Let It Flow

A talk about Functional Reactive Programming (FRP) that I gave at LifeWorks for people from different backgrounds (iOS, Android, Web, servers)

Transcript

1. LET IT FLOW FUNCTIONAL REACTIVE PROGRAMMING
2. DISCLAIMERS NOT A TALK ABOUT... > Monads ! > Category theory > Advanced FRP > Concrete implementations
3. ALEJANDRO MARTINEZ http://alejandromp.com @alexito4
4. DISCLAIMERS - THERE ARE BETTER WAYS The ways we have been taught are not the panacea
5. BE OPEN MINDED
6. The purpose of abstraction is not to be vague, but to create a new semantic level in which one can be absolutely precise. Edsger Dijkstra
7. STATE IS THE ENEMY
8. STATE? !
9. STATE STORED VALUES AT ANY GIVEN TIME JUSTIN SPAHR-SUMMERS ENEMY OF THE STATE
10. > Is easy, but not simple > O.O.P. just moved the problem around > Is the single most complex part of any application > reboot? > database inconsistencies? race conditions? UI not updated everywhere? out of order operations? > complex code!
11. > essential complexity, is hard enough > state adds incidental complexity
12. Programs must be written for people to read, and only incidentally for machines to execute. — Harold Abelson, Structure and Interpretation of Computer Programs
13. GROWS WITH EXPONENTIAL COMPLEXITY > var visible → 2 states > var enabled → 4 states > var selected → 8 states > var highlighted → 16 states
14. STATE IS A GLORIFIED CACHE! AND CACHE INVALIDATION IS A REALLY HARD PROBLEM
15. func addRow() { items.append("") let indexPath = IndexPath(row: items.count, section: 0) tableView.insertRows(at: [indexPath], with: .automatic) }
17. STATE IS HARD > complex to manage > unpredictable > hard to test BUT NECESSARY > documents, preferences, UI...
18. CAN WE TALK ABOUT FRP ALREADY PLEASE?
19. NOT REAL FRP functional reactive programming operates on values that change continuously over time — The essence and origins of FRP
20. PROGRAMMING WITH ASYNCHRONOUS DATA STREAMS.
21. FUNCTIONAL Pure functions and immutability Each functions transforms the data REACTIVE changes in one thing → affect other things
22. STREAMS OF DATA
23. STREAMS TRANSFORMATIONS BINDINGS ASH FURROW FUNCTIONAL REACTIVE AWESOMENESS WITH SWIFT
24. IMPLEMENTATIONS > Reactive Extensions > ReactiveCocoa > ReactiveX Many languages (Java, Javascript, PHP, Swift, Ruby...) and platforms (Cocoa, Android)
25. STREAMS TRANSFORMATIONS BINDINGS
26. REACTIVE
27. > Observable<T> is the core of FRP > Push instead of pull
28. SENDS VALUES OVER TIME INDEFINITELY OR UNTIL THEY COMPLETE OR ERROR OUT
29. UNIFIES DIFFERENT EVENT/ASYNCHRONOUS SYSTEMS > notifications, delegates, callbacks, KVO.... > superset of Promise/Future
30. STREAMS TRANSFORMATIONS BINDINGS
31. FUNCTIONAL
32. > data is transformed over time with operators > transformations return a new stream > Types also flow trough the chain > the goal is conceal state, not being able to access it and only reacting to its changes
33. RAILWAY PROGRAMMING F# FOR FUN AN PROFIT RAILWAY ORIENTED PROGRAMMING
34. OPERATORS
35. > map: converts each value sent to another value.
36. > filter: only lets some values pass trough
37. > reduce: applies a function to each value in the stream, sequntially, and emits the final value
38. > combineLatest: combines two or more signals into one > timeout, retry, buffer, debounce...
