Swift DispatchGroup notify before task finish

问题

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

回答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.")
}

回答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.
}

来源：https://stackoverflow.com/questions/49376157/swift-dispatchgroup-notify-before-task-finish

标签

ios

swift

dispatch