Waiting until the task finishes

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.

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}

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)") }