Introduction to Functional Reactive Programming with ReactiveCocoa Ian Dundas @id github.com/iandundas/reactivecocoa-presentation 

Coming up.. • Problems of imperative programming • How Functional Reactive programming addresses these issues • ReactiveCocoa, an implementation of FRP 

[self.overlayYAxisLayoutConstraint setConstant:200];
 [self.tapGestureRecogniser setEnabled:NO]; BOOL isSyncing= NO; 
 BOOL overlayIsShowing= YES;
 BOOL userIsActive=YES;
 
 x= 1 y= 2
 z= x + y; # z is 3
 y= 4; #change value of y 
 print z: "3" # z is still 3 

–Justin Spahr-Summers, co-author, on NSBrief podcast "ReactiveCocoa comes from the world of Functional Reactive Programming, which is kinda like a paradigm shift on top of a paradigm shift: it’s Functional Programming with Reactive Programming added on top, which makes it intimidating because it’s a big change from what we’re used to" 

Functional Programming - from 1000 miles high • Distilled down to an “abhorrence of state” • Functions don’t have side-effects • Immutability • Side-effects are useful, however 

Reactive Programming x= 1 y= 2
 z= x + y # z is 3
 y= 4 # change value of y 
 print z: “5” # z was updated z “reacts” to the changes of x and y 

Functional Reactive Programming • Just a combination of functional and reactive paradigms • “We model user input as a function that changes over time, abstracting away the idea of mutable state.” • Can now model difficult things like *user input* 

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Signals & Subscriptions • Signal can send: • “Next”, one or many times • “Completed”, or “Error”, only once • We can subscribe to receive these values from a Signal, -> we become a Subscriber. 

So - what is ReactiveCocoa? • ReactiveCocoa (RAC) is an open source library that brings Functional Reactive Programming paradigm to Objective-C. • It was created by Josh Abernathy & Justin Spahr- Summers in the development of GitHub for Mac. 

Signals in RAC @property(nonatomic, copy) NSString *firstName; RACSignal *firstNameSignal= RACObserve(self, firstName); 

“Subscribing” to a Signal [RACObserve(self, firstName)
 subscribeNext:^(NSString *newName) {
 NSLog(@"Name updated: %@", newName);
 }
 ]; 
 

Signal Operators: Map: [[RACObserve(self, isLoggedIn) 
 map:^id(NSNumber *boolValue){
 BOOL enabled= [boolValue boolValue];
 return enabled? @"Logged in": @"Logged out";
 }] 
 subscribeNext:^(NSString *loggedInString){
 NSLog(@"We are currently %@", loggedInString);
 // output: "We are currently: Logged in” [self.tableView reloadData]; // side-effect
 }]; @property(nonatomic) BOOL *isLoggedIn; 

Signal Operators: Filter: [[RACObserve(self, username)
 filter:^(NSString *newName) {
 return newName.length > 4;
 }]
 subscribeNext:^(NSString *newName) {
 NSLog(@"%@", newName);
 }];
 @property (nonatomic, copy) NSString *username; 

Binding with RAC() RAC(self.firstNameLabel, text) = RACObserve(self, firstName); binds self.firstNameLabel’s text property to the signal produces a signal which sends a new value each time self.firstName changes 

Signal Operators: combineLatest:reduce: 

! Powerful Binding ! & example of combineLatest:reduce: RACSignal *loginShouldBeEnabledSignal = [RACSignal 
 combineLatest:@[
 self.usernameTextField.rac_textSignal, 
 self.passwordTextField.rac_textSignal,
 RACObserve(LoginManager.sharedManager, loggingIn)
 ] 
 reduce: (id) ^(NSString *username, NSString *password, NSNumber *loggingIn) {
 BOOL shouldBeEnabled= 
 (username.length > 0 && password.length > 0 && !loggingIn.boolValue);
 
 return @(shouldBeEnabled);
 }
 ];
 // Bind signal to `enabled` property on loginButton
 RAC(self.loginButton, enabled) = loginShouldBeEnabledSignal; 

–ReactiveCocoa README.md “By chaining, combining, and reacting to signals, software can be written declaratively, without the need for code that continually observes and updates values (manually).” 

