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我正在尝试使用 rx Java 中的背压在我的 Android 应用程序中创建无限滚动。我希望它只调用外部服务请求的次数（调用后request(1)）。但是在使用 flatmap 之后，每次subscribe加载 16 页。

在我的代码下面有预期的结果。几乎每个测试都因为第一次请求而失败（n=16）

import org.junit.Before;
import org.junit.Test;
import org.mockito.Mockito;
import rx.Observable;
import rx.observers.TestSubscriber;

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

import static java.util.Collections.emptyList;
import static org.mockito.Mockito.*;
import static rx.internal.util.UtilityFunctions.identity;

public class ServiceObservablesTest {


    public static <T> Observable<List<T>> create(DataProvider<T> dataProvider) {
        Observable<Observable<List<T>>> metaObservalble = Observable.create(subscriber -> {
            AtomicInteger pageNumber = new AtomicInteger();
            subscriber.setProducer(n -> {
                // at subscribe rxJava makes request for 16 elements - probably because of flatMap
                // after first request with 16 elements everything seems to work fine even if i ignore the 'n' param

                Observable<List<T>> page = dataProvider.requestPage(pageNumber.getAndIncrement());
                subscriber.onNext(page);

            });
        });
        return metaObservalble.flatMap(identity()).takeWhile(page -> !page.isEmpty());
    }

    public interface DataProvider<T> {
        Observable<List<T>> requestPage(int page);
    }


    private DataProvider provider;

    @Before
    public void setUp() throws Exception {
        provider = Mockito.mock(DataProvider.class);
        List<Object> list = Arrays.asList(new Object());
        when(provider.requestPage(anyInt())).thenReturn(Observable.just(list));
    }

    @Test
    public void shouldRequestOnlyFirstPageOnSubscribe() {
        //given

        TestSubscriber<List<Object>> subscriber = new TestSubscriber<>(1);
        Observable<List<Object>> flightsObservable = create(provider);

        //when
        flightsObservable.subscribe(subscriber);

        //then
        subscriber.assertValueCount(1);
        subscriber.assertNotCompleted();

        verify(provider, times(1)).requestPage(0);
        verify(provider, never()).requestPage(1);
    }


    @Test
    public void shouldRequestNumberOfPagesSpecified() {
        //given

        int requested_pages = 5;
        TestSubscriber<List<Object>> subscriber = new TestSubscriber<>(0);
        Observable<List<Object>> flightsObservable = create(provider);

        //when
        flightsObservable.subscribe(subscriber);
        subscriber.requestMore(requested_pages);

        //then
        subscriber.assertValueCount(requested_pages);
        subscriber.assertNotCompleted();


        for (int i = 0; i < requested_pages; i++) {
            verify(provider, times(1)).requestPage(i);
        }
        verify(provider, never()).requestPage(requested_pages);

    }


    @Test
    public void shouldCompleteAfterRetrievingEmptyResult() {
        //given

        int emptyPage = 2;
        when(provider.requestPage(emptyPage)).thenReturn(Observable.just(emptyList()));

        TestSubscriber<List<Object>> subscriber = new TestSubscriber<>(100);
        Observable<List<Object>> flightsObservable = create(provider);


        //when
        flightsObservable.subscribe(subscriber);

        //then
        subscriber.assertValueCount(emptyPage);
        subscriber.assertCompleted();


        verify(provider, times(1)).requestPage(0); //requested at subscribe
        for (int i = 1; i <= emptyPage; i++) {
            verify(provider, times(1)).requestPage(i);
        }
        verify(provider, never()).requestPage(emptyPage + 1);

    }

    @Test
    public void shouldRequestNextPageWhenRequestedMore() {
        //given

        TestSubscriber<List<Object>> subscriber = new TestSubscriber<>(1);
        Observable<List<Object>> flightsObservable = create(provider);

        //when
        flightsObservable.subscribe(subscriber);
        subscriber.requestMore(1);

        //then
        subscriber.assertValueCount(2);
        verify(provider, times(1)).requestPage(0);
        verify(provider, times(1)).requestPage(1);
        verify(provider, never()).requestPage(2);

        //when
        subscriber.requestMore(1);

        //then
        subscriber.assertValueCount(3);
        subscriber.assertNotCompleted();

        verify(provider, times(1)).requestPage(0);
        verify(provider, times(1)).requestPage(1);
        verify(provider, times(1)).requestPage(2);
        verify(provider, never()).requestPage(3);

    }

    @Test
    public void shouldWorkWithMultipleSubscribers() {

        //given

        TestSubscriber<List<Object>> subscriber1 = new TestSubscriber<>(1);
        TestSubscriber<List<Object>> subscriber2 = new TestSubscriber<>(1);
        Observable<List<Object>> flightsObservable = create(provider);

        //when
        flightsObservable.subscribe(subscriber1);
        flightsObservable.subscribe(subscriber2);

        //then
        subscriber1.assertValueCount(1);
        subscriber2.assertValueCount(1);

        verify(provider, times(2)).requestPage(0);
        verify(provider, never()).requestPage(1);

        //when
        subscriber1.requestMore(1);
        //then
        subscriber1.assertValueCount(2);
        subscriber2.assertValueCount(1);

        verify(provider, times(2)).requestPage(0);
        verify(provider, times(1)).requestPage(1);
        verify(provider, never()).requestPage(2);

        //when
        subscriber2.requestMore(1);
        //then
        subscriber1.assertValueCount(2);
        subscriber2.assertValueCount(2);

        verify(provider, times(2)).requestPage(0);
        verify(provider, times(2)).requestPage(1);
        verify(provider, never()).requestPage(2);
    }

}
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1 回答 1

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背压旨在协商并发消费者生产者的行为，并允许程序作者设置策略以解决当产生的数据速率超过数据消耗的速率时要做什么。

也就是说，您会看到组合 observables 的运算符merge将为您提供与您需要的数据量不对应的请求量。外部的 observable（Observable 的 Observable）在合并时总是会在 RxAndroid 上收到 16（RxJava 中为 128）的请求。然后，当它接收到 List 的内部 Observable 时，每个内部 Observable 都会收到一个请求，该请求基于来自下游订阅者的请求数量。如果您尝试编写 an ，Observable<Observable<T>>您将被迫编写一个OnSubscribe<Observable<List<T>>>内部管理合并行为的函数，以便它是 anObservable<List<T>>而不是Observable<Observable<List<T>>. 写这个会强制你订阅你的数据提供者返回的 observable 来 unwrap 和 onNext List<T>。

我建议您改为将屏幕 y 位置映射到 End-Of-Page 事件，然后使用扫描将其转换为单调递增的数字，然后将该数字 concatMap 转换为对DataProvider.requestPage().

screenYPositions
    .map(this::isUninitializedOrNearEndOfPage)
    .scan(1, (event, pageNumber) -> pageNumber + 1 )
    .concatMap(dataProvider::requestPage)
    .subscribe(testSubscriber);
于 2015-08-12T18:54:38.033 回答