Introduction to reactive programming & ReactiveCocoa

(Func&onal)+Reac&ve+Programming
with%Reac*veCocoa
Florent(Pillet
fpillet@gmail.com
@fpillet

Reac%ve'programming'is'essen%ally
working'with
asynchronous*data*streams

Data!changes!over!$me!and!flows!in!streams!
processed!with!an!asynchronous,!operator5
based!logic.

They%are%all%separate,%yet%usually%found%at%the%
same%place%in%the%code.

Build&modern,&highly&interac3ve,&dynamic&
applica3ons
with%minimal%pain.

Learning(curve(is(steep1
Need$to$let$go$of$impera/ve$habits,$learn$new$abstrac/ons$and$
pa8erns
but...
1"If"you"read"only"one,"it"should"be"The"introduc6on"to"reac6ve"programming"you've"been"missing

Pays%off%big%+me
Modular
Reusable
Expressive
Testable(code

Reac%ve'programming'paves'the'way'to'
elimina'ng)state)and)mutability'from'your'
code.2
2"Suggested"reading:"Enemy"of"the"state"presenta6on"by"Jus6n"Sparh9Summmers.

So#what#does#this#reac.ve#thing#look#like?

Signal of current GPS position (CLLocation)
@100 @450 @300 @140
time
Signal of numerical values (NSNumber)
@270
Signal of boolean values (NSNumber)
@YES @NO @YES

@100 @450 @300 @140
time
@270
Signal of boolean values (NSNumber)
@YES @NO @YES
Signals(send(events(over(
&me
A"signal"is"a"push,driven"stream.
It#produces#three#types#of#events:
• next"values
• an"error
• a"comple+on

next!values
A"stream"of"values"delivered"over"0me.
Signals(produce(zero,(one(or(many(next(values(before(either(
comple*ng(or(producing(an(error.
time
next next next next next next

signal'comple'on
time
Signal with result of network request
next completed

signal'error
time
Signal of network request emits error on request failure
error
(emits a NSError)

Opera&ons
• A#signal#can#have#mul0ple#subscribers
• Opera0ons#subcribe#to#a#signal#and#return#a#new#signal
• Easy#mul0;staged#transforma0ons
• Async,#async,#async!

map
time
B"="A.map(fn)
@100 @270
@450 @300 @140
A
B

filter
@100 @270
@450 @300 @140
B"="A.filter(fn)
@100 @140
time
A
B

concat
time
A
B
C
C"="A.concat(B)

merge
time
A
B
C
C"="merge(A,B)

combineLatest
time
A 1 3
B
C
6 7 8
2
C"="combineLatest(A,B)
2
6
3
6
3
7
3
8
Signal of tuples

switchToLatest
time
D A C B
E
E"="D.switchToLatest()
A
B
C

Other&opera*ons
• flatten flattenMap zip zipWith startWith
• delay throttle sample timeout
• take takeUntil skip ignoreValues
• try catch then switch if and or not
• replay replayLast repeat retry
..."and"a"lot"more"...

Main%classes%in%Reac+veCocoa
• Signals:*RACSignal
• Tuples:*RACTuple
• Disposables:*RACDisposable
• Schedulers:*RACScheduler
• Commands:*RACCommand*(not*covered*here)

Subscribing*to*signals
[someSignal subscribeNext:^(id nextValue) {
// this is called asynchronously, every time a new value
// is available on the signal
NSLog(@"Signal emitted new value= %@", nextValue);
}];

Subscribing*to*signals
} error:^(NSError *error) {
NSLog(@"Signal emitted error %@", error);
} completed:^{
NSLog(@"Signal completed");
}];

Unsubscribing*from*signals
// subscribe
RACDisposable *disposable =
}];
// can cancel subscription at any time
[disposable dispose];

Crea%ng(signals
• Using'KVO
• Transforming'exis3ng'signals'into'a'new'signal
• Dynamically'with'a'generator'block
• Li>ing'from'the'impera3ve'world
• Manually'producing'events

