Building iOS apps
with ReactiveCocoa

About me
Eliasz Sawicki
Blog: http://eluss.github.io/
Twitter: @EliSawic

Let's begin

ReactiveCocoa

Functional Reactive
Programming

Wikipedia
Functional 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).

Reactive
Programming

Reactive Programming
• Working with asynchronous dataflow
• Reacting to state changes

Functional
Programming

Functional Programming
• Immutable

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

A person
class Person {
var name: String
init(name: String) {
self.name = name
}
}

Mutable
func personWithReversedName(person: Person) -> Person {
person.name = String(person.name.characters.reverse())
return person
}
let person = Person(name: "John")
let reversedA = personWithReversedName(person)
print(reversedA.name) // nhoJ
let reversedB = personWithReversedName(person)
print(reversedB.name) // John

Immutable
func personWithReversedName(person: Person) -> Person {
let name = String(person.name.characters.reverse())
let newPerson = Person(name: name)
return newPerson
}
let person = Person(name: "John")
let reversedA = personWithReversedName(person)
print(reversedA.name) // nhoJ
let reversedB = personWithReversedName(person)
print(reversedB.name) // nhoJ

Functional Programming
• Immutable
• Stateless

Stateful
var value = 0
func increment() {
value += 1
}

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

Declarative

Imperative
vs
Declarative

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

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

Back to
ReactiveCocoa

Event streams

Event Stream

Event

Non-Terminating
• Next
Terminating
• Completed
• Failed
• Interrupted

Observer

Signal

What is it?
• Represents events over time
• Must be observed in order to access it's events
• Observing a signal does not trigger any side effects (push
based)
• No random access to events

Signal's lifetime
• Passes any number of Next events
• "Dies" when terminating event arrives
• Any new observer will receive Interrupted event

Observing
signal.observe { (event) in
print(event)
}
signal.observeNext { (value) in
print(value)
}
signal.observeCompleter {
print("Completed")
}

Creating Signals

Basic signal
Signal<String, NSError> { (observer) -> Disposable? in
observer.sendNext("test")
observer.sendCompleted()
return ActionDisposable(action: {
print("Signal disposed")
})
}

Pipe

let (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 test
observer.sendCompleted() // Test completed

SignalProducer

What is it?
• Represents tasks
• Creates signals
• Performs side effects
• Does not start it's work if not started

Injecting side effects
let producer = signalProducer
.on(started: {
print("Started")
}, event: { event in
print("Event: (event)")
}, failed: { error in
print("Failed: (error)")
}, completed: {
print("Completed")
}, interrupted: {
print("Interrupted")
}, terminated: {
print("Terminated")
}, disposed: {
print("Disposed")
}, next: { value in
print("Next: (value)")
})

Creating Signal
Producers

Basic Signal Producer
SignalProducer<String, NSError> { (observer, composite) in
composite.addDisposable({
print("Clearing work")
})
observer.sendNext("In Progres...")
observer.sendCompleted()
}

Buffer
let (producer, observer) = SignalProducer<String, NoError>.buffer(3)
observer.sendNext("test")
observer.sendCompleted()
producer.startWithSignal { (signal, disposable) in
signal.observeNext({ (text) in
print(text) // test
})
signal.observeCompleted({
print("Test completed") // Test completed
})
}
observer.sendNext("is interrupted")
observer.sendInterrupted()
producer.startWithSignal { (signal, disposable) in
signal.observeNext({ (text) in
print(text) // test, is interrupted
})
signal.observeInterrupted({
print("Test interrupted") // Test interrupted
})
}

Manipulating signals

Map

Map
let (numberSignal, observer) = Signal<Int, NoError>.pipe()
let textSignal = numberSignal.map { (number) -> String in
return "Number is (number)"
}
numberSignal.observeNext { (number) in
print(number) // 5
}
textSignal.observeNext { (text) in
print(text) // Number is 5
}
observer.sendNext(5)

