introduction to functional reactive programming

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Eliasz SawickiBlog: www.eliaszsawicki.comTwitter: @EliSawic

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Introduction to Functional Reactive

Programming

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Agenda• What is Functional Reactive Programming?

• Let's take a look at ReactiveCocoa

• Working with streams

• Real life example

• Conclusion

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Functional Reactive Programming

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WikipediaFunctional reactive programming (FRP) is a programming paradigm for reactive programming (asynchronous dataflow programming) using the building blocks of functional programming (e.g. map, reduce, filter).

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Reactive Programming

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Asynchronous Dataflow

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Reacting to state changes

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Functional Programming

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Immutable

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assert(f(x) == f(x))

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A personclass Person { let name: String let phoneNumber: String init(name: String, phoneNumber: String) { self.name = name self.phoneNumber = phoneNumber }}

class MobilePhone { func call(person: Person) -> Bool { // implementation }}

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Mutablelet mobilePhone = MobilePhone()let john = Person(name: "John", phoneNumber: "123456789")

func makeAPhoneCall(device: Phone, person: Person, countryCode: String) -> Bool { person.phoneNumber = countryCode + person.phoneNumber let success = device.call(person) return success}

makeAPhoneCall(device: mobilePhone, person: john, countryCode: "+48") // truemakeAPhoneCall(device: mobilePhone, person: john, countryCode: "+48") // falsemakeAPhoneCall(device: mobilePhone, person: john, countryCode: "+48") // false

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Immutablelet mobilePhone = MobilePhone()let john = Person(name: "John", phoneNumber: "123456789")

func makeAPhoneCall(device: Phone, person: Person, countryCode: String) -> Bool { let prefixedPhoneNumber = countryCode + person.phoneNumber let newPerson = Person(name: person.name, phoneNumber: prefixedPhoneNumber) let success = device.call(newPerson) return success}

makeAPhoneCall(device: mobilePhone, person: john, countryCode: "+48") // truemakeAPhoneCall(device: mobilePhone, person: john, countryCode: "+48") // truemakeAPhoneCall(device: mobilePhone, person: john, countryCode: "+48") // true

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Stateless

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Statefulvar value = 0

func increment() -> Int { value += 1 return value}

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Statelessfunc increment(value: Int) -> Int { return value + 1}

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Functional Reactive Programming

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Imperative vs

Declarative

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Imperative

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Imperativelet array = [0, 1, 2, 3, 4, 5]var evenNumbers = [Int]()for element in array { if element % 2 == 0 { evenNumbers.append(element) }}

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Declarative

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Declarativelet array = [0, 1, 2, 3, 4, 5]let evenNumbers = array.filter { $0 % 2 == 0 }

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ReactiveCocoa

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Event streams

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Event Stream

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Event

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Non-Terminating• Next

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Terminating

• Completed

• Failed

• Interrupted (Reactive Cocoa)

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Signal

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Events over time

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No side effects

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No random access to events

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Must be observed in order to access it's

events

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Hot

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Signal's lifetime• Passes any number of Next events

• "Dies" when terminating event

• Any new observer will receive Interrupted event

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Observinglet someWork = Signal<Int, NoError> { observer in observer.sendNext(1) observer.sendNext(2) .... observer.sendNext(1000000) observer.sendCompleted()}

signal.observe { (event) in print(event)}signal.observeNext { (value) in print(value)}

signal.observeCompleted { print("Completed")}

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Pipe

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Pipelet (signal, observer) = Signal<String, NoError>.pipe()

signal.observeNext({ text in print(text)})

signal.observeCompleted({ print("Test completed")})

observer.sendNext("It's a test") // It's a testobserver.sendCompleted() // Test completed

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SignalProducer

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Represents tasks

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Creates signal

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Possible side effects

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Does not start it's work if not started

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Cold

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Using Signal Producerlet producer = SignalProducer<String, NSError> { (observer, composite) in observer.sendNext("In Progress...") // ...... observer.sendCompleted()}

producer.startWithSignal { (signal, _) in signal.observeNext({ (text) in print(text) // In Progress... }) signal.observeCompleted({ print("Test completed") // Test completed })}

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Cold vs Hot

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Properties

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MutablePropertylet name = MutableProperty("Bob")

name.producer.startWithNext { (text) in print(text)}

name.value = "Lisa"

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Bindings

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Basic bindinglet property = MutableProperty<String>("")let (signal, _) = Signal<String, NoError>.pipe()property <~ signal

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Schedulers

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Memory Management

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Disposables

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Manipulating signals

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Map

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Filter

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Aggregating

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Skip repeats

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Manipulating multiple signals

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Combine latest

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Zip

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Merge

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Chaining operators

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Chain them all!let newSignalX = signalX.skipRepeats() .filter { x > 2 } .map { x * 10 }

let newSignalY = signalY.filter { x > 10 }

let combined = combineLatest(newSignalX, newSignalY)

combined.observeNext { xValue, yValue in print("Update: \(xValue) : \(yValue)")}

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Example

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How does it work?

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Is name valid?let isValidName = nameSignal.map { (name) -> Bool in return input.characters.count > 2}

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Is mail valid?let isValidMail = mailSignal.map { (mail) -> Bool in let emailRegEx = "[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}" let emailTest = NSPredicate(format:"SELF MATCHES %@", emailRegEx) return emailTest.evaluateWithObject(mail)}

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Combine Latestlet formData = combineLatest(isValidName, isValidSurname, isValidMail)

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Is form valid?disposables += isFormValid <~ formData.map { (isValidName, isValidSurname, isValidMail) -> Bool in return isValidMail && isValidSurname && isValidMail}

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Button statelet producer = isFormValid.producer.skipRepeats()disposables += producer.startWithNext {[unowned self] (isValid) in self.updateAcceptButtonWithState(isValid)}

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Conclusion

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Thank you for your attention!

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