How to use DispatchGroup in Swift?

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DispatchGroup allows you to add a set of tasks and schedule them for asynchronous execution. This behaviour can be helpful when progress can’t be made until all of the specified tasks are complete.

For instance, If you have several long-running tasks to perform and you’d like to run some further logic, you need DispatchGroup. However, You can run each task sequentially, but it is not very efficient.

let group = DispatchGroup()

func downloadImages() {
    // Mimicking the list of image URLs
    for i in 0..<5 {
        group.enter()
        print("Image download start - \(i)")

        // Mimicking the download fucntion
        DispatchQueue.global().async {
            sleep(arc4random() % 5)
            print("Image downloaded - \(i)")
            group.leave()
        }
    }

    group.notify(queue: DispatchQueue.global()) {
        print("All images are downloaded.")
        // ... makeCollage()
    }
}

downloadImages()

//Output
//Image download start - 0
//Image download start - 1
//Image download start - 2
//Image download start - 3
//Image download start - 4
//Image downloaded - 0
//Image downloaded - 4
//Image downloaded - 1
//Image downloaded - 3
//Image downloaded - 2
//All images are downloaded.

In the code example, I have downloaded many images concurrently, followed by the makeCollage function. When all the images are downloaded, notify will be called.

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Each call to enter() must be matched later on with a call to leave(), after which the group will call the closure provided to notify(). If you do not want to wait for the groups to finish, but instead want to run a function once all the tasks have completed, use the notify function in place of the group.wait()

DispatchGroup wait() key take ways –

• wait() – blocks the current thread until the group’s tasks have completed.
• wait(timeout) – blocks the current thread, but after the timeout specified.
• wait(timeout) – returns an enum that can be used to determine whether the group completed or timed out.

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