android/arch/lifecycle/LiveDataReactiveStreams.java

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.arch.lifecycle;

import android.arch.core.executor.ArchTaskExecutor;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;

import org.reactivestreams.Publisher;
import org.reactivestreams.Subscriber;
import org.reactivestreams.Subscription;

import java.util.concurrent.atomic.AtomicReference;

/**
 * Adapts {@link LiveData} input and output to the ReactiveStreams spec.
 */
@SuppressWarnings("WeakerAccess")
public final class LiveDataReactiveStreams {
    private LiveDataReactiveStreams() {
    }

    /**
     * Adapts the given {@link LiveData} stream to a ReactiveStreams {@link Publisher}.
     *
     * <p>
     * By using a good publisher implementation such as RxJava 2.x Flowables, most consumers will
     * be able to let the library deal with backpressure using operators and not need to worry about
     * ever manually calling {@link Subscription#request}.
     *
     * <p>
     * On subscription to the publisher, the observer will attach to the given {@link LiveData}.
     * Once {@link Subscription#request) is called on the subscription object, an observer will be
     * connected to the data stream. Calling request(Long.MAX_VALUE) is equivalent to creating an
     * unbounded stream with no backpressure. If request with a finite count reaches 0, the observer
     * will buffer the latest item and emit it to the subscriber when data is again requested. Any
     * other items emitted during the time there was no backpressure requested will be dropped.
     */
    public static <T> Publisher<T> toPublisher(
            final LifecycleOwner lifecycle, final LiveData<T> liveData) {

        return new LiveDataPublisher<>(lifecycle, liveData);
    }

    private static final class LiveDataPublisher<T> implements Publisher<T> {
        final LifecycleOwner mLifecycle;
        final LiveData<T> mLiveData;

        LiveDataPublisher(final LifecycleOwner lifecycle, final LiveData<T> liveData) {
            this.mLifecycle = lifecycle;
            this.mLiveData = liveData;
        }

        @Override
        public void subscribe(Subscriber<? super T> subscriber) {
            subscriber.onSubscribe(new LiveDataSubscription<T>(subscriber, mLifecycle, mLiveData));
        }

        static final class LiveDataSubscription<T> implements Subscription, Observer<T> {
            final Subscriber<? super T> mSubscriber;
            final LifecycleOwner mLifecycle;
            final LiveData<T> mLiveData;

            volatile boolean mCanceled;
            // used on main thread only
            boolean mObserving;
            long mRequested;
            // used on main thread only
            @Nullable
            T mLatest;

            LiveDataSubscription(final Subscriber<? super T> subscriber,
                    final LifecycleOwner lifecycle, final LiveData<T> liveData) {
                this.mSubscriber = subscriber;
                this.mLifecycle = lifecycle;
                this.mLiveData = liveData;
            }

            @Override
            public void onChanged(T t) {
                if (mCanceled) {
                    return;
                }
                if (mRequested > 0) {
                    mLatest = null;
                    mSubscriber.onNext(t);
                    if (mRequested != Long.MAX_VALUE) {
                        mRequested--;
                    }
                } else {
                    mLatest = t;
                }
            }

            @Override
            public void request(final long n) {
                if (mCanceled) {
                    return;
                }
                ArchTaskExecutor.getInstance().executeOnMainThread(new Runnable() {
                    @Override
                    public void run() {
                        if (mCanceled) {
                            return;
                        }
                        if (n <= 0L) {
                            mCanceled = true;
                            if (mObserving) {
                                mLiveData.removeObserver(LiveDataSubscription.this);
                                mObserving = false;
                            }
                            mLatest = null;
                            mSubscriber.onError(
                                    new IllegalArgumentException("Non-positive request"));
                            return;
                        }

                        // Prevent overflowage.
                        mRequested = mRequested + n >= mRequested
                                ? mRequested + n : Long.MAX_VALUE;
                        if (!mObserving) {
                            mObserving = true;
                            mLiveData.observe(mLifecycle, LiveDataSubscription.this);
                        } else if (mLatest != null) {
                            onChanged(mLatest);
                            mLatest = null;
                        }
                    }
                });
            }

