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Combine: Asynchronous Programming with Swift

First Edition · iOS 13 · Swift 5.1 · Xcode 11

Before You Begin

Section 0: 3 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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6. Time Manipulation Operators
Written by Florent Pillet

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Timing is everything. The core idea behind reactive programming is to model asynchronous event flow over time. In this respect, the Combine framework provides a range of operators that allow you to deal with time. In particular, how sequences react to and transform values over time.

As you’ll see throughout this chapter, managing the time dimension of your sequence of values is easy and straightforward. It’s one of the great benefits of using a framework like Combine.

Getting started

To learn about time manipulation operators, you’ll practice with an animated Xcode Playground that visualizes how data flows over time. This chapter comes with a starter playground you’ll find in the projects folder.

The playground is divided into several pages. You’ll use each page to exercise one or more related operators. It also includes some ready-made classes, functions and sample data that’ll come in handy to build the examples.

If you have the playground set to show rendered markup, at the bottom of each page there will be a Next link that you can click to go to the next page.

Note: To toggle showing rendered markup on and off, select Editor ▸ Show Rendered/Raw Markup from the menu.

You can also select the page you want from the Project navigator in the left sidebar or even the jump bar at the top of the page. There are lots of ways to get around in Xcode!

Look at Xcode, you can see controls at top-right of the window:

  1. Make sure the left sidebar button is enabled so you can see the list of Playground pages.
  2. Show the editor with Live View. This will display a live view of the sequences you build in code. This is where the real action will happen! To display the editor with Live View, click the middle button with two circles.

Also, remember that playgrounds can run manually or automatically. Showing the Debug area and configuring the playground for manual / automatic run is done using the controls at bottom left of the editor:

  1. Click the vertical arrow at left to show/hide the Debug area. This is where the debug prints go.
  2. Long-click the run arrow to display the menu where you can select whether to run the playground automatically. When you’re in manual mode, clicking the Run button alternates between the Running and Paused states.

Playground not working?

From time to time Xcode may “act up” and not run properly your playground. If this happens to you, open the Preferences dialog in Xcode and select the Locations tab. Click the arrow next to the Derived Data location, depicted by the red circled 1 in the screenshot below. It shows the DerivedData folder in the Finder.

Shifting time

Every now and again you need time traveling. While Combine can’t help with fixing your past relationship mistakes, it can freeze time for a little while to let you wait until self-cloning is available.

Collecting values

In certain situations, you may need to collect values emitted by a publisher at specified intervals. This is a form of buffering that can be useful. For example, when you want to average a group of values over short periods of time and output the average.

Collecting values (part 2)

The second option offered by the collect(_:options:) operator allows you to keep collecting values at regular intervals. It also allows you to limit the number of collected values.

Holding off on events

When coding user interfaces, you frequently deal with text fields. Wiring up text field contents to an action using Combine is a common task. For example, you may want to send a search URL request that returns a list of items matching what’s typed in the text field.

Debounce

Switch to the playground page named Debounce. Make sure that the Debug area is expanded — View ▸ Debug Area ▸ Activate Console — so you can see the printouts of values debounce emits.

Throttle

The kind of holding-off pattern that debounce allows is so useful that Combine provides a close relative: throttle(for:scheduler:latest:). It’s very close to debounce, but the differences justify the need for two operators.

Timing out

Next in this roundup of time manipulation operators is a special one: timeout. Its primary purpose is to semantically distinguish an actual timer from a timeout condition. Therefore, when a timeout operator fires, it either completes the publisher or emits an error you specify. In both cases, the publisher terminates.

Measuring time

To complete this roundup of time manipulation operators, you’ll look at one particular operator which doesn’t manipulate time but just measures it. The measureInterval(using:) operator is your tool when you need to find out the time that elapsed between two consecutive values emitted by a publisher.

Challenge

Challenge: Data

If time allows, you may want to try a little challenge to put this new knowledge to good use!

Solution

You’ll find the solution to this challenge in the challenge/final folder.

Key points

In this chapter, you looked at time from a different angle. In particular, you learned that:

Where to go from here?

This was a lot to learn. To put events in their right order, move along to the next chapter and learn about sequence operators!

5. Combining Operators
7. Sequence Operators
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