Waiting until the task finishes

Solution 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.

Solution 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
}

Solution 3:

Swift 5 version of the solution

func myCriticalFunction() {
    var value1: String?
    var value2: String?

    let group = DispatchGroup()


    group.enter()
    //async operation 1
    DispatchQueue.global(qos: .default).async { 
        // Network calls or some other async task
        value1 = //out of async task
        group.leave()
    }


    group.enter()
    //async operation 2
    DispatchQueue.global(qos: .default).async {
        // Network calls or some other async task
        value2 = //out of async task
        group.leave()
    }

    
    group.wait()

    print("Value1 \(value1) , Value2 \(value2)") 
}

Solution 4:

Use dispatch group

dispatchGroup.enter()
FirstOperation(completion: { _ in
    dispatchGroup.leave()
})
dispatchGroup.enter()
SecondOperation(completion: { _ in
    dispatchGroup.leave()
})
dispatchGroup.wait() // Waits here on this thread until the two operations complete executing.