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Basic Operators

This document explains some of the most common operators used in ReactiveCocoa, and includes examples demonstrating their use.

Note that “operators,” in this context, refers to functions that transform signals and signal producers, not custom Swift operators. In other words, these are composable primitives provided by ReactiveCocoa for working with event streams.

This document will use the term “event stream” when dealing with concepts that apply to both Signal and SignalProducer. When the distinction matters, the types will be referred to by name.

Performing side effects with event streams

  1. Observation
  2. Injecting effects

Operator composition

  1. Lifting
  2. Pipe

Transforming event streams

  1. Mapping
  2. Filtering
  3. Aggregating

Combining event streams

  1. Combining latest values
  2. Zipping

Flattening producers

  1. Concatenating
  2. Merging
  3. Switching to the latest

Handling errors

  1. Catching errors
  2. Mapping errors
  3. Retrying

Performing side effects with event streams

Observation

Signals can be observed with the observe function. It takes an Observer as argument to which any future events are sent.

signal.observe(Signal.Observer { event in
    switch event {
    case let .Next(next):
        println("Next: \(next)")
    case let .Error(error):
        println("Error: \(error)")
    case .Completed:
        println("Completed")
    case .Interrupted:
        println("Interrupted")
    }
})

Alternatively, callbacks for the Next, Error, Completed and Interrupted events can be provided which will be called when a corresponding event occurs.

signal.observe(next: { next in
    println("Next: \(next)")
}, error: { error in
    println("Error: \(error)")
}, completed: {
    println("Completed")
}, interrupted: {
    println("Interrupted")
})

Note that it is not necessary to provide all four parameters - all of them are optional, you only need to provide callbacks for the events you care about.

observe is also available as operator that can be used with |>

Injecting effects

Side effects can be injected on a SignalProducer with the on operator without actually subscribing to it.

let producer = signalProducer
    |> on(started: {
        println("Started")
    }, event: { event in
        println("Event: \(event)")
    }, error: { error in
        println("Error: \(error)")
    }, completed: {
        println("Completed")
    }, interrupted: {
        println("Interrupted")
    }, terminated: {
        println("Terminated")
    }, disposed: {
        println("Disposed")
    }, next: { next in
        println("Next: \(next)")
    })

Similar to observe, all the parameters are optional and you only need to provide callbacks for the events you care about.

Note that nothing will be printed until producer is started (possibly somewhere else).

Operator composition

Pipe

The |> operator can be used to apply a primitive to an event stream. Multiple operators can be chained after each other using the |> operator:

intSignal
    |> filter { num in num % 2 == 0 }
    |> map(toString)
    |> observe(next: { string in println(string) })

Lifting

Signal operators can be lifted to operate upon SignalProducers using the lift method.

This will create a new SignalProducer which will apply the given operator to every Signal created, just as if the operator had been applied to each produced Signal individually.

The |> operator implicitly lifts Signal operators, so it can be used to apply them directly to SignalProducers.

Transforming event streams

These operators transform an event stream into a new stream.

Mapping

The map operator is used to transform the values in a event stream, creating a new stream with the results.

let (signal, sink) = Signal<String, NoError>.pipe()

signal
    |> map { string in string.uppercaseString }
    |> observe(next: println)

sendNext(sink, "a")     // Prints A
sendNext(sink, "b")     // Prints B
sendNext(sink, "c")     // Prints C

Interactive visualisation of the map operator.

Filtering

The filter operator is used to only include values in an event stream that satisfy a predicate.

let (signal, sink) = Signal<Int, NoError>.pipe()

signal
    |> filter { number in number % 2 == 0 }
    |> observe(next: println)

sendNext(sink, 1)     // Not printed
sendNext(sink, 2)     // Prints 2
sendNext(sink, 3)     // Not printed
sendNext(sink, 4)     // prints 4

Interactive visualisation of the filter operator.

Aggregating

The reduce operator is used to aggregate a event stream’s values into a single combined value. Note that the final value is only sent after the input stream completes.

let (signal, sink) = Signal<Int, NoError>.pipe()

signal
    |> reduce(1) { $0 * $1 }
    |> observe(next: println)

sendNext(sink, 1)     // nothing printed
sendNext(sink, 2)     // nothing printed
sendNext(sink, 3)     // nothing printed
sendCompleted(sink)   // prints 6

The collect operator is used to aggregate a event stream’s values into a single array value. Note that the final value is only sent after the input stream completes.

let (signal, sink) = Signal<Int, NoError>.pipe()

let collected = signal |> collect

collected.observe(next: println)

sendNext(sink, 1)     // nothing printed
sendNext(sink, 2)     // nothing printed
sendNext(sink, 3)     // nothing printed
sendCompleted(sink)   // prints [1, 2, 3]

Interactive visualisation of the reduce operator.

