Using Protocols in Async iOS Development

When developing iOS applications, it's essential to have the ability to perform asynchronous tasks. Swift has several advanced features that allow us to execute methods in the background and handle their results when they are ready. In this tutorial, we will focus on using protocols to manage async calls.

What are Protocols?

Protocols are a fundamental part of Swift's type system. They allow us to define methods and properties that a class must implement if it wants to conform. A protocol provides a blueprint of functionality that must be implemented by a conforming class.

Async Calls in iOS

When executing a method asynchronously, we typically pass a completion block that will be called when the task is completed. This block can take parameters that represent the result of the call.

Here's an example of a method that performs an asynchronous network call:

func fetchData(completion: @escaping (Result) -> Void) {
    DispatchQueue.global(qos: .default).async {
        // Perform network call here.
        
        // Invoke the completion block with the result.
        DispatchQueue.main.async {
            completion(.success(data))
        }
    }
}

In this example, fetchData takes a completion block that will be called when the network call is finished. We use the global() method on the DispatchQueue class to run the call on a background thread. When the call is complete, we switch back to the main thread and invoke the completion block with the result.

Using Protocols to Define Completion Blocks

Instead of passing a closure as the completion block, we can define a protocol that defines the required methods for the completion block:

protocol DataFetcherDelegate {
    func didFetchData(data: Data)
    func didFailWithError(error: Error)
}

In this example, we define a DataFetcherDelegate protocol with two methods. Classes that conform to this protocol must implement both methods.

We can now modify the fetchData method to take a DataFetcherDelegate parameter instead of a closure:

func fetchData(delegate: DataFetcherDelegate) {
    DispatchQueue.global(qos: .default).async {
        // Perform network call here.
        
        // Invoke the appropriate delegate method.
        DispatchQueue.main.async {
            delegate.didFetchData(data)
        }
    }
}

When the network call is finished, we invoke the appropriate delegate method instead of the closure passed in the original example.

Conclusion

Using protocols to define completion blocks provides a more structured and type-safe way to handle asynchronous tasks in iOS development. By defining a protocol, we have a clear definition of what needs to be implemented when the async task is complete.