我可以区分“DispatchGroup”异步任务完成吗？

Question

Suppose I have three async functions A, B, C.

I want a callback after completion of these three, I will use DispatchGroup .

But I have a batch of these async tasks A, B, C. Suppose, 6 of them.

Now I want a callback after each of these A, B, C async task completion.

Like,

A
B
C
Callback
A
B
C
Callback
A
B
C
Callback

Using DispatchGroup , I am getting,

A
B
C
A
B
C
A
B
C
Callback

as group,notify() works only when everyone have group.leave() .

Sample,

// Called 6 times

private func fetchLists(in room: Room, 
completion: @escaping(                                                                   _ ownerList: [Any], 
_ memberList: [Any],
_ subscribersList: [Any]) -> Void) {

        var ownerList = [Any]()
        var memberList = [Any]()
        var subscribersList = [Any]()
        
        let group = DispatchGroup()

        group.enter()
        // Asyncronous
        fetchOwners(in: room, completion: { list in
            ownerList = list
            group.leave()
        })
        
        group.enter()
        // Asyncronous
        fetchMembers(in: room, completion: { list in
            memberList = list
            group.leave()
        })
        
        group.enter()
        // Asyncronous
        fetchSubscribers(in: room, completion: { list in
            subscribersList = list
            group.leave()
        })
    
        debug("Called 6 times")
        group.notify(queue: .main) {
            debug("Called 1 time. Want it to be called 6 times each with 3 batch.")
            completion(memberList, ownerList, subscribersList)
        }
    }

Answer 1

As per DispatchGroup 's documentation the notify method is called when group is empty, which means you have to call group.leave() as many times as you have called group.enter() . For your scenario, since you want to notify group each time a batch operation completes you have to use a separate group for each batch of tasks.