DispatchGroup in Swift

What is DispatchGroup?

DispatchGroup provides a mechanism to track the completion group of tasks.

How DispatchGroup works?

DispatchGroup has three main methods, enter, leave and notify, that allow you to control the completion of a specific task.

let dispatchGroup = DispatchGroup()
dispatchGroup.enter()
dispatchGroup.leave()
dispatchGroup.notify(queue: .main) {}

Let`s talk about each of these methods.

enter — manually indicate a block has entered group. leave — manually indicate a block in the group has been completed. notify(queue: ) — schedule a block to be submitted to a queue when all the blocks associated with a group have been completed. The queue parameter is the queue to which the supplied block will be submitted when the group is complete.

How to implement DispatchGroup?

You can implement DispatchGroup following these steps:

let dispatchGroup = DispatchGroup()

dispatchGroup.enter()
fetchGitHubUser1 {
    print("fetchGitHubUser1 task completed")
    dispatchGroup.leave()
}

dispatchGroup.enter()
fetchGitHubUser2 {
    print("fetchGitHubUser2 task completed")
    dispatchGroup.leave()
}

dispatchGroup.notify(queue: .main) {
    print("All tasks completed")
}

// prints
fetchGitHubUser1 task started
fetchGitHubUser2 task started
fetchGitHubUser1 task completed
fetchGitHubUser2 task completed
All tasks completed

Pros

You can create a group of tasks and track when all tasks finish their work. You can specify a queue where you want to be notified about completed operations.

Cons

You should manually manage enter and leave operations that increase complexity and the chance of error. You can accidentally forget to write the leave operation, which can cause unpredictable behavior.