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Reactive Extensions for Java
There is a newer version: 3.1.9
/*
 * 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 io.reactivex.rxjava3.internal.operators.observable;

import io.reactivex.rxjava3.core.*;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.exceptions.Exceptions;
import io.reactivex.rxjava3.functions.*;
import io.reactivex.rxjava3.internal.disposables.*;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;

import java.util.Objects;

public final class ObservableCollect extends AbstractObservableWithUpstream {
    final Supplier initialSupplier;
    final BiConsumer collector;

    public ObservableCollect(ObservableSource source,
            Supplier initialSupplier, BiConsumer collector) {
        super(source);
        this.initialSupplier = initialSupplier;
        this.collector = collector;
    }

    @Override
    protected void subscribeActual(Observer t) {
        U u;
        try {
            u = Objects.requireNonNull(initialSupplier.get(), "The initialSupplier returned a null value");
        } catch (Throwable e) {
            Exceptions.throwIfFatal(e);
            EmptyDisposable.error(e, t);
            return;
        }

        source.subscribe(new CollectObserver<>(t, u, collector));

    }

    static final class CollectObserver implements Observer, Disposable {
        final Observer downstream;
        final BiConsumer collector;
        final U u;

        Disposable upstream;

        boolean done;

        CollectObserver(Observer actual, U u, BiConsumer collector) {
            this.downstream = actual;
            this.collector = collector;
            this.u = u;
        }

        @Override
        public void onSubscribe(Disposable d) {
            if (DisposableHelper.validate(this.upstream, d)) {
                this.upstream = d;
                downstream.onSubscribe(this);
            }
        }

        @Override
        public void dispose() {
            upstream.dispose();
        }

        @Override
        public boolean isDisposed() {
            return upstream.isDisposed();
        }

        @Override
        public void onNext(T t) {
            if (done) {
                return;
            }
            try {
                collector.accept(u, t);
            } catch (Throwable e) {
                Exceptions.throwIfFatal(e);
                upstream.dispose();
                onError(e);
            }
        }

        @Override
        public void onError(Throwable t) {
            if (done) {
                RxJavaPlugins.onError(t);
                return;
            }
            done = true;
            downstream.onError(t);
        }

        @Override
        public void onComplete() {
            if (done) {
                return;
            }
            done = true;
            downstream.onNext(u);
            downstream.onComplete();
        }
    }
}