This document provides an overview of ReactiveCocoa, a framework for working with asynchronous data streams in a reactive and declarative way. It discusses key concepts in reactive and functional programming like streams, signals, sequences, and transformations like map, filter, and fold. It shows how to combine streams, inject side effects safely, and work with subjects. It recommends resources for learning more about ReactiveCocoa and reactive programming best practices.

1. Learn You A ReactiveCocoa
For Great Good

2. Jason Larsen (@jarsen)
Software Engineer at Instructure

3. Reactive Cocoa
More Than Fancy KVO For Hipsters

4. Saying that ReactiveCocoa is just KVO/bindings
is like saying that CoreData is just a SQLite
wrapper.

5. Functional Programming
• No state
• No side effects
• Immutability
• First class functions

6. Reactive Programming
1 3
2 4
3 7 10

7. Reactive Programming
1 3
5 4
6 7 13

8. FRP
Functional Reactive Programming

9. RAC(self, label.text) = RACObserve(self, name);
Declarative
tell the code what to do without telling it how to do it.

10. Reduce State
(Not eliminate—Obj-C is not purely functional)

11. RACStream
RACSequence
(pull driven)
RACSignal
(push driven)

12. A RACStream is like a pipe—new values flow
through it. You can subscribe to these values
and then transform them as you please.

13. RACSequence
Pull-Driven Stream

14. Sequences
NSArray *numbers = @[ @1, @2, @3 ];
RACSequence *sequence = numbers.rac_sequence;

15. Transforming Streams

16. Map
RACSequence *numbersSquared = [numbers.rac_sequence
map:^id(NSNumber *number) {
return @([number intValue] * [number intValue]);
}];

17. Map
Mapping
Function
@[@1, @2, @3] @[@1, @4, @9]

18. But… for loops!
NSArray *numbers = @[@1,@2,@3];
NSMutableArray *mutableNumbers = [numbers mutableCopy];
for (NSNumber *number in numbers) {
[mutableNumbers addObject:@([number intValue] *
[number intValue])];
}

19. Lazy Evaluation
NSArray *strings = @[ @"A", @"B", @"C" ];
RACSequence *sequence = [strings.rac_sequence
map:^(NSString *str) {
return [str stringByAppendingString:@"_"];
}];
!
sequence.head; // evaluates @"A_"
sequence.tail.head; // evaluates @"B_"
sequence.eagerSequence; // evaluates sequence

20. Filter
RACSequence *evenNumbers = [numbers.rac_sequence
filter:^BOOL(NSNumber *number) {
return @([number intValue] % 2 == 0);
}];

21. Filter
Filtering
Function
@[@1, @2, @3] @[@2]

22. Fold (a.k.a. reduce)
NSNumber *sum = [numbers.rac_sequence foldLeftWithStart:@0
reduce:^id(id accumulator, id value) {
return @([accumulator intValue] + [value intValue]);
}];

23. Fold
Folding
Function
@[@1, @2, @3] @6
@0

24. Fold Left
Sequence
!
@[@1, @2, @3, @4]
@[@1, @2, @3, @4]
@[@1, @2, @3, @4]
@[@1, @2, @3, @4]
@[@1, @2, @3, @4]
Accumulator
!
@0
@1
@3
@6
@10

25. Combining Streams

26. Concatenating
// concatenate letters at end of numbers
[numbers concat:letters];

27. Flattening Sequences
RACSequence *sequenceOfSequences =
@[letters,numbers].rac_sequence;
!
// flattening sequences concatenates them
RACSequence *flattened = [sequenceOfSequences flatten];

28. RACSignal
Push-Driven Stream

29. Map
// set the label to always be equal to the formatted
// string of “12 units”, where 12 is whatever the
// current value of self.total
RACSignal *signal = RACObserve(self, total);
RAC(self, totalLabel.text) = [signal map:^id(NSNumber
*total) {
[NSString stringWithFormat:@"%i units", total];
}];

30. Filter
// only set total to the even numbers in
// the stream
RAC(self, total) = [RACSignal(self, cell1)
filter:^BOOL(NSNumber *number) {
return @([number intValue] % 2 == 0);
}];

31. Creating Signals
[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber)
{
NSURLSessionDataTask *task = [self PUT:path parameters:parameters
success:^(NSURLSessionDataTask *task, id responseObject) {
[subscriber sendNext:responseObject];
} failure:^(NSURLSessionDataTask *task, NSError *error) {
[subscriber sendError:error];
}];
return [RACDisposable disposableWithBlock:^{
[task cancel];
}];
}];

32. Combining Signals
Yo dawg, I heard you like signals…

33. Sequencing
// when signal 1 completes, do signal 2
[[signal doNext:^(id x) {
NSLog(@"value: %@", x);
}]
then:^RACSignal *{
return signal2;
}];

