Where the Reactive Rubber Meets the Road

Square's simple idiom for binding RxJava to the UI without boilerplate or side effects.

https://www.youtube.com/watch?v=va1d4MqLUGY

Transcript

1. Where the
   Reactive Rubber
   Meets the Road
   

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2. What could possibly go
   wrong?
   Hard Learned Lesson
   // We want to ensure that the animation end 

   // event happens after the transition has

   // actually finished.
   

   newChild.post(new Runnable() {

   @Override public void run() {

   animationEnded(newChild);

   }

   });
   

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3. What could possibly go
   wrong?
   Hard Learned Lesson
   // We want to ensure that the animation end 

   // event happens after the transition has

   // actually finished.
   

   newChild.installLandMine(new Runnable() {

   @Override public void run() {

   animationEnded(newChild);

   }

   });
   

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4. Every Post is a Race
   java.lang.NullPointerException:
   at com.squareup.ui.root.HomeDrawerPresenter.closeDrawer(HomeDrawerPresenter.java:303)
   at com.squareup.ui.root.HomeScreen$Presenter.onFlowAnimationEnd(HomeScreen.java:226)
   at com.squareup.ui.root.HomeView.onFlowAnimationEnd(HomeView.java:81)
   at com.squareup.flow.RegisterFlowContainerSupport.flowAnimationEnded(RegisterFlowContainerSupport.java:258)
   at com.squareup.flow.RegisterFlowContainerSupport.access$000(RegisterFlowContainerSupport.java:45)
   at com.squareup.flow.RegisterFlowContainerSupport$1.run(RegisterFlowContainerSupport.java:120)
   at android.os.Handler.handleCallback(Handler.java:733)
   at android.os.Handler.dispatchMessage(Handler.java:95)
   at android.os.Looper.loop(Looper.java:136)
   at android.app.ActivityThread.main(ActivityThread.java:5103)
   at java.lang.reflect.Method.invokeNative(Method.java:-2)
   at java.lang.reflect.Method.invoke(Method.java:515)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:606)
   at dalvik.system.NativeStart.main(NativeStart.java:-2)
   

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5. ‣ Millions of downloads
   ‣ Every code path, every corner case
   will be hit every day
   ‣ Each post() adds another
   combinatorial dimension
   Every Race
   Will be Run
   

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6. Keep your main thread synchronous
   

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7. Embrace the net’s asynchrony
   Keep your main thread synchronous
   

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8. Belly Burgers
   Unlike any Burger You’ve Ever Had.
   $1.99 fee • 40 - 60 min • $$
   Taqueria Cazado…
   Ole!
   $1.99 fee • 40 - 60 min • $
   

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9. Belly Burgers
   Unlike any Burger You’ve Ever Had.
   $1.99 fee • 40 - 60 min • $$
   Taqueria Cazado…
   Ole!
   $1.99 fee • 40 - 60 min • $
   Picasso
   

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10. Belly Burgers
    Unlike any Burger You’ve Ever Had.
    $1.99 fee • 40 - 60 min • $$
    Taqueria Cazado…
    Ole!
    $1.99 fee • 40 - 60 min • $
    Model
    Picasso
    

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11. RestaurantListView.java
    Ain’t Rx Grand?
    restaurants.restaurant(restaurantId)

    .observeOn(mainThread())
    .subscribe(new Action1() {

    @Override public void call(Restaurant
    restaurant) {

    nameView.setText(restaurant.name);
    // etc.

    }

    }));
    

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12. RestaurantListView.java
    Command-minus
    Ain’t Rx Grand?
    restaurants.restaurant(restaurantId)

    .observeOn(mainThread())
    .subscribe((Action1) (restaurant) -> {

    nameView.setText(restaurant.name);
    // etc.

    }));
    

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13. RestaurantListView.java
    Command-minus
    Ain’t Rx Grand?
    restaurants.restaurant(restaurantId)

    .observeOn(mainThread())
    .subscribe((Action1) (restaurant) -> {

    nameView.setText(restaurant.name);
    // etc.

