Functional Reactive Programming in the Mobile World

Transcript

1. Functional Reactive
   Programming in the
   Mobile World
   !
   Edward Dale
   @scompt
   

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2. Agenda
   • Theory
   • The mobile world
   • Functional Reactive
   Programming (FRP)
   • Practice
   • ReactiveExtensions (RX)
   • ReactiveCocoa
   • RxJava
   !2
   http://knowyourmeme.com/memes/business-cat
   

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3. The Mobile World
   • (or maybe the desktop world)
   • Event-based (onTouch,
   onLongClick, network
   connection lost, etc)
   • Concurrent
   • Messy
   !3
   

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4. Lots of solutions to
   concurrency complexity
   • Grand Central Dispatch
   • NSOperationQueue
   • Loader
   • AsyncTask
   • ExecutorService
   • Thread
   !4
   http://www.flickr.com/photos/30630175@N07/2950272850
   

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5. Lots of solutions to
   concurrency complexity
   • Grand Central Dispatch
   • NSOperationQueue
   • Loader
   • AsyncTask
   • ExecutorService
   • Thread
   Still using!
   imperative!
   programming
   !4
   http://www.flickr.com/photos/30630175@N07/2950272850
   

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6. Functional Reactive
   Programming (FRP)
   • Functional
   • Emphasize functions as the building
   blocks of a program
   • Avoid mutable data and side effects
   • x′ = f(x)
   • Reactive
   • Emphasize data flow and
   propagation of change
   • Similar to Observer pattern
   • a := b + c
   λ
   !5
   

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7. FRP in Practice
   • Reactive Extensions (Rx)
   • from Microsoft
   • Rx = Observables + Operators +
   Schedulers
   • Observables represent
   asynchronous data streams
   • Operators query 

   asynchronous data streams
   • Schedulers parameterize

   the concurrency in

   asynchronous data streams
   !6
   

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8. Observer
   1 //Provides a mechanism for receiving push-based notifications.
   2 public interface IObserver {
   3 //Provides the observer with new data.
   4 void OnNext(T value);
   5
   6 // Notifies the observer that the provider has
   7 // experienced an error condition.
   8 void OnError(Exception error);
   9
   10 // Notifies the observer that the provider has
   11 // finished sending push-based notifications.
   12 void OnCompleted();
   13 }
   !7
   

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9. Marble Diagrams
   !8
   https://raw.github.com/wiki/Netflix/RxJava/images/rx-operators/legend.png
   

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10. ReactiveCocoa
    • Implementation of Reactive
    Extensions for Objective C
    • Originally by GitHub
    !9
    

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11. ReactiveCocoa Example 1
    1 - (id)init
    2 {
    3 self = [super initWithFrame:CGRectZero];
    4 if (self) {
    5 // Build objects...
    6 RAC(backButton, enabled) = RACObserve(self, backEnabled);
    7 RAC(forwardButton, enabled) = RACObserve(self, forwardEnabled);
    8 RAC(titleLabel, text) = RACObserve(self, title);
    9
    10 RACSignal *loadingSignal = RACObserve(self, loading);
    11 RAC(refreshButton, hidden) = loadingSignal;
    12 RAC(stopButton, hidden) = [loadingSignal map:^id(NSNumber *enabled) {
    13 return [NSNumber numberWithBool:![enabled boolValue]];
    14 }];
    15 }
    16 return self;
    17 }
    !10
    

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12. 1 - (RACSignal *) savedPages {
    2 return [[[RACSignal createSignal:^RACDisposable *(id subsc) {
    3 NSURLSessionDataTask *op =
    4
    5 [self GET:@"saved_pages"
    6 parameters:nil
    7 success:^(NSURLSessionDataTask *operation, NSXMLParser *parser) {
    8 GVSavedPagesParserDelegate *parDel =
    9 [GVSavedPagesParserDelegate new];
    10 parser.delegate = parDel;
    11 if([parser parse]) {
    12 [subsc sendNext:RACTuplePack(parDel.savedPages,
    13 parDel.homepage)];
    14 [subsc sendCompleted];
    15 } else {
    16 [subsc sendError:parser.parserError];
    17 }
    18 }
    19 failure:^(NSURLSessionDataTask *operation, NSError *error) {
    20 [subsc sendError:error];
    21 }];
    22
    23 return [RACDisposable disposableWithBlock:^{
    24 [op cancel];
    25 }];
    26 }] replayLazily] subscribeOn:[RACScheduler scheduler]];
    27 }
    ReactiveCocoa Example 2
    WIP
    !11
    

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13. RxJava
    • Implementation of Reactive
    Extensions for Java
    • Originally by Netflix
    !12
    

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14. RxJava Example 1
    1 @Override
    2 public void onCreate(Bundle savedInstanceState) {
    3
    4 // ...
    5
    6 final Button sendButton = (Button) findViewById(R.id.send);
    7 final EditText bodyEdit = (EditText) findViewById(R.id.body);
    8 final Observable bodyText = Events.text(bodyEdit);
    9
    10 // send button is only enabled when we have some body content
    11 bodyText.map(new Func1() {
    12 @Override
    13 public Boolean call(String text) {
    14 return !text.trim().equals("");
    15 }
    16 }).subscribe(Properties.enabledFrom(sendButton));
    17 }
    !13
    https://github.com/andrewhr/rxjava-android-example/blob/master/app/src/main/java/com/example/rx/ComposeMessageActivity.java
    

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15. RxJava Example 2
    1 public Observable getWeatherData(final String city) {
    2 return Observable.create(new Observable.OnSubscribeFunc() {
    3 @Override
    4 public Subscription onSubscribe(Observer super WeatherData> obs) {
    5 try {
    6 obs.onNext(apiManager.getWeather(city, "metric"));
    7 obs.onCompleted();
    8 } catch (Exception e) {
    9 obs.onError(e);
    10 }
    11
    12 return Subscriptions.empty();
    13 }
    14 }).subscribeOn(Schedulers.threadPoolForIO());
    15 }
    !14
    http://howrobotswork.wordpress.com/2013/10/28/using-rxjava-in-android/
    

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16. Questions?
    Edward Dale
    @scompt
    

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