Combining event streams

These operators combine values from multiple event streams into a new, unified stream.

Combining latest values

The combineLatest function combines the latest values of two (or more) event streams.

The resulting stream will only send its first value after each input has sent at least one value. After that, new values on any of the inputs will result in a new value on the output.

let (numbersSignal, numbersSink) = Signal<Int, NoError>.pipe()
let (lettersSignal, lettersSink) = Signal<String, NoError>.pipe()

combineLatest(numbersSignal, lettersSignal)
    |> observe(next: println, completed: { println("Completed") })

sendNext(numbersSink, 0)    // nothing printed
sendNext(numbersSink, 1)    // nothing printed
sendNext(lettersSink, "A")  // prints (1, A)
sendNext(numbersSink, 2)    // prints (2, A)
sendCompleted(numbersSink)  // nothing printed
sendNext(lettersSink, "B")  // prints (2, B)
sendNext(lettersSink, "C")  // prints (2, C)
sendCompleted(lettersSink)  // prints "Completed"

The combineLatestWith operator works in the same way, but as an operator.

Interactive visualisation of the combineLatest operator.

Zipping

The zip function joins values of two (or more) event streams pair-wise. The elements of any Nth tuple correspond to the Nth elements of the input streams.

That means the Nth value of the output stream cannot be sent until each input has sent at least N values.

let (numbersSignal, numbersSink) = Signal<Int, NoError>.pipe()
let (lettersSignal, lettersSink) = Signal<String, NoError>.pipe()

zip(numbersSignal, lettersSignal)
    |> observe(next: println, completed: { println("Completed") })

sendNext(numbersSink, 0)    // nothing printed
sendNext(numbersSink, 1)    // nothing printed
sendNext(lettersSink, "A")  // prints (0, A)
sendNext(numbersSink, 2)    // nothing printed
sendCompleted(numbersSink)  // nothing printed
sendNext(lettersSink, "B")  // prints (1, B)
sendNext(lettersSink, "C")  // prints (2, C) & "Completed"

The zipWith operator works in the same way, but as an operator.

Interactive visualisation of the zip operator.

Flattening producers

The flatten operator transforms a SignalProducer-of-SignalProducers into a single SignalProducer whose values are forwarded from the inner producer in accordance with the provided FlattenStrategy.

To understand, why there are different strategies and how they compare to each other, take a look at this example and imagine the column offsets as time:

let values = [
// imagine column offset as time
[ 1,    2,      3 ],
   [ 4,      5,     6 ],
         [ 7,     8 ],
]

let merge =
[ 1, 4, 2, 7,5, 3,8,6 ]

let concat = 
[ 1,    2,      3,4,      5,     6,7,     8]

let latest =
[ 1, 4,    7,     8 ]

Note, how the values interleave and which values are even included in the resulting array.

Merging

The .Merge strategy immediately forwards every value of the inner SignalProducers to the outer SignalProducer. Any error sent on the outer producer or any inner producer is immediately sent on the flattened producer and terminates it.

let (producerA, lettersSink) = SignalProducer<String, NoError>.buffer(5)
let (producerB, numbersSink) = SignalProducer<String, NoError>.buffer(5)
let (signal, sink) = SignalProducer<SignalProducer<String, NoError>, NoError>.buffer(5)

signal |> flatten(FlattenStrategy.Merge) |> start(next: println)

sendNext(sink, producerA)
sendNext(sink, producerB)
sendCompleted(sink)

sendNext(lettersSink, "a")    // prints "a"
sendNext(numbersSink, "1")    // prints "1"
sendNext(lettersSink, "b")    // prints "b"
sendNext(numbersSink, "2")    // prints "2"
sendNext(lettersSink, "c")    // prints "c"
sendNext(numbersSink, "3")    // prints "3"

Interactive visualisation of the flatten(.Merge) operator.