Signals(from(variables((KVO)
Use$the$RACObserve(object,path)$macro$to$update$our$
unread$count$label3:
[RACObserve(self.model, unreadCount) subscribeNext:^(NSNumber *value) {
self.unreadCountLabel.text = [value stringValue];
}];
3"Real"developers"use"NSNumberForma+er

Transforming+exis.ng+signals
- (RACSignal *)colorForUnreadCount {
return [RACObserve(self.model,unreadCount) // this is a signal
// 'map' subscribes to the signal above and returns
// a new signal that sends a color for each new unreadCount
map:^id(NSNumber *unread) {
NSInteger count = unread.integerValue;
return count < 10 ? [UIColor blackColor] :
count < 20 ? [UIColor orangeColor] :
[UIColor redColor];
}];
}

Transforming+exis.ng+signals
// using the signal created in the previous slide
[[model colorForUnreadCount] subscribeNext:^(UIColor *color) {
self.unreadLabel.textColor = color;
}];
// a shorter way to write this (for simple binding cases)
// see <ReactiveCocoa/RACSusbscriptingAssignmentTrampoline.h>
RAC(self.unreadLabel, textColor) = model.colorForUnreadCount;

Dynamic(signals
- (RACSignal *)post:(NSDictionary *)formData toURL:(NSURL *)url {
return [RACSignal createSignal:^(id<RACSubscriber> subscriber)] {
// use AFNetworking to post form data
NSURLSessionDataTask *task = [self.sessionManager POST:url parameters:data
success:^(NSURLSessionDataTask *t, NSDictionary *responseObject) {
if (responseObject)
[subscriber sendNext:responseObject];
[subscriber sendCompleted];
}
failure:^(NSURLSessionDataTask *t, NSError *error) {
[subscriber sendError:error];
}
];
return [RACDisposable disposableWithBlock:^{
[task cancel];
}];
}];
}

Dynamic(signals
// use signal defined in previous slide
RACSignal *postSignal = [manager post:@{@"name": @"Florent"} toURL:someURL];
[postSignal subscribeNext:^(NSDictionary *response) {
NSLog(@"Server answered POST with %@", response);
} error:^(NSError *error) {
NSLog(@"POST failed with error: %@", error);
} completed:{
NSLog(@"POST was successful");
}]

Li#ing&to&the&reac.ve&world
[[[[[self
rac_signalForSelector:@selector(locationManager:didRangeBeacons:inRegion:)
fromProtocol:@protocol(CLLocationManagerDelegate)]
reduceEach:^(CLLocationManager *manager, NSArray *beacons, CLBeaconRegion *region) {
return [[beacons sortedArrayUsingFunction:proximityComparator context:NULL]
firstObject] ?: [NSNull null];
}]
filter:^BOOL(id value) {
return [value isKindOfClass:[CLBeacon class]];
}]
distinctUntilChanged]
subscribeNext:^(CLBeacon *beacon) {
NSLog(@"Last closest beacon: %@.%@", beacon.major, beacon.minor);
}];

Manual&signals
@property (strong) RACSubject *manualSignal;
- (id)init {
if (self = [super init]) {
self.manualSignal = [[RACSubject alloc] init];
}
return self;
}
- (void)dealloc {
[self.manualSignal sendCompleted];
}
- (RACSignal *)dataSignal {
return self.manualSignal;
}
- (void)newDataObtained:(id)data {
[self.manualSignal sendNext:data];
}

Manual&signals
Note%that:
• **RACSubject**#doesn't#automa.cally#emit#a#completed#
event#on#dealloc.#You#must#do#it#manually.
• Use#**RACReplaySubject**#to#create#a#subject#that#can#
resend#one#or#more#of#the#last#next#values#to#new#subscribers.
• Avoid#using#subjects#if#you#have#alterna.ves.

