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Where the Reactive Rubber Meets the Road

rjrjr
November 10, 2015

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

rjrjr

November 10, 2015
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  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

    View full-size slide

  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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