Filter

Filter
let (numberSignal, observer) = Signal<Int, NoError>.pipe()
let fiveSignal = numberSignal.filter { (number) -> Bool in
return number == 5
}
numberSignal.observeNext { (number) in
print(number) // 6, 5
}
fiveSignal.observeNext { (number) in
print(number) // 5
}
observer.sendNext(6)
observer.sendNext(5)

Aggregating

Aggregating
let (numberSignal, observer) = Signal<Int, NoError>.pipe()
let aggregtingSignal = numberSignal.reduce(0) { (currentValue, addedValue) -> Int in
return currentValue + addedValue
}
numberSignal.observeNext { (number) in
print(number) // 5, 6
}
aggregtingSignal.observeNext { (number) in
print("Aggregated (number)") // Aggregated 11
}
observer.sendNext(5)
observer.sendNext(6)
observer.sendCompleted()

Skip repeats

Skip repeats
let (numberSignal, observer) = Signal<Int, NoError>.pipe()
numberSignal.observeNext { (number) in
print(number) // 1, 2, 2, 3
}
numberSignal.skipRepeats().observeNext { (number) in
print(number) // 1, 2, 3
}
observer.sendNext(1)
observer.sendNext(2)
observer.sendNext(2)
observer.sendNext(3)

Skip until

Skip until
let (numberSignal, observer) = Signal<Int, NoError>.pipe()
numberSignal.observeNext { (number) in
print(number) // 5, 6
}
let (trigger, triggerObserver) = Signal<Void, NoError>.pipe()
numberSignal.skipUntil(trigger).observeNext { (number) in
print("Triggered (number)") // Triggered 6
}
observer.sendNext(5)
triggerObserver.sendNext()
observer.sendNext(6)

Collect
let (numberSignal, observer) = Signal<Int, NoError>.pipe()
numberSignal.observeNext { (number) in
print(number) // 1, 2, 3, 4, 5
}
numberSignal.collect { (values) -> Bool in
return values.reduce(0, combine: +) > 4
}.observeNext { (values) in
print(values) // [1, 2, 3], [4 ,5]
}
observer.sendNext(1)
observer.sendNext(2)
observer.sendNext(3)
observer.sendNext(4)
observer.sendNext(5)

Manipulating
multiple signals

Combine latest

Combine latest

let (numberSignal, numberObserver) = Signal<Int, NoError>.pipe()
let (textSignal, textObserver) = Signal<String, NoError>.pipe()
combineLatest(numberSignal, textSignal).observeNext { (number, text) in
print("(text) - (number)")
}
numberObserver.sendNext(1) // Nothing printed
textObserver.sendNext("John") // John - 1
numberObserver.sendNext(2) // John - 2
textObserver.sendNext("Mary") // Mary - 2

Zip

Zip
let (menSignal, menObserver) = Signal<String, NoError>.pipe()
let (womenSignal, womenObserver) = Signal<String, NoError>.pipe()
let zippedSignal = zip(menSignal, womenSignal)
zippedSignal.observeNext { (man, woman) in
print("New couple - (man) and (woman)")
}
zippedSignal.observeCompleted({
print("Completed")
})
menObserver.sendNext("John") // Nothing printed
menObserver.sendNext("Tom") // Nothing printed
womenObserver.sendNext("Lisa") // New couple - John and Lisa
menObserver.sendNext("Greg") // Nothing printed
menObserver.sendCompleted()
womenObserver.sendNext("Sandra") // New couple - Tom and Sandra
womenObserver.sendNext("Mary") // New couple - Greg and Mary, Completed

Merge

Merge
let (menSignal, menObserver) = Signal<String, NoError>.pipe()
let (womenSignal, womenObserver) = Signal<String, NoError>.pipe()
let (peopleSignal, peopleObserver) = Signal<Signal<String, NoError>, NoError>.pipe()
peopleSignal.flatten(.Merge).observeNext { (name) in
print(name)
}
peopleObserver.sendNext(menSignal)
peopleObserver.sendNext(womenSignal)
menObserver.sendNext("John") // John
womenObserver.sendNext("Lisa") // Lisa

