RxJava return 错误为 onNext 并继续流

Question

所以我尝试使用 onErrorReturn 来 return 我想要的结果，但它会在之后完成流，我如何捕获错误 return as Next 并继续流？

使用下面的代码，出现错误时它不会到达 retryWhen，如果我翻转它，如果出现错误

，它不会重新订阅 retryWhen

fun process(): Observable<State> {
    return publishSubject
        .flatMap { intent ->
            actAsRepo(intent) // Might return error
                .map { State(data = it, error = null) }
        }
        .onErrorReturn { State(data = "", error = it) } // catch the error
        .retryWhen { errorObs -> 
            errorObs.flatMap {
                Observable.just(State.defaultState()) // continue subscribing
            }
        }
}

private fun actAsRepo(string: String): Observable<String> {
    if (string.contains('A')) {
        throw IllegalArgumentException("Contains A")
    } else {
        return Observable.just("Wrapped from repo: $string")
    }
}

订阅者将是

viewModel.process().subscribe(this::render)

Answer 1

onError 是终端操作员。如果发生 onError，它将在操作员之间传递。您可以使用 onError 运算符来捕获 onError 并提供回退。

在您的示例中，onError 发生在 flatMap 的内部流中。 onError 将向下游传播到 onErrorReturn 运算符。如果您查看实现，您会看到将调用 onErrorReturn lambda，结果将在 onComplete

之后通过 onNext 推送到下游

    @Override
    public void onError(Throwable t) {
        T v;
        try {
            v = valueSupplier.apply(t);
        } catch (Throwable e) {
            Exceptions.throwIfFatal(e);
            downstream.onError(new CompositeException(t, e));
            return;
        }

        if (v == null) {
            NullPointerException e = new NullPointerException("The supplied value is null");
            e.initCause(t);
            downstream.onError(e);
            return;
        }

        downstream.onNext(v); // <--------
        downstream.onComplete(); // <--------
    }

你的解决方案的结果是什么？

您的流完成是因为：#retryWhen JavaDoc

If the upstream to the operator is asynchronous, signalling onNext followed by onComplete immediately may result in the sequence to be completed immediately. Similarly, if this inner {@code ObservableSource} signals {@code onError} or {@code onComplete} while the upstream is active, the sequence is terminated with the same signal immediately.

你应该做什么：

将onErrorReturn 放在flatMap 的地图操作符后面。使用此顺序，当 inner-flatMap 流 onErrors.

时，您的流将无法完成

这是为什么？

当外部（来源：publishSubject）和内部流（订阅）都完成时，flatMap 运算符完成。在这种情况下，外部流 (publishSubject) 发出 onNext，内部流将在通过 onNext 发送 {State(data = "", error = it) } 后完成。因此，流将保持打开状态。

interface ApiCall {
    fun call(s: String): Observable<String>
}

class ApiCallImpl : ApiCall {
    override fun call(s: String): Observable<String> {
        // important: warp call into observable, that the exception is caught and emitted as onError downstream
        return Observable.fromCallable {
            if (s.contains('A')) {
                throw IllegalArgumentException("Contains A")
            } else {
                s
            }
        }
    }
}

data class State(val data: String, val err: Throwable? = null)

apiCallImpl.call 将 return 一个懒惰的可观察对象，它会在订阅时抛出错误，而不是在可观察的组装时间。

// no need for retryWhen here, except you want to catch onComplete from the publishSubject, but once the publishSubject completes no re-subscription will help you, because the publish-subject is terminated and onNext invocations will not be accepted anymore (see implementation). 
fun process(): Observable<State> {
    return publishSubject
        .flatMap { intent ->
            apiCallImpl.call(intent) // Might return error
                .map { State(data = it, err = null) }
                .onErrorReturn { State("", err = it) }
        }
}

测试

lateinit var publishSubject: PublishSubject<String>
lateinit var apiCallImpl: ApiCallImpl

@Before
fun init() {
    publishSubject = PublishSubject.create()
    apiCallImpl = ApiCallImpl()
}

@Test
fun myTest() {
    val test = process().test()

    publishSubject.onNext("test")
    publishSubject.onNext("A")
    publishSubject.onNext("test2")

    test.assertNotComplete()
        .assertNoErrors()
        .assertValueCount(3)
        .assertValueAt(0) {
            assertThat(it).isEqualTo(State("test", null))
            true
        }
        .assertValueAt(1) {
            assertThat(it.data).isEmpty()
            assertThat(it.err).isExactlyInstanceOf(IllegalArgumentException::class.java)
            true
        }
        .assertValueAt(2) {
            assertThat(it).isEqualTo(State("test2", null))
            true
        }
}

备选

此替代方案的行为与第一个解决方案略有不同。 flatMap-Operator 采用布尔值 (delayError)，这将导致吞下 onError 消息，直到源完成。当源完成时，将发出错误。

您可以使用 delayError true，当异常没有用并且不能在出现时记录下来

进程

fun process(): Observable<State> {
    return publishSubject
        .flatMap({ intent ->
            apiCallImpl.call(intent)
                .map { State(data = it, err = null) }
        }, true)
}

测试

只发出了两个值。错误不会转换为回退值。

@Test
fun myTest() {
    val test = process().test()

    publishSubject.onNext("test")
    publishSubject.onNext("A")
    publishSubject.onNext("test2")

    test.assertNotComplete()
        .assertNoErrors()
        .assertValueAt(0) {
            assertThat(it).isEqualTo(State("test", null))
            true
        }
        .assertValueAt(1) {
            assertThat(it).isEqualTo(State("test2", null))
            true
        }
        .assertValueCount(2)
}

注意：我想你想在这种情况下使用 switchMap，而不是 flatMap。