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DispatchGroup vs DispatchSemaphore in iOS (Swift)

 In this post, I will share my knowledge about how to synchronise async code in swift programming language. Puzzled? Let read it to clarify :)

In my current project, I had to develop a solution where I need to perform multiple api calls in a loop, but synchronously and application should not freeze.

Initially, I tried with DispatchGroup, but this approach was successful only if I have to run all apis calls asynchronously. Let’s look at below code snippet which illustrate the difference between DispatchGroup and DispatchSemaphore.

DispatchGroup:

In below example, api.performSomeAction() will be executed concurrently.

This method call api in asynchronous manner.

    /// - parameter ids: Array of identifer which will pass to api's post data

    private func insertData(ids: [String]) {

        

        let group = DispatchGroup()

        

        ///Method which will execute when below if loop completes
         // (When dispatch group is notified)

        func completion() {

            //Perform some action in the end

        }

        

        ///All api calls will be parallel.

        for identifier in ids {

            group.enter()

            api.performSomeAction(id: identifier, success: { [weak self] _ in

                group.leave()

             }, failure: { _ in

                group.leave()

            })

        }

        

        group.notify(queue: .main) {

            completion()

        }

    }

DispatchSemaphore

In below example, api.performSomeAction() will be executed sequentially.

   /// This method call api in synchronous manner without blocking the UI

    /// - parameter ids: Array of identifer which will pass to api's post data

    private func insertData(ids: [String]) {

        

        let group = DispatchGroup()

        

        ///Method which will execute when below if loop completes

        func completion() {

            //Perform some action in the end

        }

        

        DispatchQueue.global().async {

            let semaphore = DispatchSemaphore(value: 0)

            

            ///All api calls will be sequential.

            for identifier in ids {

                api.performSomeAction(id: identifier, success: { [weak self] _ in

                    semaphore.signal()

                 }, failure: { (_) in

                    semaphore.signal()

                })

                _ = semaphore.wait(timeout: .distantFuture)

            }

            completion()

        }

    }

Conclusion:

Dispatch Semaphore: A shared resource is not accessed at the same time using semaphore. Dispatch Group: If tasks are independent of one another, use dispatch group.

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