            @Override
            public void cancel() {
                if (mCanceled) {
                    return;
                }
                mCanceled = true;
                ArchTaskExecutor.getInstance().executeOnMainThread(new Runnable() {
                    @Override
                    public void run() {
                        if (mObserving) {
                            mLiveData.removeObserver(LiveDataSubscription.this);
                            mObserving = false;
                        }
                        mLatest = null;
                    }
                });
            }
        }
    }

    /**
     * Creates an Observable {@link LiveData} stream from a ReactiveStreams publisher.
     *
     * <p>
     * When the LiveData becomes active, it subscribes to the emissions from the Publisher.
     *
     * <p>
     * When the LiveData becomes inactive, the subscription is cleared.
     * LiveData holds the last value emitted by the Publisher when the LiveData was active.
     * <p>
     * Therefore, in the case of a hot RxJava Observable, when a new LiveData {@link Observer} is
     * added, it will automatically notify with the last value held in LiveData,
     * which might not be the last value emitted by the Publisher.
     * <p>
     * Note that LiveData does NOT handle errors and it expects that errors are treated as states
     * in the data that's held. In case of an error being emitted by the publisher, an error will
     * be propagated to the main thread and the app will crash.
     *
     * @param <T> The type of data hold by this instance.
     */
    public static <T> LiveData<T> fromPublisher(final Publisher<T> publisher) {
        return new PublisherLiveData<>(publisher);
    }

    /**
     * Defines a {@link LiveData} object that wraps a {@link Publisher}.
     *
     * <p>
     * When the LiveData becomes active, it subscribes to the emissions from the Publisher.
     *
     * <p>
     * When the LiveData becomes inactive, the subscription is cleared.
     * LiveData holds the last value emitted by the Publisher when the LiveData was active.
     * <p>
     * Therefore, in the case of a hot RxJava Observable, when a new LiveData {@link Observer} is
     * added, it will automatically notify with the last value held in LiveData,
     * which might not be the last value emitted by the Publisher.
     *
     * <p>
     * Note that LiveData does NOT handle errors and it expects that errors are treated as states
     * in the data that's held. In case of an error being emitted by the publisher, an error will
     * be propagated to the main thread and the app will crash.
     *
     * @param <T> The type of data hold by this instance.
     */
    private static class PublisherLiveData<T> extends LiveData<T> {
        private final Publisher mPublisher;
        final AtomicReference<LiveDataSubscriber> mSubscriber;

        PublisherLiveData(@NonNull final Publisher publisher) {
            mPublisher = publisher;
            mSubscriber = new AtomicReference<>();
        }

        @Override
        protected void onActive() {
            super.onActive();
            LiveDataSubscriber s = new LiveDataSubscriber();
            mSubscriber.set(s);
            mPublisher.subscribe(s);
        }

        @Override
        protected void onInactive() {
            super.onInactive();
            LiveDataSubscriber s = mSubscriber.getAndSet(null);
            if (s != null) {
                s.cancelSubscription();
            }
        }

        final class LiveDataSubscriber extends AtomicReference<Subscription>
                implements Subscriber<T> {

            @Override
            public void onSubscribe(Subscription s) {
                if (compareAndSet(null, s)) {
                    s.request(Long.MAX_VALUE);
                } else {
                    s.cancel();
                }
            }

            @Override
            public void onNext(T item) {
                postValue(item);
            }

            @Override
            public void onError(final Throwable ex) {
                mSubscriber.compareAndSet(this, null);

                ArchTaskExecutor.getInstance().executeOnMainThread(new Runnable() {
                    @Override
                    public void run() {
                        // Errors should be handled upstream, so propagate as a crash.
                        throw new RuntimeException("LiveData does not handle errors. Errors from "
                                + "publishers should be handled upstream and propagated as "
                                + "state", ex);
                    }
                });
            }

            @Override
            public void onComplete() {
                mSubscriber.compareAndSet(this, null);
            }

            public void cancelSubscription() {
                Subscription s = get();
                if (s != null) {
                    s.cancel();
                }
            }
        }
    }
}