ReactiveCocoa provides a common interface for handling asynchronous events from different sources like UI controls, network requests, and notifications. It uses signals that emit events to represent asynchronous data over time. Operations like filtering, mapping, and combining signals allow defining reactive workflows. Swift's support for functional programming makes it a good fit for the declarative style of ReactiveCocoa.

1. ReactiveCocoa and Swift
Better Together
@ColinEberhardt
ShinobiControls

2. www.shinobicontrols.com

4. What is
ReactiveCocoa?

6. Functional Programming
+
Reactive Programming
=
Functional Reactive
Programming

7. ReactiveCocoa
In my own words

8. Every line of code we write is
executed in reaction to an event

9. But … these events come in
many different forms
KVO
delegates
NSNotification
target-action
callbacks

10. ReactiveCocoa provides a
common interface for all events!

11. … this allows us to define a
language for manipulating,
transforming and coordinating
events

12. ReactiveCocoa
Made Simple

13. Get Reactive
let textSignal: RACSignal =
usernameTextField.rac_textSignal()
textSignal.subscribeNext {
(text: AnyObject!) -> Void in
let textString = text as String
println(textString)
}

14. Get Reactive

15. Objective-C Friction
let textSignal: RACSignal =
usernameTextField.rac_textSignal()
textSignal.subscribeNext {
(text: AnyObject!) -> Void in
let textString = text as String
println(textString)
}

16. Simplified with Swift
func subscribeNextAs<T>(nextClosure:(T) -> ()) -> () {
self.subscribeNext {
textSignal.subscribeNextAs {
(text: String) -> () in
println(text)
}
(next: AnyObject!) -> () in
let nextAsT = next! as T
nextClosure(nextAsT)
}
}
textSignal.subscribeNext {
(text: AnyObject!) -> Void in
let textString = text as String
println(textString)
}

17. Signals
• A signal emits events
• next
• error
• completed
• A signal can have none,
one or more subscribers
textSignal.subscribeNextAs({
(text: String) in
println(text)
}, error: {
(error) in
// ...
}, completed: {
// ...
})

18. Events
Signals can emit none, one or more next events,
optionally followed by either an error or
completed
COMPLETED
NEXT NEXT ERROR
NEXT NEXT NEXT NEXT …
intervals do not
have to be
regular!

19. Signal all things
• Network request
• A single next, followed by a completed
• Large download
• Multiple next events, representing partial data,
followed by completed
• UI control
• An infinite stream of next events

20. filter
let textSignal: RACSignal = usernameTextField.rac_textSignal()
let filteredText = textSignal.filterAs {
(text: NSString) -> Bool in
return text.length > 3
}
filteredText.subscribeNextAs {
(text: String) in
println(text)
}
A filter is a ‘gate’, filtering-out events which do not match
the given condition

22. What exactly are events?
• What does a next event actually look like?
• Anything!
• Signals are an interface for handling
asynchronous events
• The event contents are context dependant

23. map
let textSignal: RACSignal = usernameTextField.rac_textSignal()
let textLength = textSignal.mapAs {
(text: NSString) -> NSNumber in
return text.length
}
textLength.subscribeNextAs {
(length: NSNumber) in
println(length)
}
Transforms each next event (please ignore the types!)

25. Creating a pipeline
let textSignal: RACSignal =
usernameTextField.rac_textSignal()
let textLength = textSignal.mapAs {
(text: NSString) -> NSNumber in
return text.length
}
let filteredText = textLength.filterAs {
(number: NSNumber) -> Bool in
return number > 3
}
filteredText.subscribeNextAs {
(length: NSNumber) in
println(length)
}

26. Fluent syntax
usernameTextField
.rac_textSignal()
.mapAs {
(text: NSString) -> NSNumber in
return text.length
}
.filterAs {
(number: NSNumber) -> Bool in
return number > 3
}
.subscribeNextAs {
(length: NSNumber) in
println(length)
}

27. number- number-
ReactiveCocoa Signals
rac_textSignal- map- filter- subscribeNext-string-
PUSH%
Lazy Sequences
string% number% number%
lazy% map% filter% generator%
PULL$

