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What the FRP? – Using Reactive Cocoa
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Jay Baird
July 23, 2013
Programming
3
250
What the FRP? – Using Reactive Cocoa
A talk I gave on using Reactive Cocoa in the Rackspace Cloud Control application at NSMeetup July.
Jay Baird
July 23, 2013
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Transcript
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.
<RACSubscriber> • 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
None
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
None
RACSignal *deleteSignal = [button rac_signalForControlEvents:UIControlEventTouchUpInside]; [deleteSignal subscribeNext:^(UIButton *sender) { [[self.server
delete] subscribeCompleted:deleteServer]; }];
None
None