Concatenating

The .Concat strategy is used to serialize work of the inner SignalProducers. The outer producer is started immediately. Each subsequent producer is not started until the preceeding one has completed. Errors are immediately forwarded to the flattened producer.

let (producerA, lettersSink) = SignalProducer<String, NoError>.buffer(5)
let (producerB, numbersSink) = SignalProducer<String, NoError>.buffer(5)
let (signal, sink) = SignalProducer<SignalProducer<String, NoError>, NoError>.buffer(5)

signal |> flatten(FlattenStrategy.Concat) |> start(next: println)

sendNext(sink, producerA)
sendNext(sink, producerB)
sendCompleted(sink)

sendNext(numbersSink, "1")    // nothing printed
sendNext(lettersSink, "a")    // prints "a"
sendNext(lettersSink, "b")    // prints "b"
sendNext(numbersSink, "2")    // nothing printed
sendNext(lettersSink, "c")    // prints "c"
sendCompleted(lettersSink)    // prints "1", "2"
sendNext(numbersSink, "3")    // prints "3"
sendCompleted(numbersSink)

Interactive visualisation of the flatten(.Concat) operator.

Switching to the latest

The .Latest strategy forwards only values from the latest input SignalProducer.

let (producerA, sinkA) = SignalProducer<String, NoError>.buffer(5)
let (producerB, sinkB) = SignalProducer<String, NoError>.buffer(5)
let (producerC, sinkC) = SignalProducer<String, NoError>.buffer(5)
let (signal, sink) = SignalProducer<SignalProducer<String, NoError>, NoError>.buffer(5)

signal |> flatten(FlattenStrategy.Latest) |> start(next: println)

sendNext(sink, producerA)   // nothing printed
sendNext(sinkC, "X")        // nothing printed
sendNext(sinkA, "a")        // prints "a"
sendNext(sinkB, "1")        // nothing printed
sendNext(sink, producerB)   // prints "1"
sendNext(sinkA, "b")        // nothing printed
sendNext(sinkB, "2")        // prints "2"
sendNext(sinkC, "Y")        // nothing printed
sendNext(sinkA, "c")        // nothing printed
sendNext(sink, producerC)   // prints "X", "Y"
sendNext(sinkB, "3")        // nothing printed
sendNext(sinkC, "Z")        // prints "Z"

Handling errors

These operators are used to handle errors that might occur on an event stream.

Catching errors

The catch operator catches any error that may occur on the input SignalProducer, then starts a new SignalProducer in its place.

let (producer, sink) = SignalProducer<String, NSError>.buffer(5)
let error = NSError(domain: "domain", code: 0, userInfo: nil)

producer
    |> catch { error in SignalProducer<String, NSError>(value: "Default") }
    |> start(next: println)


sendNext(sink, "First")     // prints "First"
sendNext(sink, "Second")    // prints "Second"
sendError(sink, error)      // prints "Default"

Retrying

The retry operator will restart the original SignalProducer on error up to count times.

var tries = 0
let limit = 2
let error = NSError(domain: "domain", code: 0, userInfo: nil)
let producer = SignalProducer<String, NSError> { (sink, _) in
    if tries++ < limit {
        sendError(sink, error)
    } else {
        sendNext(sink, "Success")
        sendCompleted(sink)
    }
}

producer
    |> on(error: {e in println("Error")})             // prints "Error" twice
    |> retry(2)
    |> start(next: println,                           // prints "Success"
            error: { _ in println("Signal Error")})

If the SignalProducer does not succeed after count tries, the resulting SignalProducer will fail. E.g., if retry(1) is used in the example above instead of retry(2), "Signal Error" will be printed instead of "Success".

Mapping errors

The mapError operator transforms any error in an event stream into a new error.

enum CustomError: String, ErrorType {
    case Foo = "Foo"
    case Bar = "Bar"
    case Other = "Other"
    
    var nsError: NSError {
        return NSError(domain: "CustomError.\(rawValue)", code: 0, userInfo: nil)
    }
    
    var description: String {
        return "\(rawValue) Error"
    }
}

let (signal, sink) = Signal<String, NSError>.pipe()

signal
    |> mapError { (error: NSError) -> CustomError in
        switch error.domain {
        case "com.example.foo":
            return .Foo
        case "com.example.bar":
            return .Bar
        default:
            return .Other
        }
    }
    |> observe(error: println)

sendError(sink, NSError(domain: "com.example.foo", code: 42, userInfo: nil))    // prints "Foo Error"

Promote

The promoteErrors operator promotes an event stream that does not generate errors into one that can.

let (numbersSignal, numbersSink) = Signal<Int, NoError>.pipe()
let (lettersSignal, lettersSink) = Signal<String, NSError>.pipe()

numbersSignal
    |> promoteErrors(NSError)
    |> combineLatestWith(lettersSignal)

The given stream will still not actually generate errors, but this is useful because some operators to combine streams require the inputs to have matching error types.