34. Merging Signals
// creates a new signal that will send the
// values of both signals, complete when both
// are completed, and error when either errors
[RACSignal merge:@[signal1, signal2]];

35. Combine Latest Values
RACSignal *cell1Signal = RACObserve(self, cell1);
RACSignal *cell2Signal = RACObserve(self, cell2);
RAC(self, total) = [RACSignal
combineLatest:@[cell1Signal, cell2Signal] reduce:^id(id
cell1, id cell2){
return @([cell1 intValue] + [cell2 intValue]);
}];

36. Flattening Signals
RACSignal *signalOfSignals = [RACSignal
createSignal:^ RACDisposable *
(id<RACSubscriber> subscriber) {
[subscriber sendNext:letters];
[subscriber sendNext:numbers];
[subscriber sendCompleted];
return nil;
}];
!
// flattening signals merges them
RACSignal *flattened = [signalOfSignals
flatten];

37. Flattening & Mapping
// creates multiple signals of work which
// are automatically recombined, or in other words
// it maps each letter to a signal using
// saveEntriesForLetter: and then it merges them all.
letters = @[@“a”, @“b”, @“c”]
[[letters
flattenMap:^(NSString *letter) {
return [database saveEntriesForLetter:letter];
}]
subscribeCompleted:^{
NSLog(@"All database entries saved successfully.");
}];

38. Side Effects
Can’t live with ‘em, can’t live without ‘em

39. What is a “side effect?”
• logging
• making a network request
• update the UI
• changing some state somewhere

40. Subscriptions
[signal subscribeNext:^(id x) {
// do something with each value
} error:^(NSError *error) {
// do something with errors
} completed:^{
// do something with completed
}];

41. Inject Side Effects
[signal doCompleted:^{
// do some side effect after
}];
!
[signal doNext:^(id x) {
// some side effect here
}];
!
[signal doError:^(NSError *error) {
// handle error
}];

42. Inject Side Effects
[signal initially:^{
// do some side effect before signal
}];

43. Side Effects
// DO NOT DO
[signal map:^id(NSString *string) {
NSString *exclamation = [NSString
stringWithFormat:@"%@!!!", string];
[self showAlert:exclamation];
return exclamation;
}];

44. Side Effects
// DO DO
[[signal map:^id(NSString *string) {
return [NSString
stringWithFormat:@"%@!!!", string];
}] doNext:^(NSString *string) {
[self showAlert:string];
}];

45. RACSubject
Manual Signals

46. Use createSignal: over
RACSubject when possible
[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber)
{
NSURLSessionDataTask *task = [client PUT:path parameters:parameters
success:^(NSURLSessionDataTask *task, id responseObject) {
[subscriber sendNext:responseObject];
} failure:^(NSURLSessionDataTask *task, NSError *error) {
[subscriber sendError:error];
}];
return [RACDisposable disposableWithBlock:^{
[task cancel];
}];
}];

47. Concurrency

48. Scenario
• I want to run three networking calls and then when
they are all done do something
• I want to MERGE three signals and THEN do
something.

49. Scenario Solution
// basically: merging signals can replace dispatch groups
[[RACSignal merge:@[signal1, signal2, signal3]] then:^RACSignal * {
return [self someSignalToDoAfter];
}];

50. Replay/Multicasting

51. Replace Delegation
• rac_signalForSelector:fromProtocol:

52. Other Cool Methods
• -throttle:
• -takeUntil: can be used to automatically dispose of a
subscription when an event occurs (like a "Cancel"
button being pressed in the UI).
• -setNameWithFormat: for debugging
• -logNext, -logError, -logCompleted, -logAll automatically
log signal events as they occur, and include the name
of the signal in the messages. This can be used to
conveniently inspect a signal in real-time.

54. Your hammer
99% of the time
• -subscribeNext:error:completed:
• -doNext: / -doError: / -doCompleted:
• -map:
• -filter:
• -concat:
• -flattenMap:
• -then:
• +merge:
• +combineLatest:reduce:
• -switchToLatest:

55. We Want More!

56. Specifically, read DesignGuidelines.md
and BasicOperators.md
https://github.com/ReactiveCocoa/
ReactiveCocoa/tree/master/Documentation

57. https://leanpub.com/iosfrp
If you’re into books, this one’s decent.

58. https://github.com/ReactiveCocoa/
ReactiveViewModel

59. Github’s Obj-C API Lib
https://github.com/octokit/octokit.objc
!
(and Instructure’s API Lib modeled after it)
https://github.com/instructure/canvaskit

60. Don’t Cross the Streams

61. Type Safety
A word of warning

62. Non KVO Compliance
like textLabel.text

63. viewDidLoad Bloat
break stuff up into setupMethods