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
– vadian, Commented Oct 21, 2017 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
– Palle, Commented Oct 21, 2017 at 13:15
@Palle downloadProductsByCategory itself is async method using alamofire — Timeless
– Timeless, Commented Oct 21, 2017 at 13:16
Then just put dispatchSemaphore.wait() after the call to downloadProductsByCategory and dispatchSemaphore.signal() in the completion handler. — Palle
– Palle, Commented Oct 21, 2017 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
– Timeless, Commented Oct 21, 2017 at 13:24

Palle · Accepted Answer · 2020-04-01 12:20:37Z

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 1, 2020 at 12:20

answered Oct 21, 2017 at 13:22

Palle

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6 Comments

Timeless Over a year ago

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?

Palle Over a year ago

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.

Brandon Over a year ago

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.

Jonny Over a year ago

THe dispatchSemaphore in this answer is not created anywhere.

Palle Over a year ago

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.

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