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For NLSR’s use case, I’d suggest an additional signal:

Signal<SegmentFetcher, Data> afterSegmentValidated;

This is emitted only if the Data passes validation.

I'm not against using signals in general, but the current SegmentFetcher API is based on callbacks. This proposal mixes callbacks and signals, and makes the API ugly and inconsistent. Any reasons we're not augmenting the API by adding more callbacks? Or alternatively, why not make the whole API signal-based? (in the latter case, existing callbacks can be deprecated and eventually removed).

Any reasons we're not augmenting the API by adding more callbacks?

There are already too many of them.

why not make the whole API signal-based?

I agree with this. In this case, fetch function shall be renamed as start and the function does not take any callbacks.
The fetch function becomes deprecated.

Junxiao Shi wrote:

Any reasons we're not augmenting the API by adding more callbacks?

There are already too many of them.

why not make the whole API signal-based?

I agree with this. In this case, fetch function shall be renamed as start and the function does not take any callbacks.
The fetch function becomes deprecated.

I think it should be in a different commit. What do you think? I can crate an issue for this.

Guys, I don't disagree with moving to signal based, but I would consider it is "beautification" that we can do later in due time. We have kind of an emergency to make this specific signal work, so let's not stall.

Created an issue #4464 to make SegmentFetcher Signal based.