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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.

example(of: "startWith") {
  // 1
  let numbers = Observable.of(2, 3, 4)

  // 2
  let observable = numbers.startWith(1)
  _ = observable.subscribe(onNext: { value in
    print(value)
  })
}

——— Example of: startWith ———
1
2
3
4


example(of: "Observable.concat") {
  // 1
  let first = Observable.of(1, 2, 3)
  let second = Observable.of(4, 5, 6)

  // 2
  let observable = Observable.concat([first, second])

  observable.subscribe(onNext: { value in
    print(value)
  })
}

example(of: "concat") {
  let germanCities = Observable.of("Berlin", "Münich", "Frankfurt")
  let spanishCities = Observable.of("Madrid", "Barcelona", "Valencia")

  let observable = germanCities.concat(spanishCities)
  _ = observable.subscribe(onNext: { value in
    print(value)
  })
}

example(of: "concatMap") {
  // 1
  let sequences = [
    "German cities": Observable.of("Berlin", "Münich", "Frankfurt"),
    "Spanish cities": Observable.of("Madrid", "Barcelona", "Valencia")
  ]

  // 2
  let observable = Observable.of("German cities", "Spanish cities")
    .concatMap { country in sequences[country] ?? .empty() }

  // 3
  _ = observable.subscribe(onNext: { string in
      print(string)
    })
}

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?

example(of: "merge") {
  // 1
  let left = PublishSubject<String>()
  let right = PublishSubject<String>()

  // 2
  let source = Observable.of(left.asObservable(), right.asObservable())

  // 3
  let observable = source.merge()
  _ = observable.subscribe(onNext: { value in
    print(value)
  })

  // 4
  var leftValues = ["Berlin", "Munich", "Frankfurt"]
  var rightValues = ["Madrid", "Barcelona", "Valencia"]
  repeat {
      switch Bool.random() {
      case true where !leftValues.isEmpty:
          left.onNext("Left:  " + leftValues.removeFirst())
      case false where !rightValues.isEmpty:
          right.onNext("Right: " + rightValues.removeFirst())
      default:
          break
      }
  } while !leftValues.isEmpty || !rightValues.isEmpty

  // 5
  left.onCompleted()
  right.onCompleted()
}

——— Example of: merge ———
Right: Madrid
Left:  Berlin
Right: Barcelona
Right: Valencia
Left:  Munich
Left:  Frankfürt

Combining elements

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

example(of: "combineLatest") {
  let left = PublishSubject<String>()
  let right = PublishSubject<String>()

  // 1
  let observable = Observable.combineLatest(left, right) {
    lastLeft, lastRight in
    "\(lastLeft) \(lastRight)"
  }

  _ = observable.subscribe(onNext: { value in
    print(value)
  })

  // 2
  print("> Sending a value to Left")
  left.onNext("Hello,")
  print("> Sending a value to Right")
  right.onNext("world")
  print("> Sending another value to Right")
  right.onNext("RxSwift")
  print("> Sending another value to Left")
  left.onNext("Have a good day,")

  left.onCompleted()
  right.onCompleted()
}

let observable = Observable
  .combineLatest(left, right) { ($0, $1) }
  .filter { !$0.0.isEmpty }

example(of: "combine user choice and value") {
  let choice: Observable<DateFormatter.Style> = Observable.of(.short, .long)
  let dates = Observable.of(Date())

  let observable = Observable.combineLatest(choice, dates) {
    format, when -> String in
    let formatter = DateFormatter()
    formatter.dateStyle = format
    return formatter.string(from: when)
  }

  _ = observable.subscribe(onNext: { value in
    print(value)
  })
}

  // 1
  let observable = Observable.combineLatest([left, right]) {
    strings in strings.joined(separator: " ")
  }


example(of: "zip") {
  enum Weather {
    case cloudy
    case sunny
  }
  let left: Observable<Weather> = Observable.of(.sunny, .cloudy, .cloudy, .sunny)
  let right = Observable.of("Lisbon", "Copenhagen", "London", "Madrid", "Vienna")

  let observable = Observable.zip(left, right) { weather, city in
    return "It's \(weather) in \(city)"
  }
  _ = observable.subscribe(onNext: { value in
    print(value)
  })
}

——— Example of: zip ———
It's sunny in Lisbon
It's cloudy in Copenhagen
It's cloudy in London
It's sunny in Madrid

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!

example(of: "withLatestFrom") {
  // 1
  let button = PublishSubject<Void>()
  let textField = PublishSubject<String>()

  // 2
  let observable = button.withLatestFrom(textField)
  _ = observable.subscribe(onNext: { value in
    print(value)
  })

  // 3
  textField.onNext("Par")
  textField.onNext("Pari")
  textField.onNext("Paris")
  button.onNext(())
  button.onNext(())
}

Paris
Paris


  // 2
  let observable = textField.sample(button)

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.

example(of: "amb") {
  let left = PublishSubject<String>()
  let right = PublishSubject<String>()

  // 1
  let observable = left.amb(right)
  _ = observable.subscribe(onNext: { value in
    print(value)
  })

  // 2
  left.onNext("Lisbon")
  right.onNext("Copenhagen")
  left.onNext("London")
  left.onNext("Madrid")
  right.onNext("Vienna")

  left.onCompleted()
  right.onCompleted()
}


example(of: "switchLatest") {
  // 1
  let one = PublishSubject<String>()
  let two = PublishSubject<String>()
  let three = PublishSubject<String>()

  let source = PublishSubject<Observable<String>>()

  // 2
  let observable = source.switchLatest()
  let disposable = observable.subscribe(onNext: { value in
    print(value)
  })

  // 3
  source.onNext(one)
  one.onNext("Some text from sequence one")
  two.onNext("Some text from sequence two")

  source.onNext(two)
  two.onNext("More text from sequence two")
  one.onNext("and also from sequence one")

  source.onNext(three)
  two.onNext("Why don't you see me?")
  one.onNext("I'm alone, help me")
  three.onNext("Hey it's three. I win.")

  source.onNext(one)
  one.onNext("Nope. It's me, one!")

  disposable.dispose()
}

——— Example of: switchLatest ———
Some text from sequence one
More text from sequence two
Hey it's three. I win.
Nope. It's me, one!

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.

example(of: "reduce") {
  let source = Observable.of(1, 3, 5, 7, 9)

  // 1
  let observable = source.reduce(0, accumulator: +)
  _ = observable.subscribe(onNext: { value in
    print(value)
  })
}

  // 1
  let observable = source.reduce(0) { summary, newValue in
    return summary + newValue
  }


example(of: "scan") {
  let source = Observable.of(1, 3, 5, 7, 9)

  let observable = source.scan(0, accumulator: +)
  _ = observable.subscribe(onNext: { value in
    print(value)
  })
}

——— Example of: scan ———
1
4
9
16
25

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.

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

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