Monix Observer调用疑问：同调用链为何一示例合法一示例非法？

Great question! This difference boils down to two key things: the purpose of Observer.dump and the Reactive Streams contract that Monix strictly follows. Let's break it down step by step:

1. The Reactive Streams Core Rule #

In Reactive Streams (which Monix implements), an Observer's methods (onNext, onComplete, onError) are meant to be called exclusively by the corresponding Publisher (or Observable in Monix terms).

When you subscribe an Observer to an Observable, the Observable takes responsibility for emitting events in a valid sequence:

• Zero or more onNext calls, followed by exactly one onComplete or onError call.
• Once onComplete/onError is called, no further events can be sent to the Observer.

Manually calling these methods on a regular Observer (the kind you pass to Observable.subscribe) violates this contract—because you're bypassing the Observable's control over event emission, which can lead to race conditions, resource leaks, or undefined behavior.

2. Observer.dump Is a Special Debug Utility #

Observer.dump is built specifically for manual testing and debugging. Monix designed it to:

• Let developers manually send events to see how an observer would react (perfect for validating stream logic without hooking up a full observable).
• Internally enforce the Reactive Streams contract automatically:
  • It tracks its own state (whether it's been terminated by onComplete/onError).
  • It ignores any invalid events (like onNext after onComplete) without throwing errors.

So when you call onNext(1) -> onNext(2) -> onComplete() on an Observer.dump instance, you're using it exactly as intended—manual event emission is a valid use case here.

3. Why the "Direct Observer" Call Is Illegal #

The second example refers to a regular Observer (e.g., one created with Observer.create, or passed to Observable.subscribe). For these observers:

• Manual calls to onNext/onComplete break the Reactive Streams contract, since the Observable is supposed to be the sole driver of event emission.
• Even if your call sequence looks valid, bypassing the Observable's control can cause issues:
  • The observer might not have initialized resources properly (if it depends on the subscription process).
  • It might not handle backpressure correctly (a core requirement of Reactive Streams).
  • It could leave the stream in an inconsistent state.

In short: Observer.dump is a special debug tool that allows manual event emission, while regular Observers are meant to be driven by Observables—manual calls on them violate the Reactive Streams contract.

内容的提问来源于stack exchange，提问作者St.Antario