Swift 4 async call with for loop execute in order using DispatchGroup, DispatchQueue and DispatchSemaphore

Question

I want to run a for loop in swift in order, DispatchGroup will fire them together, so I want to use DispatchQueue and DispatchSemaphore to achieve my goal. I failed to make my program work, how can I force them to wait and run one by one?

let dispatchGroup = DispatchGroup()
let dispatchQueue = DispatchQueue(label: "taskQueue")
let dispatchSemaphore = DispatchSemaphore(value: 1)

for c in self.categories {

    dispatchSemaphore.wait()

    dispatchQueue.async(group: dispatchGroup) {

        if let id = c.categoryId {

            dispatchGroup.enter()

            self.downloadProductsByCategory(categoryId: id) { success, data in

                if success, let products = data {

                    self.products.append(products)
                }

                dispatchSemaphore.signal()
                dispatchGroup.leave()
            }
        }
    }
}

dispatchGroup.notify(queue: dispatchQueue) {

    self.refreshOrderTable { _ in

        self.productCollectionView.reloadData()

        NVActivityIndicatorPresenter.sharedInstance.stopAnimating()
    }
}

Thanks to Palle, here is my final code:

let dispatchGroup = DispatchGroup()
let dispatchQueue = DispatchQueue(label: "taskQueue")
let dispatchSemaphore = DispatchSemaphore(value: 0)

dispatchQueue.async {

    for c in self.categories {

        if let id = c.categoryId {

            dispatchGroup.enter()

            self.downloadProductsByCategory(categoryId: id) { success, data in

                if success, let products = data {

                    self.products.append(products)
                }

                dispatchSemaphore.signal()
                dispatchGroup.leave()
            }

            dispatchSemaphore.wait()
        }
    }
}

dispatchGroup.notify(queue: dispatchQueue) {

    DispatchQueue.main.async {

        self.refreshOrderTable { _ in

            self.productCollectionView.reloadData()
        }
    }
}

Unrelated but don't make the `weak self` - `strong self` dance. It's nonsensical because GCD does **not** cause retain cycles. — vadian, Oct 21 '17 at 13:07
If you want to execute in order, why use concurrency anyway? Just create a loop in a background queue and execute your `downloadProductsByCategory` function directly in the loop. — Palle, Oct 21 '17 at 13:15
@Palle `downloadProductsByCategory` itself is async method using alamofire — Timeless, Oct 21 '17 at 13:16
Then just put `dispatchSemaphore.wait()` after the call to `downloadProductsByCategory` and `dispatchSemaphore.signal()` in the completion handler. — Palle, Oct 21 '17 at 13:17
@Palle for some reason, when `dispatchSemaphore.wait()` called for the *second* time, the whole program will be blocked there forever. and `downloadProductsByCategory` never receive a callback — Timeless, Oct 21 '17 at 13:24
You have to create your semaphore with an initial value of 0, otherwise there will always be two concurrent download tasks. Is your code the same as in my answer? — Palle, Oct 21 '17 at 13:26
Why is there a `semaphore.wait()` call in the first line of the for loop? It should not be there. If `categoryId` is nil, this will cause more `wait` calls than `signal` calls and you will get a deadlock. The `wait` call has to be **after** the call to `downloadProductsByCategory`. — Palle, Oct 21 '17 at 13:30
There is no need for `DispatchGroup` when executing tasks synchronously.. https://pastebin.com/6MgAaJSq — Brandon, Dec 29 '17 at 01:13
@Brandon could you please answer the question and provide your solution? Thanks. — Timeless, Oct 13 '18 at 00:54
If you want asynchronous but still ordered then: https://pastebin.com/rqrA5Fxm — Brandon, Oct 13 '18 at 16:24
Why are you using a dispatch group and a semaphore at the same time? It makes absolutely no sense. — Peter Schorn, Nov 07 '20 at 18:49

Palle · Accepted Answer · 2020-04-01T12:20:37.793

19

You can put the whole loop in a block instead of putting just the download function in a block:

dispatchQueue.async {
    for c in self.categories {
        if let id = c.categoryId {
            self.downloadProductsByCategory(categoryId: id) { success, data in
                if success, let products = data {
                    self.products.append(products)
                }

                dispatchSemaphore.signal()
            }
            dispatchSemaphore.wait()
        }
    }
}

You can simplify your code by using compactMap to unwrap your product ids:

for id in self.categories.compactMap({$0.categoryId}) { ... }

edited Apr 01 '20 at 12:20

answered Oct 21 '17 at 13:22

Palle

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thanks, it works. but one more thing, how can I get notified when I get all data? can I use `DispatchGroup` with the current code? – Timeless Oct 21 '17 at 13:39
Just put a `DispatchQueue.main.async { ... }` in the block when you're done with the loop. A dispatch group with a single `enter` and `leave` will of course also work. – Palle Oct 21 '17 at 13:53
There really is NO need for a dispatch group when the code itself is synchronous. It doesn't make sense. Just call your completion block right after your for-loop. – Brandon Dec 29 '17 at 00:59
THe `dispatchSemaphore` in this answer is not created anywhere. – Jonny Apr 22 '19 at 07:55
The semaphore is initialized with a value of 0: `let dispatchSemaphore = DispatchSemaphore(value: 0)` before. You can see the full final solution in the question. – Palle Apr 22 '19 at 16:50
@Palle could you have a lookt at this question? It is similiar to this one but a bit different and I am stuck: https://stackoverflow.com/questions/62954319/run-for-loop-with-asynchronous-calls-in-order?noredirect=1#comment111329717_62954319 – Chris Jul 17 '20 at 14:57

Swift 4 async call with for loop execute in order using DispatchGroup, DispatchQueue and DispatchSemaphore

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