RxSwift RetryWhen导致可重入性异常

Question

我一直在尝试测试RxSwift上的retryWhen操作符，我遇到了Reentrancy Anomaly问题，代码如下：

Observable<Int>.create { observer in
    observer.onNext(1)
    observer.onNext(2)
    observer.onNext(3)
    observer.onNext(4)
    observer.onError(RequestError.dataError)
    return Disposables.create()
    }
    .retryWhen { error in
        return error.enumerated().flatMap { (index, error) -> Observable<Int> in
        let maxRetry = 1
        print("index: \(index)")
        return index < maxRetry ? Observable.timer(1, scheduler: MainScheduler.instance) : Observable.error(RequestError.tooMany)
        }
    }
    .subscribe(onNext: { value in
        print("This: \(value)")
    }, onError: { error in
        print("ERRRRRRR: \(error)")
    })
    .disposed(by: disposeBag)

使用上面的代码，它提供了：

This: 1
This: 2
This: 3
This: 4
index: 0
This: 1
This: 2
This: 3
This: 4
index: 1
⚠️ Reentrancy anomaly was detected.
  > Debugging: To debug this issue you can set a breakpoint in /Users/tony.lin/Documents/Snippet/MaterialiseTest/Pods/RxSwift/RxSwift/Rx.swift:97 and observe the call stack.
  > Problem: This behavior is breaking the observable sequence grammar. `next (error | completed)?`
    This behavior breaks the grammar because there is overlapping between sequence events.
    Observable sequence is trying to send an event before sending of previous event has finished.
  > Interpretation: This could mean that there is some kind of unexpected cyclic dependency in your code,
    or that the system is not behaving in the expected way.
  > Remedy: If this is the expected behavior this message can be suppressed by adding `.observeOn(MainScheduler.asyncInstance)`
    or by enqueing sequence events in some other way.

⚠️ Reentrancy anomaly was detected.
  > Debugging: To debug this issue you can set a breakpoint in /Users/tony.lin/Documents/Snippet/MaterialiseTest/Pods/RxSwift/RxSwift/Rx.swift:97 and observe the call stack.
  > Problem: This behavior is breaking the observable sequence grammar. `next (error | completed)?`
    This behavior breaks the grammar because there is overlapping between sequence events.
    Observable sequence is trying to send an event before sending of previous event has finished.
  > Interpretation: This could mean that there is some kind of unexpected cyclic dependency in your code,
    or that the system is not behaving in the expected way.
  > Remedy: If this is the expected behavior this message can be suppressed by adding `.observeOn(MainScheduler.asyncInstance)`
    or by enqueing sequence events in some other way.

ERRRRRRR: tooMany

只是想知道有没有人知道这个问题的原因？

Answer 1

正如控制台注释所解释的那样，可以使用.observeOn(MainScheduler.asyncInstance)取消此警告，如下所示：

Observable<Int>.from([1, 2, 3, 4]).concat(Observable.error(RequestError.dataError))
    .observeOn(MainScheduler.asyncInstance) // this is the magic that makes it work.
    .retryWhen { error in
        return error.enumerated().flatMap { (index, error) -> Observable<Int> in
            let maxRetry = 1
            print("Index:", index)
            guard index < maxRetry else { throw RequestError.tooMany }
            return Observable.timer(1, scheduler: MainScheduler.instance)
        }
    }
    .subscribe(onNext: { value in
        print("This: \(value)")
    }, onError: { error in
        print("ERRRRRRR: \(error)")
    })

我擅自对您的示例代码做了一些细微的调整，以展示编写代码的另一种方法。

更多信息

您要求解释(a)添加ObserveOn的原因和(b)为什么需要它。

.observeOn(MainScheduler.asyncInstance)所做的是将请求路由到另一个线程，在那里事件可以完成，然后在主线程上再次发出事件。换句话说，就像这样做：

.observeOn(backgroundScheduler).observeOn(MainScheduler.instance)

其中backgroundScheduler的定义如下：

let backgroundScheduler = SerialDispatchQueueScheduler(qos: .default)

至少这是我的理解。

至于为什么需要它，我不能说。您可能在库中发现了一个bug，因为使用1秒的延迟可以在没有observeOn的情况下工作得很好。