39. STREAMS TRANSFORMATIONS BINDINGS
40. > observers a.k.a subscriber > subscribe to a stream to perform side effects > bind a property to the last value sent by a stream > subscribing starts the observable (not always )
41. A SONG OF ICE AND FIRE hot ! active !produce notifications regardless of subscriptions! Mouse ! Console vs. cold ❄ passive ❄start producing notifications on request❄ WebSocket ❄ Network calls
42. SHOW ME THE CODE
43. A SIMPLE STREAM Observable.of("Feel", "Loved", "™", "!") .subscribe { event in print(event) } Feel Loved ™ !
44. MAPPING STRINGS TO THEIR LENGTH Observable.of("Feel", "Loved", "™", "!") .map({ $0.characters.count }) .subscribe(onNext: { print($0) }) 4 5 1 4
45. ONLY WITH MORE THAN 2 CHARACTERS let disposeBag = DisposeBag() Observable.of("Feel", "Loved", "™", "!") .map({ $0.characters.count }) .filter({ $0 > 2 }) .subscribe(onNext: { print($0) }) .addDisposableTo(disposeBag) 4 5 4
46. > subscribing returns a disposable > disposing this token releases any resources and stops the stream > tie it to object deallocation
47. KEEP IT UP TO DATE ! bind the identifier c to a value calculated from a and b if some condition is satisfied var c: String var a = 1 var b = 2 if a + b >= 0 { c = "\(a + b) is positive" // 3 is positive } a = -4 // 3 is positive ⁉
48. KEEP IT UP TO DATE ! let c = Observable .combineLatest(a.asObservable(), b.asObservable()) { $0 + $1 } .filter { $0 >= 0 } .map { "\($0) is positive" } c.subscribe(onNext: { print($0) })
49. MODERNIZE APIS Observable.create { observer in doSomethingAsync { result in observer.on(.next(result)) observer.on(.completed) } return Disposables.create() }
50. PLATFORM INTEGRATION primeTextField.rx.text .map { WolframAlphaIsPrime(Int($0) ?? 0) } .concat() .map { "number \($0.n) is prime? \($0.isPrime)" } .bindTo(resultLabel.rx.text) // user interaction primeTextField.text = "43"
51. FLATMAP primeTextField.rx.text // Observable<String> .map { WolframAlphaIsPrime(Int($0) ?? 0) } // Observable<Observable<Prime>> primeTextField.rx.text // Observable<String> .flatMap `{ WolframAlphaIsPrime(Int($0) ?? 0) } // Observable<Prime>
52. EMAIL VALIDATION emailTextField.rx.text // Observable<String> .map(isEmail) // Observable<Bool> .bindTo(submitButton.rx.enabled)
53. USERNAME AVAILABLE? (valid: Bool?, message: String) RESPONSE TYPE
54. USERNAME AVAILABLE? self.usernameField.rx.text
55. USERNAME AVAILABLE? .map { username in if username.isEmpty { return Observable.just((valid: false, message: "Username can't be empty.")) }
56. USERNAME AVAILABLE? let loadingValue : LoadingInfo = (valid: nil, message: "Checking availability ...") return API.usernameAvailable(username) .map { available in if available { return (true, "Username available") } else { return (false, "Username already taken") } } .startWith(loadingValue)
57. USERNAME AVAILABLE? self.usernameField.rx.text .map { username in if username.isEmpty { return Observable.just((valid: false, message: "Username can't be empty.")) } typealias LoadingInfo = (valid: String?, message: String?) let loadingValue : LoadingInfo = (valid: nil, message: "Checking availability ...") return API.usernameAvailable(username) .map { available in if available { return (true, "Username available") } else { return (false, "Username already taken") } } .startWith(loadingValue) } .switchLatest() .subscribe(onNext: { valid in errorLabel.textColor = validationColor(valid) errorLabel.text = valid.message })
58. COOL RANDOM TRICKS
59. AUTO UPDATE TOKENS storage // Observable<UserToken> .userToken
