OnNext

OnNext is a term used in reactive programming to deliver the next element in a data stream to an observer. In the .NET Reactive Extensions, OnNext is a method of the IObserver<T> interface.

Role in the contract: An IObservable<T> sequence pushes data to observers by calling OnNext for each item. The sequence is terminated by either OnCompleted or OnError. OnNext may be called multiple times, and is the main channel for data items.

Usage constraints: The producer should not call OnNext after OnCompleted or OnError. In many implementations, OnNext should not throw exceptions; if it does, the sequence is typically terminated with an error.

Signature and variants: In .NET, OnNext has the signature void OnNext(T value). In other ecosystems there is a similar method with different casing, such as onNext in RxJava.

Examples and semantics: An observer implementing OnNext might update a UI, accumulate values, or forward them to downstream operators. Operators in Rx compose around OnNext to transform, filter, or combine streams.

Related concepts: OnError and OnCompleted; backpressure considerations differ between Rx implementations (for example, RxJava provides Flowable with backpressure support). OnNext is central to the push-based model of reactive streams.

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