Handling errors

Catching errors
let (producer, observer) = SignalProducer<String, NSError>.buffer(5)
let error = NSError(domain: "domain", code: 0, userInfo: nil)
producer
.flatMapError { _ in SignalProducer<String, NoError>(value: "Default") }
.startWithNext { next in print(next) }
observer.sendNext("First") // prints "First"
observer.sendNext("Second") // prints "Second"
observer.sendFailed(error) // prints "Default"

Retry
var tries = 0 let limit = 2 let error = NSError(domain: "domain", code: 0, userInfo: nil)
let producer = SignalProducer<String, NSError> { (observer, _) in
if tries++ < limit {
observer.sendFailed(error)
} else {
observer.sendNext("Success")
observer.sendCompleted()
}
}
producer
.on(failed: {e in print("Failure")}).retry(2).start { event in // prints "Failure" twice
switch event {
case let .Next(next):
print(next) // prints "Success"
case let .Failed(error):
print("Failed: (error)")
}
}

Promoting
let (numbersSignal, numbersObserver) = Signal<Int, NoError>.pipe()
let (lettersSignal, lettersObserver) = Signal<String, NSError>.pipe()
numbersSignal
.promoteErrors(NSError)
.combineLatestWith(lettersSignal)

Mapping errors

Properties

Properties
• AnyProperty
• ConstantProperty
• MutableProperty
• DynamicProperty

MutableProperty
let name = MutableProperty<String>("Bob")
name.producer.startWithNext { (text) in
print(text)
}
name.modify { (name) -> String in
return name + "!"
}
name.value = "Lisa"

DynamicProperty
let textProperty = DynamicProperty(object: textField, keyPath: "text")
textProperty.producer.startWithNext { (text) in
print(text)
}
textProperty.value = "Textfield text"

Bindings

Basic binding
let property = MutableProperty<String>("")
let (producer, _) = SignalProducer<String, NoError>.buffer(1)
let (signal, _) = Signal<String, NoError>.pipe()
property <~ producer
property <~ signal

Action

Create Action
let action = Action<Int, String, NSError>({ (number) -> SignalProducer<String, NSError> in
return SignalProducer<String, NSError> {observer, disposable in
observer.sendNext("Number is (number)")
observer.sendCompleted()
}
})

Create signal producer
let producer = action.apply(1)

Execute action
prodcuer.startWithSignal { (signal, disposable ) in
signal.observeNext({ (value) in
print("(value)")
})
signal.observeFailed({ (actionError) in
print("(actionError)")
})
}

Observing actions
let action = Action<Int, String, NSError>({ (number) -> SignalProducer<String, NSError> in
return SignalProducer<String, NSError> {observer, disposable in
observer.sendNext("Number is (number)")
observer.sendCompleted()
}
})
action.values.observe { (value) in
print("Value: (value)")
}
action.errors.observe { (error) in
print("Error: (error)")
}
action.events.observe { (event) in
print("Event: (event)")
}
action.apply(5).startWithSignal { (_ , _ ) in }

CocoaAction

Prepare Action
var text = MutableProperty<String>("Switch is on")
let switchControl = UISwitch()
let switchAction = Action<Bool, String, NoError>({
(isOn) -> SignalProducer<String, NoError> in
return SignalProducer<String, NoError> { observer, disposable in
observer.sendNext(isOn ? "Switch is on" : "Switch is off")
observer.sendCompleted()
}
})

Create CocoaAction
let switchCocoaAction = CocoaAction(switchAction, { (control) -> Bool in
let control = control as! UISwitch
return control.on
})
switchControl.addTarget(switchCocoaAction, action: CocoaAction.selector,
forControlEvents: .ValueChanged)
text <~ switchAction.values

Schedulers
• SchedulerType
• ImmediateScheduler
• UIScheduler
• DateSchedulerType
• QueueScheduler
• TestScheduler

