Add `Observer::add_component`

[Shatur]

What problem does this solve or what need does it fill?

We have Observer::with_entities and Observer::with_entity, but for components it's only Observer::with_component. But for dynamic case I don't think with_ is convenient. In bevy_replicon I store ComponentId after user registrations and I need to create an observer for then during Plugin::finish.

What solution would you like?

Extend the API by adding Observer::add_component. It's not a builder and just pushes a component into the list of components.

What alternative(s) have you considered?

Add Observer::with_components that accepts an iterator, similar to Observer::with_entities. But for my use case add_component would be more convenient.

[SkiFire13]

Note that Observer::with_component already works by pushing into a list of components, it does not replace the existing ones.

[Shatur]

Yep, my main inconvenience is that it takes self. It's not a problem to write observer = observer.with_component(id) in a loop, but a bit awkward.

[Shatur]

Decided to add Observer::with_components. It's consistent with the API for entities.