Swift Combine Series: Part 1 — Getting Started with Combine

Reactive programming has transformed the way developers handle asynchronous events and data streams. Swift Combine, introduced by Apple in iOS 13, is a robust framework designed to simplify this process for iOS and macOS developers. In this first part of our series, we’ll dive into the basics of Combine, exploring its core concepts and setting the foundation for tackling real-world problems in the upcoming parts.

Photo by Sahej Brar on Unsplash

What is Combine?

Combine is Apple’s declarative Swift framework for processing values over time. It provides tools to work with asynchronous events by combining publishers, subscribers, and operators. Combine is particularly powerful when dealing with network responses, user input, and state synchronization across your app.

Key benefits of Combine:

• Declarative code that’s easier to read and maintain.
• Built-in support for handling asynchronous and event-driven programming.
• Tight integration with Swift and Foundation.

Core Concepts

Before diving into code, it’s essential to understand the building blocks of Combine:

1. Publisher

A Publisher emits a sequence of values over time. Examples include notifications, network responses, or user inputs. Publishers in Combine are types that conform to the Publisher protocol.

2. Subscriber

A Subscriber receives and reacts to values or completion events from a publisher. You can think of a subscriber as the listener or consumer of the data stream.

3. Operators

Operators are methods used to transform, filter, or combine values emitted by publishers. They’re the core of Combine’s declarative approach.

4. Cancellable

When a subscriber subscribes to a publisher, it returns a Cancellable object. You use this to cancel the subscription and free resources when they’re no longer needed.

5. Subjects

Subjects act as both publishers and subscribers, allowing you to inject values into a data stream manually.

• PassthroughSubject: Emits values to its subscribers when explicitly told.
• CurrentValueSubject: Emits the current value to new subscribers and allows manual value injection.

Getting Started: A Basic Example

Let’s create a simple example using Combine to understand how publishers and subscribers work.

Example: A Simple Number Stream

import Combine

// 1. Create a PassthroughSubject
let numberSubject = PassthroughSubject<Int, Never>()

// 2. Create a Subscriber
let cancellable = numberSubject
    .map { $0 * 2 } // Multiply each number by 2
    .sink(receiveCompletion: { completion in
        switch completion {
        case .finished:
            print("Stream completed.")
        case .failure(let error):
            print("Error: \(error)")
        }
    }, receiveValue: { value in
        print("Received value: \(value)")
    })

// 3. Send values
numberSubject.send(1)
numberSubject.send(2)
numberSubject.send(3)
numberSubject.send(completion: .finished)

Output:

Received value: 2
Received value: 4
Received value: 6
Stream completed.

Practical Tips

1. Memory Management: Always store the Cancellable returned by a subscription. If you don’t, the subscription will be canceled immediately.
2. Debugging: Use the print() operator in your Combine chain to debug data streams.

let numberSubject = PassthroughSubject<Int, Never>()
numberSubject
    .print()
    .sink { ... }

3. Error Handling: Define clear error types when working with publishers that may fail. Use operators like catchor retry to manage failures.

What’s Next?

In the next part of this series, we’ll explore how to use Combine for network requests, handling JSON responses, and binding data to UI elements in SwiftUI. With the foundational concepts covered here, you’re ready to take the next step toward mastering Swift Combine.

Stay tuned for Part 2: Networking with Combine!