    }));
    

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14. rx.android.schedulers.
    HandlerScheduler
    Uh Oh
    public Subscription schedule(Action0 action, long delayTime, TimeUnit unit) {

    if(this.compositeSubscription.isUnsubscribed()) {

    return Subscriptions.unsubscribed();

    } else {

    action = RxAndroidPlugins.getInstance().getSchedulersHook().onSchedule(action);

    final ScheduledAction scheduledAction = new ScheduledAction(action);

    scheduledAction.addParent(this.compositeSubscription);

    this.compositeSubscription.add(scheduledAction);

    this.handler.postDelayed(scheduledAction,
    unit.toMillis(delayTime));

    scheduledAction.add(Subscriptions.create(new Action0() {

    public void call() {

    HandlerWorker.this.handler.removeCallbacks(scheduledAction);

    }

    }));

    return scheduledAction;

    }

    }

    

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15. rx.android.schedulers.
    HandlerScheduler
    Uh Oh
    public Subscription schedule(Action0 action, long delayTime, TimeUnit unit) {

    if(this.compositeSubscription.isUnsubscribed()) {

    return Subscriptions.unsubscribed();

    } else {

    action = RxAndroidPlugins.getInstance().getSchedulersHook().onSchedule(action);

    final ScheduledAction scheduledAction = new ScheduledAction(action);

    scheduledAction.addParent(this.compositeSubscription);

    this.compositeSubscription.add(scheduledAction);

    this.handler.postDelayed(scheduledAction,
    unit.toMillis(delayTime));

    scheduledAction.add(Subscriptions.create(new Action0() {

    public void call() {

    HandlerWorker.this.handler.removeCallbacks(scheduledAction);

    }

    }));

    return scheduledAction;

    }

    }

    Just make it block if
    we’re already on the
    handler’s thread.
    

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16. rx.android.schedulers.
    HandlerScheduler
    Uh Oh
    public Subscription schedule(Action0 action, long delayTime, TimeUnit unit) {

    if(this.compositeSubscription.isUnsubscribed()) {

    return Subscriptions.unsubscribed();

    } else {

    action = RxAndroidPlugins.getInstance().getSchedulersHook().onSchedule(action);

    final ScheduledAction scheduledAction = new ScheduledAction(action);

    scheduledAction.addParent(this.compositeSubscription);

    this.compositeSubscription.add(scheduledAction);

    this.handler.postDelayed(scheduledAction,
    unit.toMillis(delayTime));

    scheduledAction.add(Subscriptions.create(new Action0() {

    public void call() {

    HandlerWorker.this.handler.removeCallbacks(scheduledAction);

    }

    }));

    return scheduledAction;

    }

    }

    If I could then
    I would…
    Just make it block if
    we’re already on the
    handler’s thread.
    

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17. Belly Burgers
    Unlike any Burger You’ve Ever Had.
    $1.99 fee • 40 - 60 min • $$
    Taqueria Cazado…
    Ole!
    $1.99 fee • 40 - 60 min • $
    Model
    

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18. Belly Burgers
    Unlike any Burger You’ve Ever Had.
    $1.99 fee • 40 - 60 min • $$
    Taqueria Cazado…
    Ole!
    $1.99 fee • 40 - 60 min • $
    Model
    

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19. Rx is single-threaded by default
    

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20. What starts on the main thread,
    stays on the main thread.
    restaurants.restaurant(restaurantId)

    .observeOn(mainThread())
    .subscribe((Action1) (restaurant) -> {

    nameView.setText(restaurant.name);
    // etc.

    }));
    

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21. RestaurantList.java
    Observables all the way down
    public Observable
    restaurant(final String id) {

    return restaurantsSubject

    .map((Func1) (restaurants) -> {

    for (Restaurant r : restaurants) {

    if (r.id.equals(id)) return r;

    }

    return null;

    }) 

    .filter(notNull());

    }
    

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22. RestaurantList.java
    Promise Keeper
    /** May be called from any thread. */
    public void update(
    List restaurants) {

    Observable.just(restaurants)

    .observeOn(mainThread())

    .subscribe((Action1) (list) {

    restaurantsSubject.onNext(list);

    });

    }

    

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23. RestaurantList.java
    Better: Promise Enforcer
    /** Must be called from the main thread. */
    public void update(
    List restaurants) {

    checkMainThread();

    restaurantsSubject.onNext(restaurants);

    }

    void checkMainThread() {

    if (myLooper() != getMainLooper()) {
    throw new AssertionError(
    “Must be called from main thread.”);
    }

    }

    

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24. Keep your main thread synchronous
    Embrace the net’s asynchrony
    Pick two
    

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25. Bonus Hack Round: Cleaner stack traces
    service.doThing(thing)

    .subscribe((Action1) (response) {

    onResponse(response);

    },

    (e) {

    if (e instanceof RetrofitError) {

    onNetworkError(e);

    } else {

    throw new
    OnErrorNotImplementedException(e);

    }

    });

    If you must handle errors
    

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26. @rjrjr
    https://squareup.com/careers
    

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