IOS DEV-Disclosure of GCD Secret Weapons, explore lesser known features to upgrade your dev life

3 min readMar 4, 2023

Grand Central Dispatch (GCD) is a powerful framework that lets you manage and execute tasks in parallel on different threads. While GCD is often used to run tasks asynchronously on different queues, there are a number of lesser-known features of GCD that can simplify complex concurrency tasks. In this article, we’ll explore some of the advanced features of GCD and how they can be used to write cleaner, simpler code.

DispatchWorkItem

One of the most powerful features of GCD is the ability to cancel tasks. With DispatchWorkItem, you can encapsulate tasks in a work item and cancel them easily. This can be especially useful when you need to cancel a task that is running on a background thread. Here’s an example:

let workItem = DispatchWorkItem {
// Perform some task
}

// Execute the work item after a delay
DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(5), execute: workItem)
// Cancel the work item
workItem.cancel()

In this example, we create a work item and execute it after a delay. If we need to cancel the work item before it’s executed, we can call the cancel() method.

DispatchGroup

Another powerful feature of GCD is the ability to group and synchronize tasks using DispatchGroup. This can be useful when you need to perform a group of tasks before you can continue with your logic. With DispatchGroup, you can synchronize tasks that can run concurrently on different queues. Here’s an example:

let group = DispatchGroup()
group.enter()
DispatchQueue.global(qos: .background).async {
// Perform some task
group.leave()
}
group.enter()
DispatchQueue.global(qos: .background).async {
// Perform some task
group.leave()
}
group.notify(queue: DispatchQueue.main) {
// All tasks are complete
}

In this example, we create a dispatch group and add two tasks to it. The notify() method is called when all tasks in the group are complete. This can be useful when you need to perform a group of network requests and combine their results.

DispatchSemaphore

DispatchSemaphore is a powerful feature of GCD that allows you to wait for a group of asynchronous tasks to complete. This can be useful when you need to perform a synchronous operation in a command-line tool or script. With DispatchSemaphore, you can wait for a group of tasks to complete before continuing. Here’s an example:

let semaphore = DispatchSemaphore(value: 0)
DispatchQueue.global(qos: .background).async {
// Perform some task
semaphore.signal()
}
// Wait for the task to complete
semaphore.wait()

In this example, we create a semaphore with a value of 0. We then perform a task on a background thread and signal the semaphore when it’s complete. We can then use the wait() method to block until the semaphore is signaled.

DispatchSource

DispatchSource is a powerful feature of GCD that lets you observe changes in a file system, a signal or a mach port. This can be useful when you need to observe changes in a file or a signal. With DispatchSource, you can easily observe changes and respond to them. Here’s an example:

let fileDescriptor = open("/path/to/file", O_EVTONLY)
let source = DispatchSource.makeFileSystemObjectSource(fileDescriptor: fileDescriptor, eventMask: .write, queue: DispatchQueue.main)

source.setEventHandler {
// Handle file changes
}
source.resume()

In this example, we create a DispatchSource to observe changes in a file. We specify the file descriptor, the event mask and the queue to run on. We then set an event handler to handle file changes and call the resume()

Conclusion

In conclusion, Grand Central Dispatch is a powerful technology that every Swift developer should be familiar with. While it is primarily known for its ability to dispatch work on different concurrent queues, it has a lot of other features that are often overlooked.

In this article, we explored some lesser-known features of GCD, such as DispatchWorkItem, DispatchGroup, DispatchSemaphore, and DispatchSource. We provided examples and explanations on how to use these features, and how they can provide simpler and more “Swifty” options to many other Foundation APIs.

By taking advantage of these features, you can write more efficient and performant code, simplify your codebase, and even build more sophisticated and powerful developer tools. So, the next time you are faced with a challenging asynchronous task, consider using GCD to help you out.