A single cache that allows evicting the value or error. Subscriptions after the value is evicted will re-trigger the upstream observer.

EvictableSingleCache.java

/**
 * Copyright (c) 2016-present, RxJava Contributors.
 *
 * 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 me.tatarka.rxcache2;

import io.reactivex.*;
import io.reactivex.disposables.Disposable;
import io.reactivex.exceptions.Exceptions;
import io.reactivex.functions.BiFunction;
import io.reactivex.functions.Function;

import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;

/**
 * A single cache that allows evicting the value or error. Subscriptions after the value is evicted will re-trigger the
 * upstream observer.
 */
public class EvictableSingleCache<T> extends Single<T> implements SingleObserver<T>, CompletableObserver {
    @SuppressWarnings("rawtypes")
    static final CacheDisposable[] EMPTY = new CacheDisposable[0];
    @SuppressWarnings("rawtypes")
    static final CacheDisposable[] TERMINATED = new CacheDisposable[0];

    final SingleSource<? extends T> source;
    final BiFunction<? super T, Throwable, Completable> evictionFunction;
    final AtomicInteger wip;
    final AtomicReference<CacheDisposable<T>[]> observers;
    T value;
    Throwable error;

    /**
     * Constructs a new cached observable that is never evicted. This is equivalent to {@link Single#cache()}
     */
    public EvictableSingleCache(Single<T> source) {
        this(source, Completable.never());
    }

    /**
     * Constructs a new cached observable that's evicted when the given completable completes. The eviction completable
     * will be subscribed to when the source emits a value or error.
     */
    public EvictableSingleCache(Single<T> source, final Completable eviction) {
        this(source, new BiFunction<T, Throwable, Completable>() {
            @Override
            public Completable apply(T t, Throwable throwable) throws Exception {
                return eviction;
            }
        });
    }

    /**
     * Constructs a new cached observable that's evicted when the given completable completes. The success completable
     * will be subscribed to if the source emits a value, and the error completable will be subscribed to if the source
     * emits an error.
     */
    public EvictableSingleCache(Single<T> source, final Completable successEviction, final Completable errorEviction) {
        this(source, new BiFunction<T, Throwable, Completable>() {
            @Override
            public Completable apply(T value, Throwable error) throws Exception {
                return value != null ? successEviction : errorEviction;
            }
        });
    }

    /**
     * Constructs a new cached observable that's evicted when the given completable completes. The success function
     * will be invoked if the source emits a value, and the error function will be invoked if the source  emits an
     * error.
     */
    public EvictableSingleCache(Single<T> source, final Function<? super T, Completable> successEvictionFunction, final Function<Throwable, Completable> errorEvictionFunction) {
        this(source, new BiFunction<T, Throwable, Completable>() {
            @Override
            public Completable apply(T value, Throwable error) throws Exception {
                return (value != null ? successEvictionFunction.apply(value) : errorEvictionFunction.apply(error));
            }
        });
    }

    /**
     * Constructs a new cached observable that's evicted when the given completable completes. The function
     * will be invoked when the source emits a value or error.
     */
    public EvictableSingleCache(Single<T> source, BiFunction<? super T, Throwable, Completable> evictionFunction) {
        this.source = source;
        this.wip = new AtomicInteger();
        this.evictionFunction = evictionFunction;
        this.observers = new AtomicReference<CacheDisposable<T>[]>(EMPTY);
    }

    @Override
    protected void subscribeActual(SingleObserver<? super T> s) {
        CacheDisposable<T> d = new CacheDisposable<>(s, this);
        s.onSubscribe(d);

        if (add(d)) {
            if (d.isDisposed()) {
                remove(d);
            }
        } else {
            Throwable ex = error;
            if (ex != null) {
                s.onError(ex);
            } else {
                s.onSuccess(value);
            }
            return;
        }

        if (wip.getAndIncrement() == 0) {
            source.subscribe(this);
        }
    }

    boolean add(CacheDisposable<T> observer) {
        for (; ; ) {
            CacheDisposable<T>[] a = observers.get();
            if (a == TERMINATED) {
                return false;
            }
            int n = a.length;
            @SuppressWarnings("unchecked")
            CacheDisposable<T>[] b = new CacheDisposable[n + 1];
            System.arraycopy(a, 0, b, 0, n);
            b[n] = observer;
            if (observers.compareAndSet(a, b)) {
                return true;
            }
        }
    }

    @SuppressWarnings("unchecked")
    void remove(CacheDisposable<T> observer) {
        for (; ; ) {
            CacheDisposable<T>[] a = observers.get();
            int n = a.length;
            if (n == 0) {
                return;
            }

            int j = -1;
            for (int i = 0; i < n; i++) {
                if (a[i] == observer) {
                    j = i;
                    break;
                }
            }

            if (j < 0) {
                return;
            }

            CacheDisposable<T>[] b;

            if (n == 1) {
                b = EMPTY;
            } else {
                b = new CacheDisposable[n - 1];
                System.arraycopy(a, 0, b, 0, j);
                System.arraycopy(a, j + 1, b, j, n - j - 1);
            }
            if (observers.compareAndSet(a, b)) {
                return;
            }
        }
    }

    @Override
    public void onSubscribe(Disposable d) {
        // not supported by this operator
    }

    @Override
    public void onSuccess(T value) {
        this.value = value;

        for (CacheDisposable<T> d : observers.getAndSet(TERMINATED)) {
            if (!d.isDisposed()) {
                d.actual.onSuccess(value);
            }
        }

        try {
            Completable eviction = evictionFunction.apply(value, null);
            if (eviction != Completable.never()) {
                eviction.subscribe(this);
            }
        } catch (Exception ex) {
            throw Exceptions.propagate(ex);
        }

    }

    @Override
    public void onError(Throwable e) {
        this.error = e;

        for (CacheDisposable<T> d : observers.getAndSet(TERMINATED)) {
            if (!d.isDisposed()) {
                d.actual.onError(e);
            }
        }

        try {
            Completable eviction = evictionFunction.apply(null, e);
            if (eviction != Completable.never()) {
                eviction.subscribe(this);
            }
        } catch (Exception ex) {
            throw Exceptions.propagate(ex);
        }
    }

    @Override
    public void onComplete() {
        observers.set(EMPTY);
        wip.set(0);
    }

    static final class CacheDisposable<T>
            extends AtomicBoolean
            implements Disposable {

        private static final long serialVersionUID = 7514387411091976596L;

        final SingleObserver<? super T> actual;
        final EvictableSingleCache<T> parent;

        CacheDisposable(SingleObserver<? super T> actual, EvictableSingleCache<T> parent) {
            this.actual = actual;
            this.parent = parent;
        }

        @Override
        public boolean isDisposed() {
            return get();
        }

        @Override
        public void dispose() {
            if (compareAndSet(false, true)) {
                parent.remove(this);
            }
        }
    }
}

Examples:

Cache value for 10 minutes.

new EvictableSingle(source, Completable.timer(10, TimeUnit.MINUTES))

Don't cache errors.

new EvictableSingle(source, Completable.never(), Completable.complete())

Clear cache on button click.

new EvictableSingle(source, RxView.clicks(button).firstOrError().toCompletable())