| id | mainCompare | video | compare | lesson | title | layout | class | preview_image | preview_image_alt | ||
|---|---|---|---|---|---|---|---|---|---|---|---|
last |
take |
242933193 |
|
10 |
Last vs Take and the « ◉ complete » notification in Reactive Programming |
default |
post |
last/content_preview.jpg |
The "last" operator |
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!
In the animated comparison above, ❚ take returns a stream with 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). ❚ last and ❚ take work with an input stream that completes a few seconds after the last value emitted.
Notes
- What happens if the stream completes without emitting a value depends on the reactive library you use.
- In RxJS, last accepts an optional predicate. Learn more ➜
- A stream can complete emitting a notification (read Take 3 and complete)
- The
◉ completenotification 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
❚ lastis based on this notification
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