Why is dispatchGroup.notify called after only the first task has exited?

Question

In the following code the output is as follows:

1) Did the other thing 

**2) Did all the things** 

3) Did one thing 

4) done waiting

I would expect:

1) Did the other thing 

2) Did one thing 

3) done waiting

**4) Did all the things**

DispatchQueue.global().async {

        let dispatchGroup = DispatchGroup()

        dispatchGroup.notify(queue: DispatchQueue.main) {
            print("Did all the things")
        }


        dispatchGroup.enter()
        DispatchQueue.global().asyncAfter(deadline: .now() + 10) {
        print("Did one thing")
            dispatchGroup.leave()

        }


        dispatchGroup.enter()
        DispatchQueue.global().async {
            print("Did the other thing")
            dispatchGroup.leave()
        }

        dispatchGroup.wait()
        print("done waiting")

    }

As a side note if I perform this on the main thread it works as expected.

user_834333 · Answer

According to the very minimal Apple docs: https://developer.apple.com/documentation/dispatch/dispatchgroup

func notify(queue: DispatchQueue, work: DispatchWorkItem) Schedules a work item to be submitted to a queue when a group of previously submitted block objects have completed.

In my example above I called dispatchQueue.notify before I had submitted my blocks to the queue. By updating the code as follows, I was able to get the expected behavior.

DispatchQueue.global().async {

        let dispatchGroup = DispatchGroup()



        dispatchGroup.enter()
        DispatchQueue.global().asyncAfter(deadline: .now() + 10) {
        print("Did one thing")
            dispatchGroup.leave()

        }


        dispatchGroup.enter()
        DispatchQueue.global().async {
            print("Did the other thing")
            dispatchGroup.leave()
        }

        dispatchGroup.notify(queue: DispatchQueue.main) {
            print("Did all the things")
        }

        dispatchGroup.wait()
        print("done waiting")

    }

Why is dispatchGroup.notify called after only the first task has exited?

1 Answers