Reactive Programming in iOS with Combine

Feb 4 2021 · Swift 5.3, macOS 11.0, Xcode 12.2

Part 4: Timing, Scheduling and Sequencing Operators

23. Delay and Collect

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Have you ever heard the saying “Timing is everything”? That is especially true in Combine, which models asynchronous event flows over time. It’s no surprise then that Combine has a set of operators that deal with time, in particular how sequences react to and transform values over time. In this episode you’ll learn about two timing operators: delay and collect.

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 / elementsPerSecond, on: .main, in: .common)
  .autoconnect()
  .subscribe(sourcePublisher)

// 4
let sourceTimeline = TimelineView(title: "Emitted values (\(elementsPerSecond) 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)


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 }