Disposables
Any$subscrip,on$returns$a$RACDiposable.$Use$it$to$cancel$the$
subscrip,on

Schedulers
• Based'on'serial'queues
• Makes'cancella&on'easy!
• Use'for'5mers'and'to'control'delivery'of'signals
RACSignal *onMainThread =
[signal deliverOn:[RACScheduler mainThreadScheduler]];
RACSignal *onSomeSerialQueue =
[signal deliverOn:[[RACTargetQueueScheduler alloc]
initWithName:@"My queue scheduler"
targetQueue:someSerialQueue]]

Schedulers
// A one-time timer that fires after 1 second on main thread
RACDisposable *timer = [[RACScheduler mainThreadScheduler]
afterDelay:1.0
schedule:^{
NSLog(@"Delayed logging");
}];
// We can cancel this at any time
[timer dispose];

Schedulers
// A cancellable periodic action
RACDisposable *timer = [[RACScheduler schedulerWithPriority:RACSchedulerPriorityDefault]
after:[NSDate dateWithTimeIntervalSinceNow:1.0]
repeatingEvery:0.5
withLeeway:0.1
schedule:^{
NSLog(@"Running periodic action on private queue");
}];
// Later: stop repeating
[timer dispose];

Other&Reac*veCocoa&gems
• @weakify @strongify @unsafeify
• @onExit
#import <ReactiveCocoa/RACExtScope.h>
@weakify(self);
[signal subscribeNext:^(id value) {
@strongify(self);
[self doSomethingWith:value];
}];

..."and"there"is"a"lot"more"...
Commands,)sequences,)signal)mul1cas1ng,)side)effects,)channels,)
backtraces)&)debugging)features,)event)materializa1on)and)
dematerializa1on,)tes1ng)are)among)topics)not)covered)here.
Framework)source)code)and)the)docset)for)Dash)are)useful)
resources.

More%usage%examples
- (RACSignal *)numberOfUnreadItems
{
@weakify(self);
return [[[[[self
itemsUpdated]
startWith:@YES]
map:^(id updated) {
@strongify(self);
return self.unreadItemsCount;
}]
distinctUntilChanged]
deliverOn:RACScheduler.mainThreadScheduler];
}];
}

More%usage%examples
// Automatically update a badge on tab bar
// when the count of unread items changes
- (void)keepNewsItemUpToDate:(UITabBarItem *)newsItem {
@weakify(newsItem);
[self.model.numberOfUnreadItems subscribeNext:^(NSNumber *count) {
@strongify(newsItem);
if (count.integerValue)
newsItem.badge = count.stringValue;
else
newsItem.badge = @"";
}];
}

Links
• Reac&veCocoa*framework
• A*demo*project:*Reac&veWeather
• The*introduc&on*to*reac&ve*programming*you've*been*missing
• Enemy*of*the*state
• Reac&ve*MVVM*(ModelFViewFViewModel):*perfect*match
Also%look%up%'Reac.ve'%on%Github%and%filter%by%langage%(Obj>C).

Scary&interlude
Yellow&parts&are&my&app.
Not$always$like$this.
This$shouldn't$put$you$off!

Reac%ve'programming'is'a'logical'way'to'
model'and'react'to
asynchronous'informa%on'flow

Reac%ve'code'clearly5'exposes'logic'and'
transforma-ons'deriving'from'new'data
5"finding"the"syntax"weird?"remember"your"first"steps"with"Objec:ve<C"and"all"these"square"brackets...

Enforcing)the)separa0on)between)data)
producers)and)consumers,)reac0ve)code)is)
more%testable

Once%trained%to%think%reac.vely,
reducing)state)and)mutability
is#the#next#logical#step#towards#code#safety,#
stability#and#predictability.

Reac%veCocoa)is)a)rich)and)powerful)reac%ve)
programming)framework)that)will)bring)your)
code)to)a)new)level

Reac%veCocoa)is)a)rich)and)powerful)reac%ve)
programming)framework)that)will)bring)your)
code)to)a)new)level
works&with&Swi+,&too4
4"See"Colin"Eberhardt's"posts"for"Swi6,"Reac:veCocoa"and"MVVM"pr0n.

Thank&You&!
Q"&"A
fpillet@gmail.com
@fpillet

Introduction to reactive programming & ReactiveCocoa

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