An Introduction to the Combine Framework in iOS

An Introduction to the Combine Framework in iOS

Apple's implementation of Functional Reactive Programming

The Combine framework in iOS is a declarative Swift API that allows developers to process values over time, which can represent various kinds of asynchronous events. It is Apple's implementation of Functional Reactive Programming (FRP) and is used to handle asynchronous code like networking, user interface events, and other types of asynchronous data.

Core Concepts of Combine

  • Publishers: These are components that produce values over time. They have two associated types: Output (the type of value they produce) and Failure (the type of error they might produce). If a publisher cannot produce an error, its Failure type is Never.

  • Subscribers: These components receive a stream of values, completion, or failure events from publishers. A subscriber can request a certain number of values from the publisher, which then sends the requested values, fewer values, or a completion event.

  • Operators: These are methods that act on values coming from a publisher and perform tasks such as mapping, filtering, reducing, and more. They can also handle errors and manage threading.

Advantages of Combine

  • Declarative Syntax: Combine allows developers to write code that clearly expresses the intent of the asynchronous operations, making it easier to understand and maintain.

  • Type Safety: Combine is type-safe, which helps catch errors at compile time rather than at runtime.

  • Composition: Combine's core concepts are simple, but when combined, they can create complex and powerful data processing chains.

  • Request-Driven: Combine is designed to allow developers to manage memory usage and performance more carefully.

Integration with SwiftUI

Combine is deeply integrated with SwiftUI. Types like ObservableObject and property wrappers like @Published use Combine under the hood, making it a crucial part of the SwiftUI ecosystem.

Getting Started with Combine

To effectively use Combine, developers should understand the basic principles and start with smaller projects to get familiar with the framework. It's recommended to explore Combine through tutorials, WWDC sessions, and Swift Playgrounds to grasp its concepts and usage.


While Combine is powerful, it comes with a learning curve. Developers and their teams need to be comfortable with the framework to avoid creating a codebase that is difficult to debug and maintain. It's important to consider whether the use of Combine is necessary for the project at hand.

Wrap Up

In summary, the Combine framework is a powerful tool for handling asynchronous programming in a declarative and type-safe manner, which is especially useful in modern Swift development, including SwiftUI.

For more check out WWDC 2019 Session 722 entitled Introducing Combine with Tony Parker Manager of the Foundation team at Apple.

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