AsyncSubject

AsyncSubject is a type of Subject in RxJS that behaves differently from other Subjects. Unlike standard Subjects that multicast all notifications to all observers, AsyncSubject only emits the last value it received to all its observers, and only after it has completed.

When an observer subscribes to an AsyncSubject, it doesn't receive any value immediately. The AsyncSubject waits for the source Observable to complete. Once the source completes, the AsyncSubject emits the very last value that the source produced, and then it completes itself. If the source never emits a value before completing, the AsyncSubject will not emit any value. Similarly, if the source never completes, the AsyncSubject will never emit its last value.

This makes AsyncSubject useful in scenarios where you only care about the final result of an asynchronous operation. For example, if you have a network request that returns a single value upon completion, an AsyncSubject can be used to ensure that all subscribers receive that final value only when the request is finished.

It's important to note that an AsyncSubject, once completed, will no longer emit any new values or accept new subscribers. Any new subscriptions after completion will also receive the last emitted value and the completion notification immediately.