A Combine Kickstart

Combine is about to replace all those techniques we've used for decades for passing information around in an iOS or macOS App asynchronously: Target-Action, callbacks, closures, delegates, notifications... all of them.



This hand-on, fast-moving kickstart introduces you to the future of declarative and reactive programming on Apple platforms. We focus on core concepts and building discrete, easy-to-understand, pieces of a pipeline that allows your app to react to changes in the state.



Updated for Xcode 12 and Swift 5.3 and tested on Apple Silicon.



Chapter 1: The Forest



We begin the book with a simple example that doesn't use Combine at all and see how it works using UIKit and SwiftUI. We then refactor the UIKit example to use Combine and to introduce the four pillars of Combine: Publishers, Subscribers, Subscriptions, and Operators.



Chapter 2: Moving Out



In this chapter we distance the model from the view to make the setting more realistic. You'll learn to publish and subscribe across this distance, to use assign(to:on:) and assign(to:) in addition to the sink() we used in Chapter 1. We break the publishing chain into pieces and learn to republish bits to keep our controllers small and focused. We end the chapter with a deeper look at operators and we create and use a couple of our own.



Chapter 3: Sending Messages



In this chapter we focus on a variety of ways of publishing information. If you already use KVO or NotificationCenter, Combine makes this experience much nicer. If you want to add Combine to existing code, you can use a PassthroughSubject or CurrentValueSubject. We conclude with my favorite technique of creating a custom publishing chain and exposing it without revealing all of its details.



Chapter 4: Not Everything Lasts Forever



So far we've worked with simple publishers that last forever and never fail. In this chapter we look more closely at the publisher chain and show you how to handle publishers that fail either in sink() or using a catch() operator. We'll wrap the problem parts in a flatMap() for some extra magic at the end of the chapter.



Chapter 5: Single Streams



Foundation's Timer provides a publisher that we can use to emit values at regular intervals. Too fast for you? Use debounce(), throttle(), or removeDuplicates() to control the flow. In addition we use print() and breakpoint() to investigate and debug our flow and handleEvents() to kick off new processes.



Chapter 6: Multiple Streams



We take our Dice example from Chapter 5 and share() our source with multiple subscribers. Some, like count(), reduce(), and collect(), perform actions which don't publish until the source finishes. Want results right away? Use scan() instead. We bring multiple publishers together with merge(), combineLatest(), and zip().



Chapter 7: Very Far Away



Our journey ends with network calls. We use URLSession's custom publisher to retrieve values followed by operators to transform the returned output. If the data is JSON we use the decoder() operator to easily produce instances of custom types. We end with a look at how the Future publisher allows us to wrap methods with callbacks into Combine publishers.