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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.

let valuesPerSecond = 1.0
let delayInSeconds = 1.5

// 1
let sourcePublisher = PassthroughSubject<Date, Never>()

// 2
let delayedPublisher = sourcePublisher.delay(for: .seconds(delayInSeconds), scheduler: DispatchQueue.main)

// 3
let subscription = Timer
  .publish(every: 1.0 / valuesPerSecond, on: .main, in: .common)
  .autoconnect()
  .subscribe(sourcePublisher)

// 4
let sourceTimeline = TimelineView(title: "Emitted values (\(valuesPerSecond) per sec.):")

// 5
let delayedTimeline = TimelineView(title: "Delayed values (with a \(delayInSeconds)s delay):")

// 6
let view = VStack(spacing: 50) {
  sourceTimeline
  delayedTimeline
}

// 7
PlaygroundPage.current.liveView = UIHostingController(rootView: view)

sourcePublisher.displayEvents(in: sourceTimeline)
delayedPublisher.displayEvents(in: delayedTimeline)

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.

let valuesPerSecond = 1.0
let collectTimeStride = 4

// 1
let sourcePublisher = PassthroughSubject<Date, Never>()

// 2
let collectedPublisher = sourcePublisher
  .collect(.byTime(DispatchQueue.main, .seconds(collectTimeStride)))

let subscription = Timer
  .publish(every: 1.0 / valuesPerSecond, on: .main, in: .common)
  .autoconnect()
  .subscribe(sourcePublisher)

let sourceTimeline = TimelineView(title: "Emitted values:")
let collectedTimeline = TimelineView(title: "Collected values (every \(collectTimeStride)s):")

let view = VStack(spacing: 40) {
  sourceTimeline
  collectedTimeline
}

PlaygroundPage.current.liveView = UIHostingController(rootView: view)

sourcePublisher.displayEvents(in: sourceTimeline)
collectedPublisher.displayEvents(in: collectedTimeline)


let collectedPublisher = sourcePublisher
  .collect(.byTime(DispatchQueue.main, .seconds(collectTimeStride)))
  .flatMap { dates in dates.publisher }

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.

let collectMaxCount = 2

let collectedPublisher2 = sourcePublisher
  .collect(.byTimeOrCount(DispatchQueue.main,
                          .seconds(collectTimeStride),
                          collectMaxCount))
  .flatMap { dates in dates.publisher }

let collectedTimeline2 = TimelineView(title: "Collected values (at most \(collectMaxCount) every \(collectTimeStride)s):")

let view = VStack(spacing: 40) {
  sourceTimeline
  collectedTimeline
  collectedTimeline2
}

collectedPublisher2.displayEvents(in: collectedTimeline2)

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.

// 1
let subject = PassthroughSubject<String, Never>()

// 2
let debounced = subject
  .debounce(for: .seconds(1.0), scheduler: DispatchQueue.main)
  // 3
  .share()

public let typingHelloWorld: [(TimeInterval, String)] = [
  (0.0, "H"),
  (0.1, "He"),
  (0.2, "Hel"),
  (0.3, "Hell"),
  (0.5, "Hello"),
  (0.6, "Hello "),
  (2.0, "Hello W"),
  (2.1, "Hello Wo"),
  (2.2, "Hello Wor"),
  (2.4, "Hello Worl"),
  (2.5, "Hello World")
]

let subjectTimeline = TimelineView(title: "Emitted values")
let debouncedTimeline = TimelineView(title: "Debounced values")

let view = VStack(spacing: 100) {
	subjectTimeline
	debouncedTimeline
}

PlaygroundPage.current.liveView = UIHostingController(rootView: view)

subject.displayEvents(in: subjectTimeline)
debounced.displayEvents(in: debouncedTimeline)

let subscription1 = subject
  .sink { string in
    print("+\(deltaTime)s: Subject emitted: \(string)")
  }

let subscription2 = debounced
  .sink { string in
    print("+\(deltaTime)s: Debounced emitted: \(string)")
  }

subject.feed(with: typingHelloWorld)


+0.0s: Subject emitted: H
+0.1s: Subject emitted: He
+0.2s: Subject emitted: Hel
+0.3s: Subject emitted: Hell
+0.5s: Subject emitted: Hello
+0.6s: Subject emitted: Hello 
+1.6s: Debounced emitted: Hello 
+2.1s: Subject emitted: Hello W
+2.1s: Subject emitted: Hello Wo
+2.4s: Subject emitted: Hello Wor
+2.4s: Subject emitted: Hello Worl
+2.7s: Subject emitted: Hello World
+3.7s: Debounced emitted: Hello World

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.

let throttleDelay = 1.0

// 1
let subject = PassthroughSubject<String, Never>()

// 2
let throttled = subject
  .throttle(for: .seconds(throttleDelay), scheduler: DispatchQueue.main, latest: false)
  // 3
  .share()

let subjectTimeline = TimelineView(title: "Emitted values")
let throttledTimeline = TimelineView(title: "Throttled values")

let view = VStack(spacing: 100) {
  subjectTimeline
  throttledTimeline
}

PlaygroundPage.current.liveView = UIHostingController(rootView: view)

subject.displayEvents(in: subjectTimeline)
throttled.displayEvents(in: throttledTimeline)

let subscription1 = subject
  .sink { string in
    print("+\(deltaTime)s: Subject emitted: \(string)")
  }

let subscription2 = throttled
  .sink { string in
    print("+\(deltaTime)s: Throttled emitted: \(string)")
  }

subject.feed(with: typingHelloWorld)


