How to know when multiple asynchronous works are done?

dispatch-group.swift

/*
DispatchGroup 
Create a group, register a bunch of `enter` events, `leave` when a task completes 
and the group will automatically do code in `notify` block when all work is done
*/

import UIKit

// Example for calling API
func callAPI(delay: Int, completion: @escaping () -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + DispatchTimeInterval.seconds(delay)) {
        completion()
    }
}

let dispatchGroup = DispatchGroup()

dispatchGroup.enter()
callAPI(delay: 2) { [weak dispatchGroup] in
    defer { dispatchGroup?.leave() }
    // handle API result here
    print("Finish API 1")
}

dispatchGroup.enter()
callAPI(delay: 1) { [weak dispatchGroup] in
    defer { dispatchGroup?.leave() }
    print("Finish API 2")
}

dispatchGroup.enter()
callAPI(delay: 4) { [weak dispatchGroup] in
    defer { dispatchGroup?.leave() }
    print("Finish API 3")
}

dispatchGroup.notify(queue: .main) {
    print("Finish 3 APIs")
}

/*
Finish API 2
Finish API 1
Finish API 3
Finish 3 APIs
*/