reactive.how 2.0-alpha Menu

Last value?

As soon as the input stream emits its last value, the output stream emits it? Naaah! How could one be certain that no value comes after? Reactive programming can’t see the future.

We have to wait for a ◉ complete notification (read Take 3 and complete to learn more about it). Only when the stream has completed, we know this value was the last one – and this completion may never happen!

Last Monday, we used the card ❚ take (watch it again below). The stream it outputs has a ◉ complete notification overlapped by the 3 value, as it completes immediately. I would like to emphasize that this notification is not intended to deliver the last value of the stream. It is a separate event, without a value (though it can happen immediately after an event value).

To illustrate all those principles, let’s see how ❚ last and ❚ take work with an input stream that completes a few seconds after the last value emitted:

This is how ❚ last operates with one input stream:

  • When the input stream completes, the output stream:
    • emits the last value emitted by the input stream
    • and immediately completes

So, this card returns a new stream of at most one value.

Note: what happens if the stream completes without emitting a value depends on the reactive library you use.

Summary

  • A stream can complete emitting a notification (read Take 3 and complete)
  • The ◉ complete notification is a special event with no value
  • A listener can subscribe to a stream and act upon its completion
  • Also, the behavior of some cards like ❚ last is based on this notification

See also


takeLast vs take

💌 I create something new each week!
Learn Reactive Programming and stay tuned.

Occasional updates, plus:

Cédric Soulas - Freelance Developer Advocate. Learn more.

Follow 👨‍💻 Hire me

News

@CedricSoulas