Dispatch group notify seems to get called too early

Question

I am trying to use dispatch group as suggested here https://stackoverflow.com/a/35906703/406322

However, it seems like myGroup.notify is being called before all the iterations of the for loop is completed. What am I doing wrong?

let myGroup = DispatchGroup()

for channel in channels.subscribedChannels() {
    myGroup.enter()

    buildUser(channel) { (success, user) in
        if success {
            addUser(user)
        }

        print("Finished request \(user.id)")
        myGroup.leave()
    }
}

myGroup.notify(queue: .main) {
    print("Finished all requests.")
}

The output is this:

Finished request 1
Finished all requests.
Finished request 2

What do you get if you put `print("Entering request")` before the line `myGroup.enter()` ? — OOPer, Sep 22 '18 at 02:52

James Woodrow · Accepted Answer · 2018-09-22T02:13:21.020

Not sure, but isn't your print("Finished request \(user.id)") being called from a thread and therefore can be called after your print("Finished all requests.") since it's on a main priority queue ?

try replacing

print("Finished request \(user.id)")

by:

DispatchQueue.main.async {
    print("Finished request \(user.id)")
}

Testing it out in a playground works fine:

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class User {
    var id: Int
    init(id: Int) {
        self.id = id
    }
}

class Channel {
    var user: User
    init(user: User) {
        self.user = user
    }
}

var subscribedChannels: [Channel] = []
let user1 = User(id: 1)
let user2 = User(id: 2)
subscribedChannels.append(Channel(user: user1))
subscribedChannels.append(Channel(user: user2))
let myGroup = DispatchGroup()
let bgQueue = DispatchQueue.global(qos: .background)

func doSomething(channel: Channel, callback: @escaping (Bool, User) -> Void) {
    print("called for \(channel.user.id)")
    bgQueue.asyncAfter(deadline: .now() + 1) {
        callback(true, channel.user)
    }
}

for channel in subscribedChannels {
    myGroup.enter()
    doSomething(channel: channel) { (success, user) in
        if success {
            //
        }

        print("Finished request \(user.id)")
        myGroup.leave()
    }
}

myGroup.notify(queue: .main) {
    print("Finished all requests.")
}

this prints

called for 1
called for 2

then 1 second later

Finished request 1
Finished request 2
Finished all requests.

I don't know your classes and methods so it's hard for me to know more

Dispatch group notify seems to get called too early

1 Answers1