60. AUTO UPDATE TOKENS storage // Observable<UserToken> .userToken .flatMap { token in switch token { case .invalid: return api.refreshUserToken() case .valid: return Observable.just(token) } }
61. AUTO UPDATE TOKENS storage // Observable<UserToken> .userToken .flatMap { token in switch token { case .invalid: return api.refreshUserToken() case .valid: return Observable.just(token) } } .flatMap { validToken in return api.fetchOffers() }
62. FAIL RANDOMLY extension Observable { func randomlyFail() -> Observable<E> { return flatMapLatest { value in let fail = arc4random() % 3 == 0 return fail ? .error(NSError()) : .just(value) } } }
63. AUTO REFRESH DATA let viewIsVisible = ... let lowPowerMode = ... let shouldReload = Observable.combineLatest(viewIsVisible, lowPowerMode) { return visible && !lowPower } let reloadPeriodically = shouldReload .flatMapLatest { reload in return reload ? .interval(30) : .empty }
64. REFRESH WHEN APPEARS request .repeat( when: viewWillAppear, throttle: 60 )
65. IMPROVING PHOTOS FRAMEWORK from BingWallpapers 1. Check for authorization 2. Request authorization 3. Fetch album 4. Create album 5. Fetch album
66. IMPROVING PHOTOS FRAMEWORK // Request authorization let auth = PHPhotoLibrary.requestAuthorization() // Fetch for a collection with the given title let fetch = PHAssetCollection.fetchCollectionWithTitle(title) // Creates and fetches a new collection with the given title let createAndFetch = PHAssetCollection .createCollectionWithTitle(title) .flatMap { identifier in PHAssetCollection.fetchCollectionWithIdentifier(identifier) } // Retrieves the album with the given title. New or already created! let retreiveAlbum = auth .flatMap { fetch.flatMapError { createAndFetch } retrieveAlbum(name: "BingWallpapers") .flatMap { album in PHPhotoLibrary.saveImage(image, toCollection: collection) }
67. MVVM / MVP
68. MVVM/MVP > encapsulates all the business logic, the operators and streams > just exposes observables of what the UI needs > Subjects = Observable and Observer at the same time
69. USER SIGNUP VIEW MODEL
70. USER SIGNUP VIEW MODEL viewModel.signupEnabled .subscribe(onNext: { valid in signupButton.isEnabled = valid signupButton.alpha = valid ? 1.0 : 0.5 }) viewModel.validatedUsername .bindTo(usernameValidationLabel.rx.validationResult) viewModel.validatedEmail .bindTo(usernameValidationEmail.rx.validationResult) viewModel.validatedPassword .bindTo(passwordValidationLabel.rx.validationResult) viewModel.validatedPasswordRepeated .bindTo(repeatedPasswordValidationLabel.rx.validationResult) viewModel.signingIn .bindTo(signingUpOulet.rx.animating) viewModel.signedIn .subscribe(onNext: { signedIn in print("User signed in \(signedIn)") })
71. USER SIGNUP VIEW MODEL INPUT SignupViewModel( username: usernameField.rx.text, email: emailField.rx.text, password: passwordField.rx.text, repeatedPassword: repeatedPasswordField.rx.text, loginTaps: signupButton.rx.tap )
72. UNITS AND DRIVERS Communicate and ensure - Can't error out - Observe on main scheduler - Subscribe on main scheduler - Sharing side effects
73. REACTIVE DATASOURCE items .bindTo(tableView.rx.items(cellIdentifier: "Cell")) { index, model, cell in cell.textLabel?.text = model } !
74. GETTING STARTED ! > promise, async (network) > replace other event systems (KVO, delegates, listeners) > think twice when creating properties that change over time
75. I HOPE THAT YOU FIND FRP EASY AND SIMPLE THIS IS ONLY THE BEGINNING
76. THANKS ! http://alejandromp.com @alexito4
77. Thanks > Ash Furrow - Functional Reactive Awesomeness With Swift > Javier Soto - Building Fabric.app with ReactiveCocoa > Justin Spahr-Summers - Enemy of the State > RxMarbles > ReactiveX > Anna Port