Added SwitchTransform (#435)

Author: Ryland Degnan

rsocket-core/src/main/java/io/rsocket/internal/SwitchTransform.java

@@ -0,0 +1,81 @@
+package io.rsocket.internal;
+
+import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscription;
+import reactor.core.CoreSubscriber;
+import reactor.core.publisher.DirectProcessor;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Operators;
+
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
+import java.util.function.BiFunction;
+
+public final class SwitchTransform<T, R> extends Flux<R> {
+
+	final Publisher<? extends T> source;
+	final BiFunction<T, Flux<? extends T>, Publisher<? extends R>> transformer;
+
+	public SwitchTransform(Publisher<? extends T> source, BiFunction<T, Flux<? extends T>, Publisher<? extends R>> transformer) {
+		this.source = Objects.requireNonNull(source, "source");
+		this.transformer = Objects.requireNonNull(transformer, "transformer");
+	}
+
+	@Override
+	public void subscribe(CoreSubscriber<? super R> actual) {
+		source.subscribe(new SwitchTransformSubscriber<>(actual, transformer));
+	}
+
+	static final class SwitchTransformSubscriber<T, R> implements CoreSubscriber<T> {
+		final CoreSubscriber<? super R> actual;
+		final BiFunction<T, Flux<? extends T>, Publisher<? extends R>> transformer;
+		final DirectProcessor<T> processor = DirectProcessor.create();
+
+		Subscription s;
+
+		volatile int once;
+		@SuppressWarnings("rawtypes")
+		static final AtomicIntegerFieldUpdater<SwitchTransformSubscriber> ONCE =
+				AtomicIntegerFieldUpdater.newUpdater(SwitchTransformSubscriber.class, "once");
+
+		SwitchTransformSubscriber(CoreSubscriber<? super R> actual, BiFunction<T, Flux<? extends T>, Publisher<? extends R>> transformer) {
+			this.actual = actual;
+			this.transformer = transformer;
+		}
+
+		@Override
+		public void onSubscribe(Subscription s) {
+			if (Operators.validate(this.s, s)) {
+				this.s = s;
+
+				processor.onSubscribe(s);
+			}
+		}
+
+		@Override
+		public void onNext(T t) {
+			if (once == 0 && ONCE.compareAndSet(this, 0, 1)) {
+				try {
+					Publisher<? extends R> result = Objects.requireNonNull(transformer.apply(t, processor),
+							"The transformer returned a null value");
+					result.subscribe(actual);
+				}
+				catch (Throwable e) {
+					onError(Operators.onOperatorError(s, e, t, actual.currentContext()));
+					return;
+				}
+			}
+			processor.onNext(t);
+		}
+
+		@Override
+		public void onError(Throwable t) {
+			processor.onError(t);
+		}
+
+		@Override
+		public void onComplete() {
+			processor.onComplete();
+		}
+	}
+}