What the FRP? – Using Reactive Cocoa

A talk I gave on using Reactive Cocoa in the Rackspace Cloud Control application at NSMeetup July.

Transcript

1. What the FRP? Using Reactive Cocoa • NSMeetup July
2. Jay Baird • @skatterbean Work at Rackspace Indie iOS developer
4. Not an expert, just an enthusiast.
6. The dreaded server list view.
7. Regions Servers Images Sizes
9. make dependencies explicit
10. values can change over time
11. We wait for I/O. We wait for the disk. We wait for users.
12. Data Flow
14. a graph of signals that propagate change Reactive Programming!
15. Reactive Cocoa a github joint • https://github.com/ReactiveCocoa
18. The Players
19. RACStream •Any series of object values available immediately or in the future. •Monads and strife.
20. <RACSubscriber> • Protocol any object can adopt to receive values from a signal. • Use RACSignal’s - subscribeNext:error:completed: convenience methods
21. RACSignal • Push driven stream • As work is completed values are sent on the signal to its subscribers • Sends next, error and completed events • Can sends any number of next events, followed by one error or completed (but not both)
22. Example
23. Binding
24. RAC(self.image) = [[[client getImage:self.imageId] catch:^RACSignal *(NSError *error) { return [RACSignal empty]; // swallow the error }] map:^(NSDictionary *imageDict) { return [Image fromJSON:imageDict[@"image"]]; }];
25. RACSubject • Think of it as RACMutableSignal • provides sendNext:, sendError: and sendCompleted methods • Crazy useful for bridging other code, e.g. AFNetworking
26. Example
27. Timers
28. RACSubject *pollTimer = [RACSubject subject]; [[[RACSignal interval:60.0f] takeUntil:[pollTimer doNext:^(id x) { NSLog(@"Server poll cancelled"); }]] subscribeNext:^(id x) { NSLog(@"Tick."); } error:^(NSError *error) { NSLog(@"Handle the error condition"); } completed:^{ NSLog(@"Timer completed"); }]; // Cancels the timer. // A unit represents an empty value [pollTimer sendNext:[RACUnit defaultUnit]];
29. The dreaded server list view
30. RACSubject *subject = [RACSubject subject]; // +flavors returns a RACSignal with flavors // +images returns a RACSignal with images [[RACSignal combineLatest:@[[Flavor flavors], [Image images]]] subscribeError:^(NSError *error) { [subject sendError:error]; } completed:^{ [[client getServers] subscribeNext:^(NSArray *serverJSON) { // Process server JSON [subject sendCompleted]; } error:^(NSError *error) { [subject sendError:error]; } completed:^{ [subject sendCompleted]; }]; }]; return subject;
31. RACMulticastConnection • Signals start their work on each new subscription – great for isolating work • but bad if you have heavy side effects • Made using -multicast: on RACSignal • Creates one and only one subscription
32. Example
33. Authentication
34. RACSignal *deferredToken = [RACSignal defer:^{ return [self authWithUsername:username key:password]; }]; _tokenConnection = [deferredToken multicast:[RACReplaySubject subject]]; - (RACSignal *)withAuthToken { [_tokenConnection connect]; return _tokenConnection.signal; } - (RACSignal *)getServers { return [[self withAuthToken] flattenMap:^RACSignal *(NSString *token) { return [self enqueueRequestWithMethod:@"GET" endpoints:self.computeEndpoints path:@"/servers/detail" parameters:nil]; }]; }
35. a GUI example
36. KVO
38. NSArray *fields = @[RACAble(self.password), RACAble(self.passwordConfirmation)]; RAC(self.createEnabled) = [RACSignal combineLatest:fields reduce:^(NSString *password, NSString *passwordConfirm) { return [passwordConfirm isEqualToString:password]; }];
39. Notification Signals
41. RAC(self.logInButton.enabled) = [RACSignal combineLatest:@[ self.usernameTextField.rac_textSignal, self.passwordTextField.rac_textSignal] reduce:^(NSString *user, NSString *pass) { return (user.length > 0 && pass.length > 0); }];
42. Control Events
44. RACSignal *deleteSignal = [button rac_signalForControlEvents:UIControlEventTouchUpInside]; [deleteSignal subscribeNext:^(UIButton *sender) { [[self.server delete] subscribeCompleted:deleteServer]; }];