Swift DispatchGroup notify before task finish

Question

I'm using DispatchGroup to perform a task, but group.notify is being called before the task is completed.

My code:

let group = DispatchGroup()
let queueImage = DispatchQueue(label: "com.image")
let queueVideo = DispatchQueue(label: "com.video")
queueImage.async(group: group) {
    sleep(2)
    print("image")
}

queueVideo.async(group: group) {
    sleep(3)
    print("video")
}

group.notify(queue: .main) {
    print("all finished.")
}

Logs:

all finish.
image
video

Answer 1

Update: The question above actually runs correctly as is (as rmaddy pointed out!)

I'm saving this wrong answer below in case others get confused about DispatchQueue's async(group:) methods behavior, since Apple's swift doc on it is currently lousy.

---

The group's enter() needs to be called before each call to async(), and then the group's leave() needs to be called at end of each async() block, but within the block. It's basically like a refcount that when it reaches zero (no enters remaining), then the notify block is called.

let group = DispatchGroup()
let queueImage = DispatchQueue(label: "com.image")
let queueVideo = DispatchQueue(label: "com.video")

group.enter()
queueImage.async(group: group) {
    sleep(2)
    print("image")
    group.leave()
}

group.enter()
queueVideo.async(group: group) {
    sleep(3)
    print("video")
    group.leave()
}

group.notify(queue: .main) {
    print("all finished.")
}

Answer 2

Generic answer : (Swift 5)

let yourDispatchGroup = DispatchGroup()

yourDispatchGroup.enter()
task1FunctionCall {
  yourDispatchGroup.leave() //task 1 complete
}

yourDispatchGroup.enter()
task2FunctionCall {
  yourDispatchGroup.leave() //task 2 complete
}

.. ..
yourDispatchGroup.enter()
tasknFunctionCall {
  yourDispatchGroup.leave() //task n complete
}

dispatchGroup.notify(queue: .main) {
  //This is invoked when all the tasks in the group is completed.
}

Answer 3

If your DispatchGroup is a lazy var, try to not call the notify method inside the initialization code block.

lazy var dispatchGroup: DispatchGroup = {
    let dispatchGroup = DispatchGroup()
    
    // not call here dispatchGroup.notify(...

    return dispatchGroup
}()

You need to call all the enter methods before the notify method:

dispatchGroup.enter()

dispatchQueue.async(group: dispatchGroup) {
    // ...
    self.dispatchGroup.leave()
}

dispatchGroup.notify(queue: .main) {
    print("all finished.")
}