How to check when multiple concurrent threads have finished?

Question

I have code that goes like this:

myArray.forEach { item in
   concurentOperation(item)
}

Every item in the array goes through a concurrent operation function, which runs in different threads, I'm not sure exactly which thread or how many threads because the function is from a third party library and out of my control. I need a way to find out once all operations are finished.

How can I do this?

Answer 1

without modifying concurentOperation() this is NOT available, sorry ...

UPDATE for user @Scriptable

next snippet demonstrates, why his solution doesn't work ...

import PlaygroundSupport
import Dispatch

PlaygroundPage.current.needsIndefiniteExecution = true

let pq = DispatchQueue(label: "print", qos: .background)
func dprint(_ items: Any...) {
    pq.async {
        let r = items.map{ String(describing: $0) }.joined(separator: " ")
        print(r)
    }
}

func concurrentOperation<T>(item: T) { // dummy items
    DispatchQueue.global().async {
        // long time operation
        for i in 0..<10000 {
            _ = sin(Double(i))
        }
        dprint(item, "done")
    }
}



let myArray = [1,2,3,4,5,6,7,8,9,0]
let g = DispatchGroup()
myArray.forEach { (item) in
    DispatchQueue.global().async(group: g) {
        concurrentOperation(item: item)
    }
}
g.notify(queue: DispatchQueue.main) {
    dprint("all jobs done???")
}

UPDATE 2

without modifying the code of ConcurrentOperation()

DispatchQueue.global().async(group: g) {
            concurrentOperation(item: item)
        }

the dispatch group is entered and immediately left because concurrentOperation is the asynchronous function. If it is synchronous then the question has no sense.