Android - Rx observable subscription in view holder: How to handle lifecycle?

Question

I know that if I subscribe to an observable in an Activity or Fragment, I should bind it to their respectively lifecycle, using rxlifecycle-components or rxlifecycle-navi.

However, I'm now subscribing to an observable from inside a ViewHolder (extends RecyclerView.ViewHolder), and I'm worried what would happen if the view is destroyed and the subscription remains active.

To which lifecycle should I bind this observable and how ?

Answer 1

You don't need any third-party lib to manage Activity lifecycle. Try the following codes:

public class LifecycleBinder {

    public static <R> Observable.Transformer<R, R> subscribeUtilEvent(final Activity target, LifecycleEvent event) {
        final Application app = target.getApplication();
        final PublishSubject<LifecycleEvent> publishSubject = PublishSubject.create();
        final Application.ActivityLifecycleCallbacks callbacks = new Application.ActivityLifecycleCallbacks() {
            @Override
            public void onActivityCreated(Activity activity, Bundle savedInstanceState) {

            }

            @Override
            public void onActivityStarted(Activity activity) {

            }

            @Override
            public void onActivityResumed(Activity activity) {

            }

            @Override
            public void onActivityPaused(Activity activity) {
                if (activity == target)
                    publishSubject.onNext(LifecycleEvent.ON_PAUSED);
            }

            @Override
            public void onActivityStopped(Activity activity) {
                if (activity == target)
                    publishSubject.onNext(LifecycleEvent.ON_STOPPED);
            }

            @Override
            public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
                if (activity == target)
                    publishSubject.onNext(LifecycleEvent.ON_SAVE_INSTANCE_STATE);
            }

            @Override
            public void onActivityDestroyed(Activity activity) {
                if (activity == target)
                    publishSubject.onNext(LifecycleEvent.ON_DESTROYED);
            }
        };

        app.registerActivityLifecycleCallbacks(callbacks);
        return subscribeUtilEvent(publishSubject, event, new Action0() {
            @Override
            public void call() {
                app.unregisterActivityLifecycleCallbacks(callbacks);
            }
        });
    }

    public static <R> Observable.Transformer<R, R> subscribeUtilEvent(final Fragment target, LifecycleEvent event) {
        final FragmentManager manager = target.getFragmentManager();
        if (manager == null) {
            throw new NullPointerException("fragment manager is null!");
        }

        final PublishSubject<LifecycleEvent> publishSubject = PublishSubject.create();
        final FragmentManager.FragmentLifecycleCallbacks callbacks = manager.new FragmentLifecycleCallbacks() {

            @Override
            public void onFragmentPreAttached(FragmentManager fm, Fragment f, Context context) {
            }

            @Override
            public void onFragmentAttached(FragmentManager fm, Fragment f, Context context) {
            }

            @Override
            public void onFragmentCreated(FragmentManager fm, Fragment f, Bundle savedInstanceState) {
            }

            @Override
            public void onFragmentActivityCreated(FragmentManager fm, Fragment f, Bundle savedInstanceState) {
            }

            @Override
            public void onFragmentViewCreated(FragmentManager fm, Fragment f, View v, Bundle savedInstanceState) {
            }

            @Override
            public void onFragmentStarted(FragmentManager fm, Fragment f) {
            }

            @Override
            public void onFragmentResumed(FragmentManager fm, Fragment f) {
            }

            @Override
            public void onFragmentPaused(FragmentManager fm, Fragment f) {
                if (f == target)
                    publishSubject.onNext(LifecycleEvent.ON_PAUSED);
            }

            @Override
            public void onFragmentStopped(FragmentManager fm, Fragment f) {
                if (f == target)
                    publishSubject.onNext(LifecycleEvent.ON_STOPPED);
            }

            @Override
            public void onFragmentSaveInstanceState(FragmentManager fm, Fragment f, Bundle outState) {
                if (f == target)
                    publishSubject.onNext(LifecycleEvent.ON_SAVE_INSTANCE_STATE);
            }

            @Override
            public void onFragmentViewDestroyed(FragmentManager fm, Fragment f) {
                if (f == target)
                    publishSubject.onNext(LifecycleEvent.ON_VIEW_DESTORYED);
            }

            @Override
            public void onFragmentDestroyed(FragmentManager fm, Fragment f) {
                if (f == target)
                    publishSubject.onNext(LifecycleEvent.ON_DESTROYED);
            }

            @Override
            public void onFragmentDetached(FragmentManager fm, Fragment f) {
                if (f == target)
                    publishSubject.onNext(LifecycleEvent.ON_DESTROYED);
            }
        };
        manager.registerFragmentLifecycleCallbacks(callbacks, true);

        return subscribeUtilEvent(publishSubject, event, new Action0() {
            @Override
            public void call() {
                manager.unregisterFragmentLifecycleCallbacks(callbacks);
            }
        });
    }

    private static <R, T> Observable.Transformer<R, R> subscribeUtilEvent(final Observable<T> source, final T event, final Action0 doOnComplete) {
        return new Observable.Transformer<R, R>() {
            @Override
            public Observable<R> call(Observable<R> rObservable) {
                return rObservable.takeUntil(takeUntilEvent(source, event)).doOnCompleted(doOnComplete);
            }
        };
    }

    private static <T> Observable<T> takeUntilEvent(final Observable<T> src, final T event) {
        return src.takeFirst(new Func1<T, Boolean>() {
            @Override
            public Boolean call(T lifecycleEvent) {
                return lifecycleEvent.equals(event);
            }
        });
    }
}

Lifecycle events:

public enum LifecycleEvent {
    ON_PAUSED,
    ON_STOPPED,
    ON_SAVE_INSTANCE_STATE,
    ON_DESTROYED,
    ON_VIEW_DESTORYED,
    ON_DETACHED,
}

Usage:

myObservable
   .compose(LifecycleBinder.subscribeUtilEvent(this, LifecycleEvent.ON_DESTROYED))
   .subscribe();

Answer 2

RecyclerView.Adapter has a method onViewRecycled(ViewHolder) where you can close the subscriptions of the ViewHolder argument.

Answer 3

I, hopefully, found the answer while writing the question.

Most Adapters receive a Context parameter in their constructor, among other things. For example:

public MyAdapter(Context context, ... ) { ... }

We can change it to receive a RxActivity, which in turns works as context also:

public MyAdapter(RxActivity parent, ... ) {
  this.parent = parent;
  ...
}

And now in the ViewHolder:

myObservable
   .compose(parent.bindToLifecycle())
   .subscribe();

This ensures that any subscription will end if the parent (activity or fragment) is destroyed.