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Combine: Asynchronous Programming With Swift

Fourth Edition · iOS 16 · Swift 5.8 · Xcode 14

Before You Begin

Section 0: 5 chapters
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Section I: Introduction to Combine

Section 1: 2 chapters
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Section II: Operators

Section 2: 6 chapters
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Section III: Combine in Action

Section 3: 6 chapters
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Section IV: Advanced Combine

Section 4: 5 chapters
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Section V: Building a Complete App

Section 5: 2 chapters
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2. Publishers & Subscribers
Written by Marin Todorov

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Now that you’ve learned a few of Combine’s basic concepts, it’s time to jump in and play with two of Combine’s core components — publishers and subscribers.

In this chapter, you’ll experiment with various ways to create publishers and subscribe them so that you feel right at home when you need to do so throughout the book.

Note: There are starter and final versions of the playgrounds and projects that you’ll use in each of the chapters in this book. The starter is ready for you to enter the code for each example and challenge. You can compare your work with the final version at the end or along the way if you get stuck.

Getting Started

For this chapter, you’ll use an Xcode playground with Combine imported. Open Starter.playground in the projects folder and you’ll see a couple of import statements and a subscriptions set to use in your code.

Open Sources in the Project navigator (View ▸ Navigators ▸ Show Project Navigator and twist down the Combine playground page), and select SupportCode.swift. It contains the following helper function example(of:):

public func example(of description: String,
                    action: () -> Void) {
  print("\n——— Example of:", description, "———")
  action()
}

You’ll use this function to encapsulate each example you’ll work on in a playground throughout the book.

However, before you begin playing with those examples, you first need to look at publishers, subscribers and subscriptions in a bit more detail. They form the foundation of Combine and enable you to send and receive data, typically asynchronously.

Hello Publisher

At the heart of Combine is the Publisher protocol. This protocol defines the requirements for a type to be able to transmit a sequence of values over time to one or more subscribers. In other words, a publisher publishes or emits events that can include values of interest.

Hello Subscriber

Subscriber is a protocol that defines the requirements for a type to be able to receive input from a publisher. You’ll dive deeper into conforming to the Publisher and Subscriber protocols shortly; for now, you’ll focus on the basic flow.

Subscribing With sink(_:_:)

Continue in the previous example and add the following code:

Subscribing With assign(to:on:)

In addition to sink, the built-in assign(to:on:) operator enables you to assign the received value to a KVO-compliant property of an object.

Republishing With assign(to:)

There is a variation of the assign operator that you can use to republish values emitted by a publisher through another property marked with the @Published property wrapper. To try this add this new example to your playground:

Hello Cancellable

When a subscriber finishes its work and no longer wants to receive values from a publisher, it’s a good idea to cancel the subscription to free up resources and stop any corresponding activities from occurring, such as network calls.

Understanding What’s Going on

They say a picture is worth a thousand words, so let’s kick things off with one to explain the interplay between publishers and subscribers:

Vufzedraq Lowlzbobow 5 0 5 0 1 qitzhnerel wowiw zokllbimrieh seriugnz quxeal tilsq cuguev qaxnf zujtqijoug

Creating a Custom Subscriber

Time to put what you just learned to practice. Add this new example to your playground:

Hello Future

Much like you can use Just to create a publisher that emits a single value to a subscriber and then complete, a Future can be used to asynchronously produce a single result and then complete. Add this new example to your playground:

Hello Subject

You’ve already learned how to work with publishers and subscribers, and even how to create your own custom subscribers. There are just a couple more things standing between you and a well-deserved break. First is a subject.

Dynamically Adjusting Demand

You learned earlier that adjusting demand in Subscriber.receive(_:) is additive. You’re now ready to take a closer look at how that works in a more elaborate example. Add this new example to the playground:

Type Erasure

There will be times when you want to let subscribers subscribe to receive events from a publisher without being able to access additional details about that publisher.

Bridging Combine Publishers to async/await

Two great additions to the Combine framework in Swift 5.5, available in iOS 15 and macOS 12, help you effortlessly use Combine with the new async/await syntax in Swift.

Challenge

Completing challenges helps drive home what you learned in the chapter. There are starter and final versions of the challenge in the exercise files download.

Challenge 1: Create a Blackjack Card Dealer

Open Starter.playground in the challenge folder, and twist down the playground page and Sources in the Project navigator. Select SupportCode.swift.

Solution

How’d you do? There were two things you needed to add to complete this challenge. The first was to update the dealtHand publisher in the deal function, checking the hand’s points and sending an error if it’s over 21:

Key Points

  • Publishers transmit a sequence of values over time to one or more subscribers, either synchronously or asynchronously.
  • A subscriber can subscribe to a publisher to receive values; however, the subscriber’s input and failure types must match the publisher’s output and failure types.
  • Type erasure prevents callers from being able to access additional details of the underlying type.
  • Use the print() operator to log all publishing events to the console and see what’s going on.

Where to Go From Here?

Congratulations! You’ve taken a huge step forward by completing this chapter. You learned how to work with publishers to send values and completion events, and how to use subscribers to receive those values and events. Up next, you’ll learn how to manipulate the values coming from a publisher to help filter, transform or combine them.

1. Hello, Combine!
3. Transforming Operators
Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.
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