue finishes one task. Furthermore you can achieve your goal in different ways

One way is this:

    var a: Int?

    let queue = DispatchQueue(label: "com.app.queue")
    queue.sync {

        for  i in 0..<10 {

            print("Ⓜ️" , i)
            a = i
        }
    }

    print("After Queue \(a)")

It will wait until the loop finishes but in this case your main thread will block.

You can also do the same thing like this:

    let myGroup = DispatchGroup()
    myGroup.enter()
    //// Do your task

    myGroup.leave() //// When your task completes
     myGroup.notify(queue: DispatchQueue.main) {

        ////// do your remaining work
    }

One last thing: If you want to use completionHandler when your task completes using DispatchQueue, you can use DispatchWorkItem.

Here is an example how to use DispatchWorkItem:

let workItem = DispatchWorkItem {
    // Do something
}

let queue = DispatchQueue.global()
queue.async {
    workItem.perform()
}
workItem.notify(queue: DispatchQueue.main) {
    // Here you can notify you Main thread
}