28. TweetSentiment

30. searchTextField
.rac_textSignal()
.mapAs {
(text: NSString) -> UIColor in
text.length <= 3
? UIColor.lightRedColor()
: UIColor.whiteColor()
}
.setKeyPath("backgroundColor", onObject: searchTextField)

31. Don’t search for very short
search terms

32. Don’t issue a new query on
each key press

33. searchTextField
.rac_textSignal()
.throttle(0.5)
.filterAs {
(text: NSString) -> Bool in
text.length > 3
}
.subscribeNextAs {
(text: NSString) -> () in
println(text)
}

34. rac_textSignal-filter-
setKeyPath-thro5le-
filter- subscribeNext-

35. Creating
Signals

36. private func signalForSearchWithText(text: String) -> RACSignal {
func requestforSearchText(text: String) -> SLRequest {
return ...
}
return RACSignal.createSignal {
subscriber -> RACDisposable! in
let request = requestforSearchText(text)
let maybeTwitterAccount = self.getTwitterAccount()
if let twitterAccount = maybeTwitterAccount {
request.account = twitterAccount
request.performRequestWithHandler {
(data, response, _) -> Void in
if response != nil && response.statusCode == 200 {
let timelineData = NSJSONSerialization.parseJSONToDictionary(data)
subscriber.sendNext(timelineData)
subscriber.sendCompleted()
} else {
subscriber.sendError(TwitterInstantError.InvalidResponse.toError())
}
}
} else {
subscriber.sendError(TwitterInstantError.NoTwitterAccounts.toError())
}
return nil
}
}

37. searchTextField
.rac_textSignal()
.throttle(0.5)
.filterAs {
(text: NSString) -> Bool in
text.length > 3
}
.mapAs {
(text: NSString) -> RACStream in
self.signalForSearchWithText(text)
}
.subscribeNextAs {
...
}

38. searchTextField
.rac_textSignal()
.throttle(0.5)
.filterAs {
(text: NSString) -> Bool in
text.length > 3
}
.flattenMapAs {
(text: NSString) -> RACStream in
self.signalForSearchWithText(text)
}
.subscribeNextAs {
...
}

39. searchTextField
.rac_textSignal()
.throttle(0.5)
.filterAs {
(text: NSString) -> Bool in
text.length > 3
}
.flattenMapAs {
(text: NSString) -> RACStream in
self.signalForSearchWithText(text)
}
.deliverOn(RACScheduler.mainThreadScheduler())
.subscribeNextAs {
(tweets: NSDictionary) in
let statuses = tweets["statuses"] as [NSDictionary]
self.tweets = statuses.map { Tweet(json: $0) }
self.tweetsTableView.reloadData()
self.tweetsTableView.scrollToTop()
}

40. rac_textSignal-filter-
setKeyPath-thro5le-
filter- fla5enMap-twi5erSearch-deliverOn-
subscribeNext-

41. requestAccessToTwitterSignal()
.then {
self.searchTextField.rac_textSignal()
}
.filterAs {
...
}
.throttle(0.5)
...

42. rac_textSignal-filter-
setKeyPath-thro5le-
filter- fla5enMap-twi5erSearch-deliverOn-
subscribeNext-requestAccess-signal-
then-

43. Error
Handling

44. requestAccessToTwitterSignal()
.then {
self.searchTextField.rac_textSignal()
}
.filterAs {
...
}
.throttle(0.5)
.doNext {
...
}
.flattenMapAs {
...
self.signalForSearchWithText(text)
}
.deliverOn(RACScheduler.mainThreadScheduler())
.subscribeNextAs({
...
}, {
(error) in
println(error)
})

45. Fetching
Sentiment Data

46. Fire a sentiment API request
for each tweet and merge the
signals?

47. Fetch sentiment data when
cells become visible?

48. Fetch sentiment data when a
cell has been visible for a short
duration?

49. Signal All
Things!

50. RACSignal
.interval(0.5, onScheduler:
RACScheduler(priority: RACSchedulerPriorityBackground))
.take(1)
.takeUntil(rac_prepareForReuseSignal)
.flattenMap {
(next) -> RACStream in
self.obtainSentimentSignal(hasTweet)
}
.deliverOn(RACScheduler.mainThreadScheduler())
.subscribeNextAs {
(sentiment: String) in
NSNotificationCenter.defaultCenter()
.postNotificationName("sentiment", object: sentiment)
self.sentimentIndicator.backgroundColor =
self.sentimentToColor(sentiment)
}

