串联通量，其中第二个通量是与第一个通量的最后值即时创建的？

fluxC = fluxA.concatWith(lastA -> createFluxB(lastA))

import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Function;
import org.junit.jupiter.api.Test;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;

public class TestConcatLast {

    /**
     * Create a stream that emit all elements from input flux,
     * followed by all items from a flux created by provided function.
     * Input function is triggered using last element of source flux as input.
     *
     * @param source The flux providing elements of the first part of the concatenation.
     * @param createFromLastElement A function that provides the tail of the concatenation from a given element.
     *                              It will be triggered <em>at most once</em> using the last element of input flux.
     * @param errorIfSourceEmpty If true and input stream is empty, the returned flow will trigger an error.
     *                           If false, an empty flux is produced if input is empty.
     */
    public <T> Flux<T> concatLast(Flux<T> source, boolean errorIfSourceEmpty, Function<T, Flux<T>> createFromLastElement) {
        var sourceWithLatestCached = source.cache(1);
        final Mono<T> deferLast = Mono.defer(errorIfSourceEmpty ? sourceWithLatestCached::last : sourceWithLatestCached::next);
        return sourceWithLatestCached.concatWith(
                deferLast.flatMapMany(createFromLastElement)
        );
    }

    @Test
    public void testConcat() {
        var nextExpectedElement = new AtomicInteger(1);
        var elts = Flux.just(1, 2, 3, 4)
                // Check cache works and no element has been fetched back from source
                .doOnNext(i -> {
                    assert nextExpectedElement.compareAndSet(i, i+1);
                });

        var concatenated = concatLast(elts, true, i -> Flux.just(i + 1, i + 2, i + 3));
        StepVerifier.create(concatenated)
                .expectNext(1, 2, 3, 4, 5, 6, 7)
                .verifyComplete();
    }
}