+0.0s: Subject emitted: H
+0.0s: Throttled emitted: H
+0.1s: Subject emitted: He
+0.2s: Subject emitted: Hel
+0.3s: Subject emitted: Hell
+0.5s: Subject emitted: Hello
+0.6s: Subject emitted: Hello 
+1.0s: Throttled emitted: He
+2.2s: Subject emitted: Hello W
+2.2s: Subject emitted: Hello Wo
+2.2s: Subject emitted: Hello Wor
+2.4s: Subject emitted: Hello Worl
+2.7s: Subject emitted: Hello World
+3.0s: Throttled emitted: Hello W

let throttled = subject
  .throttle(for: .seconds(throttleDelay), scheduler: DispatchQueue.main, latest: true)
  .share()

+0.0s: Subject emitted: H
+0.0s: Throttled emitted: H
+0.1s: Subject emitted: He
+0.2s: Subject emitted: Hel
+0.3s: Subject emitted: Hell
+0.5s: Subject emitted: Hello
+0.6s: Subject emitted: Hello 
+1.0s: Throttled emitted: Hello
+2.0s: Subject emitted: Hello W
+2.3s: Subject emitted: Hello Wo
+2.3s: Subject emitted: Hello Wor
+2.6s: Subject emitted: Hello Worl
+2.6s: Subject emitted: Hello World
+3.0s: Throttled emitted: Hello World

...
+1.6s: Debounced emitted: Hello 
...
+3.7s: Debounced emitted: Hello World

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.

let subject = PassthroughSubject<Void, Never>()

// 1
let timedOutSubject = subject.timeout(.seconds(5), scheduler: DispatchQueue.main)

let timeline = TimelineView(title: "Button taps")

let view = VStack(spacing: 100) {
  // 1
  Button(action: { subject.send() }) {
    Text("Press me within 5 seconds")
  }
  timeline
}

PlaygroundPage.current.liveView = UIHostingController(rootView: view)

timedOutSubject.displayEvents(in: timeline)


enum TimeoutError: Error {
  case timedOut
}

let subject = PassthroughSubject<Void, TimeoutError>()

let timedOutSubject = subject.timeout(.seconds(5),
                                      scheduler: DispatchQueue.main,
                                      customError: { .timedOut })

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.

let subject = PassthroughSubject<String, Never>()

// 1
let measureSubject = subject.measureInterval(using: DispatchQueue.main)

let subjectTimeline = TimelineView(title: "Emitted values")
let measureTimeline = TimelineView(title: "Measured values")

let view = VStack(spacing: 100) {
  subjectTimeline
  measureTimeline
}

PlaygroundPage.current.liveView = UIHostingController(rootView: view)

subject.displayEvents(in: subjectTimeline)
measureSubject.displayEvents(in: measureTimeline)

let subscription1 = subject.sink {
  print("+\(deltaTime)s: Subject emitted: \($0)")
}

let subscription2 = measureSubject.sink {
  print("+\(deltaTime)s: Measure emitted: \($0)")
}

subject.feed(with: typingHelloWorld)

+0.0s: Subject emitted: H
+0.0s: Measure emitted: Stride(magnitude: 16818353)
+0.1s: Subject emitted: He
+0.1s: Measure emitted: Stride(magnitude: 87377323)
+0.2s: Subject emitted: Hel
+0.2s: Measure emitted: Stride(magnitude: 111515697)
+0.3s: Subject emitted: Hell
+0.3s: Measure emitted: Stride(magnitude: 105128640)
+0.5s: Subject emitted: Hello
+0.5s: Measure emitted: Stride(magnitude: 228804831)
+0.6s: Subject emitted: Hello 
+0.6s: Measure emitted: Stride(magnitude: 104349343)
+2.2s: Subject emitted: Hello W
+2.2s: Measure emitted: Stride(magnitude: 1533804859)
+2.2s: Subject emitted: Hello Wo
+2.2s: Measure emitted: Stride(magnitude: 154602)
+2.4s: Subject emitted: Hello Wor
+2.4s: Measure emitted: Stride(magnitude: 228888306)
+2.4s: Subject emitted: Hello Worl
+2.4s: Measure emitted: Stride(magnitude: 138241)
+2.7s: Subject emitted: Hello World
+2.7s: Measure emitted: Stride(magnitude: 333195273)

let subscription2 = measureSubject.sink {
  print("+\(deltaTime)s: Measure emitted: \(Double($0.magnitude) / 1_000_000_000.0)")
}

let measureSubject2 = subject.measureInterval(using: RunLoop.main)

let subscription3 = measureSubject2.sink {
  print("+\(deltaTime)s: Measure2 emitted: \($0)")
}

+0.0s: Subject emitted: H
+0.0s: Measure emitted: 0.016503769
+0.0s: Measure2 emitted: Stride(magnitude: 0.015684008598327637)
+0.1s: Subject emitted: He
+0.1s: Measure emitted: 0.087991755
+0.1s: Measure2 emitted: Stride(magnitude: 0.08793699741363525)
+0.2s: Subject emitted: Hel
+0.2s: Measure emitted: 0.115842671
+0.2s: Measure2 emitted: Stride(magnitude: 0.11583995819091797)
...

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.

// 1
let strings = subject
  // 2
  .collect(.byTime(DispatchQueue.main, .seconds(0.5)))
  // 3
  .map { array in
    String(array.map { Character(Unicode.Scalar($0)!) })
  }

// 4
let spaces = subject.measureInterval(using: DispatchQueue.main)
  .map { interval in
    // 5
    interval > 0.9 ? "👏" : ""
  }

// 6
let subscription = strings
  .merge(with: spaces)
  // 7
  .filter { !$0.isEmpty }
  .sink {
    // 8
    print($0)
  }

Combine
👏
is
👏
cool!

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
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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