52. I curried
a function!

54. func scale(domainMin: Double, domainMax: Double, screenMin: Double,
screenMax: Double, pt: Double) -> CGFloat {
let value = ((pt - domainMin) / (domainMax - domainMin))
* (screenMax - screenMin) + screenMin
return CGFloat(value)
}

55. func scale(domainMin: Double, domainMax: Double, screenMin: Double,
screenMax: Double)(pt: Double) -> CGFloat {
let value = ((pt - domainMin) / (domainMax - domainMin))
* (screenMax - screenMin) + screenMin
return CGFloat(value)
}

56. let xscale = scale(0, Double(maxValue), 35, Double(self.bounds.width - 35))
let yscale = scale(0, 3, 0, Double(self.bounds.height))
positiveLayer.frame = CGRect(x: xscale(pt: 0), y: yscale(pt: 1),
width: xdelta(pt1: Double(positive), pt2: 0.0), height: yscale(pt: 1))
neutralLayer.frame = ...
func delta(scale:(Double) -> CGFloat)(pt1: Double, pt2: Double) -> CGFloat {
return scale(pt1) - scale(pt2)
}
let xdelta = delta(xscale)

58. Better
Together

59. Swift loves
Fluent APIs

60. requestAccessToTwitterSignal()
.then {
self.searchTextField.rac_textSignal()
}
.filterAs {
(text: NSString) -> Bool in
text.length > 3
}
.doNext {
(any) in
self.tweetsTableView.alpha = 0.5
}
.throttle(0.5)
.doNext {
(any) in
NSNotificationCenter.defaultCenter().postNotificationName("sentiment", object: "reset")
}
.flattenMapAs {
(text: NSString) -> RACStream in
self.signalForSearchWithText(text)
}
.deliverOn(RACScheduler.mainThreadScheduler())
.subscribeNextAs({
(tweets: NSDictionary) in
let statuses = tweets["statuses"] as [NSDictionary]
self.tweets = statuses.map { Tweet(json: $0) }
self.tweetsTableView.reloadData()
self.tweetsTableView.scrollToTop()
self.tweetsTableView.alpha = 1.0
}, {
(error) in
println(error)
})

61. http://stackoverflow.com/questions/3124001/fluent-interface-pattern-in-objective-c

62. http://stackoverflow.com/questions/3124001/fluent-interface-pattern-in-objective-c

63. [[[[[[[[self requestAccessToTwitterSignal]
then:^RACSignal *{
@strongify(self)
return self.searchText.rac_textSignal;
}]
filter:^BOOL(NSString *text) {
@strongify(self)
return [self isValidSearchText:text];
}]
throttle:0.5]
flattenMap:^RACStream *(NSString *text) {
@strongify(self)
return [self signalForSearchWithText:text];
}]
map:^id(NSDictionary *jsonSearchResult) {
NSArray *statuses = jsonSearchResult[@"statuses"];
NSArray *tweets = [statuses linq_select:^id(id tweet) {
return [RWTweet tweetWithStatus:tweet];
}];
return tweets;
}]
deliverOn:[RACScheduler mainThreadScheduler]]
subscribeNext:^(NSArray *tweets) {
[self.resultsViewController displayTweets:tweets];
} error:^(NSError *error) {
NSLog(@"An error occurred: %@", error);
}];

64. ReactiveCocoa
hates state

65. 39 constants
12 variables
6 outlets (not my fault!)
1 UIWindow
5 UI state variables

66. ReactiveCocoa and Swift
Better Together
@ColinEberhardt
ShinobiControls
TwitterSentiment:
https://github.com/ColinEberhardt/ReactiveSwiftLondon
MVVM with ReactiveCocoa and Swift:
https://github.com/ColinEberhardt/ReactiveSwiftFlickrSearch
Tutorials:
http://www.raywenderlich.com/u/ColinEberhardt