Derived State • A core concept in Functional Reactive Programming • State is OK in ReactiveCocoa, as long as its bound to a signal • You never explicitly set the value of a bound property, but rather rely on signal transformations to derive that state for you. http://reactivecocoa.io/philosophy.html 

Getting started: 
 Replace your KVO with RAC @implementation IDViewController
 
 -(void)dealloc{
 [self teardownObservers];
 }
 
 -(void)viewDidLoad {
 [super viewDidLoad];
 [self setupObservers];
 }
 
 -(void)setupObservers{
 [self.viewModel addObserver:self
 forKeyPath:@"firstName"
 options:NSKeyValueObservingOptionNew context:NULL
 ];
 }
 
 -(void)teardownObservers{
 [self.viewModel removeObserver:self forKeyPath:@"firstName"];
 }
 
 - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
 change:(NSDictionary *)change context:(void *)context{
 
 if ([keyPath isEqualToString:@"firstName"]){
 NSLog(@"firstName did change");
 }
 }
 @end 

Getting started: 
 Replace your KVO with RAC @implementation IDViewController
 
 -(void)viewDidLoad {
 [super viewDidLoad];
 
 [RACObserve(self, firstName) subscribeNext:^(id x) {
 NSLog(@"firstName did change");
 }];
 }
 
 @end 

Throttling Cool things to do #1
 (which are otherwise tricky) 

TakeUntil Cool things to do #2
 (which are otherwise tricky) 

DeliverOn Cool things to do #3
 (which are otherwise tricky) 

[[[[[[[[self requestAccessToTwitterSignal]
 then:^RACSignal * {
 return self.searchTextField.rac_textSignal;
 }]
 http://www.raywenderlich.com/62796/reactivecocoa-tutorial-pt2 

[[[[[[[[self requestAccessToTwitterSignal]
 then:^RACSignal * {
 return self.searchTextField.rac_textSignal;
 }]
 filter:^BOOL (NSString *text) {
 return [self isValidSearchText:text];
 }] 
 http://www.raywenderlich.com/62796/reactivecocoa-tutorial-pt2 

[[[[[[[[self requestAccessToTwitterSignal]
 then:^RACSignal * {
 return self.searchTextField.rac_textSignal;
 }]
 filter:^BOOL (NSString *text) {
 return [self isValidSearchText:text];
 }]
 throttle:0.5] http://www.raywenderlich.com/62796/reactivecocoa-tutorial-pt2 

[[[[[[[[self requestAccessToTwitterSignal]
 then:^RACSignal * {
 return self.searchTextField.rac_textSignal;
 }]
 filter:^BOOL (NSString *text) {
 return [self isValidSearchText:text];
 }]
 throttle:0.5]
 flattenMap:^RACStream * (NSString *text) {
 return [self signalForSearchWithText:text];
 }]
 http://www.raywenderlich.com/62796/reactivecocoa-tutorial-pt2 

[[[[[[[[self requestAccessToTwitterSignal]
 then:^RACSignal * {
 return self.searchTextField.rac_textSignal;
 }]
 filter:^BOOL (NSString *text) {
 return [self isValidSearchText:text];
 }]
 throttle:0.5]
 flattenMap:^RACStream * (NSString *text) {
 return [self signalForSearchWithText:text];
 }]
 map:^id (NSDictionary *jsonSearchResult) {
 NSArray *statuses = jsonSearchResult[@"statuses"];
 return statuses;
 }]
 http://www.raywenderlich.com/62796/reactivecocoa-tutorial-pt2 

[[[[[[[[self requestAccessToTwitterSignal]
 then:^RACSignal * {
 return self.searchTextField.rac_textSignal;
 }]
 filter:^BOOL (NSString *text) {
 return [self isValidSearchText:text];
 }]
 throttle:0.5]
 flattenMap:^RACStream * (NSString *text) {
 return [self signalForSearchWithText:text];
 }]
 map:^id (NSDictionary *jsonSearchResult) {
 NSArray *statuses = jsonSearchResult[@"statuses"];
 return statuses;
 }]
 deliverOn:[RACScheduler mainThreadScheduler]]
 http://www.raywenderlich.com/62796/reactivecocoa-tutorial-pt2 

[[[[[[[[self requestAccessToTwitterSignal]
 then:^RACSignal * {
 return self.searchTextField.rac_textSignal;
 }]
 filter:^BOOL (NSString *text) {
 return [self isValidSearchText:text];
 }]
 throttle:0.5]
 flattenMap:^RACStream * (NSString *text) {
 return [self signalForSearchWithText:text];
 }]
 map:^id (NSDictionary *jsonSearchResult) {
 NSArray *statuses = jsonSearchResult[@"statuses"];
 return statuses;
 }]
 deliverOn:[RACScheduler mainThreadScheduler]]
 subscribeNext:^(NSArray *tweets) {
 [self displayTweets:tweets];
 } error:^(NSError *error) {
 NSLog(@"An error occurred: %@", error);
 }]; http://www.raywenderlich.com/62796/reactivecocoa-tutorial-pt2 

–Evan Coleman, http://is.gd/kTQhF2 “At first, none of it made any sense to me. I spent the next week pouring over the ReactiveCocoa docs, but no matter how much I read, signals, etc .. they were all just words to me...It wasn’t until I dove head first into the code that I started to see the light." 

–Ian Dundas, 2015 “Try it”