Memory Management

Disposables

Task
let producer = SignalProducer<String, NoError> { (observer, composite) in
let date = NSDate().dateByAddingTimeInterval(10)
composite += QueueScheduler().scheduleAfter(date, action: {
print("Doing my work") // Doing my work
observer.sendNext("Test")
observer.sendCompleted()
})
}
producer.startWithSignal { (signal, disposable) in
signal.observeNext({ (value) in
print(value) // Test
})
signal.observeCompleted({
print("Work completed") // Work completed
})
}

Cancelling work
let producer = SignalProducer<String, NoError> { (observer, composite) in
let date = NSDate().dateByAddingTimeInterval(10)
composite += QueueScheduler().scheduleAfter(date, action: {
print("Doing my work") // Not printed
observer.sendNext("Test")
observer.sendCompleted()
})
}
producer.startWithSignal { (signal, disposable) in
signal.observeNext({ (value) in
print(value) // Not printed
})
signal.observeInterrupted({
print("Work interrupted") // Work interrupted
})
let date = NSDate().dateByAddingTimeInterval(2)
QueueScheduler().scheduleAfter(date, action: {
disposable.dispose()
})
}

Cleaning
let producer = SignalProducer<String, NoError> { (observer, composite) in
composite.addDisposable({
print("I'm done")
})
let date = NSDate().dateByAddingTimeInterval(4)
composite += QueueScheduler().scheduleAfter(date, action: {
print("Doing my work") // Not printed
})
}
producer.startWithSignal { (signal, disposable) in
signal.observeInterrupted({
print("Work interrupted")
})
let date = NSDate().dateByAddingTimeInterval(2)
QueueScheduler().scheduleAfter(date, action: {
disposable.dispose() // Work interrupted, I'm done
})
}

Disposing signal
let producer = SignalProducer<String, NoError> { (observer, composite) in
composite.addDisposable({
print("I'm done")
})
let date = NSDate().dateByAddingTimeInterval(5)
composite += QueueScheduler().scheduleAfter(date, action: {
print("Doing my work") // Not printed
})
}
producer.startWithSignal { (signal, disposable) in
let signalDisposable = signal.observeInterrupted({
print("Work interrupted") // Not printed
})
let date = NSDate().dateByAddingTimeInterval(2)
QueueScheduler().scheduleAfter(date, action: {
signalDisposable!.dispose()
})
let date2 = NSDate().dateByAddingTimeInterval(4)
QueueScheduler().scheduleAfter(date2, action: {
disposable.dispose()
})
}

Closures

What's the result?
var value = 10
let closure = {
let date = NSDate().dateByAddingTimeInterval(2)
QueueScheduler().scheduleAfter(date, action: {
print(value)
})
}
closure()
value = 20

Captured value
var value = 10
let closure = { [value] in
let date = NSDate().dateByAddingTimeInterval(2)
QueueScheduler().scheduleAfter(date, action: {
print(value)
})
}
closure()
value = 20

Weak, Strong,
Unowned...

Unowned
let closure = { [unowned self] in
self.label.text = "test"
}

Weak
let closure = { [weak self] in
guard let weakSelf = self else { return }
self.label.text = "test"
}

Rex

UIButton
let cocoaAction = CocoaAction(action) { _ in }
//without Rex
button.addTarget(cocoaAction, action: CocoaAction.selector,
forControlEvents: .TouchUpInside)
//with Rex extensions
button.rex_pressed.value = cocoaAction

UITextField, UILabel, MutableProperty
var titleValue = MutableProperty<String?>(nil)
//without Rex
textField.rac_textSignal().subscribeNext {
self.titleValue.value = $0 as? String
}
titleValue.producer.startWithNext {
self.label.text = $0
self.label.hidden = $0?.characters.count < 5
}
//with Rex
titleValue <~ textField.rex_text
titleLabel.rex_text <~ titleValue
titleLabel.rex_hidden <~ titleValue.producer.map(
{ $0?.characters.count < 5 }
)

Let's see it in action