Combine: A Declarative API for Asynchronous Data Processing in Swift

Combine is a framework for Swift introduced by Apple in 2019 that provides a declarative API. This makes it ideal for working with asynchronous data, such as network requests and user input. Combine is also a powerful tool for building reactive user interfaces.

In this blog post, we will take a look at the basics of Combine, including publishers, subscribers, and operators. We will also see how Combine can be used to build asynchronous applications and reactive user interfaces.

What is Combine?

Combine is a reactive programming framework that provides a declarative API for processing values over time. This means that you can describe the desired behaviour of your code without having to worry about the details of how it will be implemented.

Combine is based on the following concepts:

• Publishers: Publishers emit values over time. They can be anything from network requests to user input.

• Subscribers: Subscribers receive values from publishers. They can do things like map values, filter values, and perform other operations.

• Operators: Operators are functions that combine publishers and subscribers. They can be used to perform common tasks, such as combining multiple publishers, filtering values, and retrying failed requests.

Using Combine to Build Asynchronous Applications in Swift

Combine is ideal for building asynchronous applications. This is because it provides a way to handle asynchronous events in a declarative way. For example, you can use Combine to make a network request and then subscribe to the response. The subscriber can then handle the response, such as mapping it to a model or displaying it in a user interface.

Here is an example of how to use Combine to make a network request:

let publisher = URLSession.shared.dataTaskPublisher(for: URL(string: "https://api.myhost.com")!)

publisher.subscribe(on: RunLoop.main) { data, _, error in
    if let data = data {
        let json = try JSONDecoder().decode(MyJSONModel.self, from: data)
        // Do something with the model
    } else if let error = error {
        // Handle the error
    }
}
    

This code creates a publisher that emits the response data from the network request. The subscriber then handles the response data, either mapping it to a model or displaying it in a user interface.

Using Combine to Build Reactive User Interfaces

Combine can also be used to build reactive user interfaces. This is because it provides a way to update user interfaces in response to changes in data. For example, you can use Combine to subscribe to a publisher that emits the current user location. The subscriber can then update the user interface to display the user's location.

Here is an example of how to use Combine to update a user interface with the current user location:

let publisher = locationManager.publisher(for: .location)

publisher.subscribe(on: RunLoop.main) { location in
    // Update the user interface with the new location
}
    

This code creates a publisher that emits the current user location. The subscriber then updates the user interface to display the user's location.

Using custom Combine implementation

Let us take a look at using PassthroughSubject to implement asynchronous declarative API.

A PassthroughSubject is a type of publisher in Combine that emits any value that is sent to it. It does not have an initial value or a buffer of the most recently-published element. This makes it ideal for use in situations where you need to react to changes in data as they happen.

import Combine

let subject = PassthroughSubject<String, Never>()

subject.sink { string in
    print(string)
}

subject.send("Hello, world!")
subject.send("This is a second message")

Here, the first line imports the Combine framework. This is needed to use the PassthroughSubject and sink operators.

The second line creates a PassthroughSubject publisher. This publisher will emit any string that is sent to it.

The third line attaches a sink subscriber to the PassthroughSubject publisher. The sink subscriber will print each string that is emitted by the publisher to the console.

The fourth and fifth lines send two strings to the PassthroughSubject publisher. These strings will be printed to the console by the sink subscriber.

Conclusion

Combine is a framework that provides a declarative API for processing values over time. This makes it ideal for working with asynchronous data and building reactive user interfaces. If you are new to Combine, I encourage you to check out the official documentation and tutorials.

I hope this blog post has given you a basic understanding of Combine. If you have any questions, please feel free to leave a comment below.