What the FRP? Using Reactive Cocoa • NSMeetup July 

Jay Baird • @skatterbean Work at Rackspace Indie iOS developer 

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Not an expert, just an enthusiast. 

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The dreaded server list view. 

Regions Servers Images Sizes 

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make dependencies explicit 

values can change over time 

We wait for I/O. We wait for the disk. We wait for users. 

Data Flow 

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a graph of signals that propagate change Reactive Programming! 

Reactive Cocoa a github joint • https://github.com/ReactiveCocoa 

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The Players 

RACStream •Any series of object values available immediately or in the future. •Monads and strife. 

• Protocol any object can adopt to receive values from a signal. • Use RACSignal’s - subscribeNext:error:completed: convenience methods 

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) 

Example 

Binding 

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"]]; }]; 

RACSubject • Think of it as RACMutableSignal • provides sendNext:, sendError: and sendCompleted methods • Crazy useful for bridging other code, e.g. AFNetworking 

Example 

Timers 

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]]; 

The dreaded server list view 

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; 

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 

Example 

Authentication 

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]; }]; } 

a GUI example 

KVO 

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NSArray *fields = @[RACAble(self.password), RACAble(self.passwordConfirmation)]; RAC(self.createEnabled) = [RACSignal combineLatest:fields reduce:^(NSString *password, NSString *passwordConfirm) { return [passwordConfirm isEqualToString:password]; }]; 

Notification Signals 

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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); }]; 

Control Events 

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RACSignal *deleteSignal = [button rac_signalForControlEvents:UIControlEventTouchUpInside]; [deleteSignal subscribeNext:^(UIButton *sender) { [[self.server delete] subscribeCompleted:deleteServer]; }]; 

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