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RxSwift: Reactive Programming with Swift

Fourth Edition · iOS 13 · Swift 5.1 · Xcode 11

Before You Begin

Section 0: 4 chapters
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Section I: Getting Started with RxSwift

Section 1: 4 chapters
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Section II: Operators & Best Practices

Section 2: 7 chapters
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Section III: iOS Apps with RxCocoa

Section 3: 2 chapters
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Section IV: Intermediate RxSwift/RxCocoa

Section 4: 4 chapters
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Section V: RxSwift Community Cookbook

Section 5: 6 chapters
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Section VI: Putting It All Together

Section 6: 3 chapters
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9. Combining Operators
Written by Florent Pillet

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In earlier chapters, you learned how to create, filter and transform observable sequences. RxSwift filtering and transformation operators behave much like Swift’s standard collection operators. You got a glimpse into the true power of RxSwift with flatMap, the workhorse operator that lets you perform a lot of tasks with very little code.

This chapter will show you several different ways to assemble sequences, and how to combine the data within each sequence. Some operators you’ll work with are very similar to Swift collection operators. They help combine elements from asynchronous sequences, just as you do with Swift arrays.

Getting started

For this chapter, you will be using an Xcode Playground set up with the basic building blocks you need to go through the chapter tasks.

To get started, open the macOS Terminal application (found in your Mac‘s Applications > Utilities folder), navigate to the current chapter’s starter project folder, then run the bootstrap script like so:

$ ./bootstrap.sh

You will again use the example(of:) construct to wrap your code in distinct blocks. Remember to show the Debug Area in Xcode (under the View and Debug Area menus), as this is where playground print(_:) statements display their output.

RxSwift is all about working with and mastering asynchronous sequences. But you’ll often need to make order out of chaos! There is a lot you can accomplish by combining observables.

Prefixing and concatenating

The first and most obvious need when working with observables is to guarantee that an observer receives an initial value. There are situations where you’ll need the “current state” first. Good use cases for this are “current location” and “network connectivity status.” These are some observables you’ll want to prefix with the current state.

Merging

RxSwift offers several ways to combine sequences. The easiest to start with is merge. Can you picture what it does from the diagram below?

Combining elements

An essential group of operators in RxSwift is the combineLatest family. They combine values from several sequences:

Triggers

Apps have diverse needs and must manage multiple input sources. You’ll often need to accept input from several observables at once. Some will simply trigger actions in your code, while others will provide data. RxSwift has you covered with powerful operators that will make your life easier. Well, your coding life at least!

Switches

RxSwift comes with two main so-called “switching” operators: amb(_:) and switchLatest(). They both allow you to produce an observable sequence by switching between the events of the combined or source sequences. This allows you to decide which sequence’s events will the subscriber receive at runtime.

Combining elements within a sequence

All cooks know that the more you reduce, the tastier your sauce will be. Although not aimed at chefs, RxSwift has the tools to reduce your sauce to its most flavorful components.

Challenge

You learned a lot about many operators in this chapter. But there is so much more to learn (and more fun to be had) about sequence combination!

Challenge: The zip case

You’ve learned about the zip family of operators that lets you go through sequences in lockstep — it’s time to start using it.

8. Transforming Operators in Practice
10. Combining Operators in Practice
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