Consider ability to 'upgrade' ViewerModel to full Viewer

[DragaDoncila]

🧰 Task

Currently, we have a single provider for the napari Viewer. This provider is used when plugin widgets request a Viewer, but also internally.

Under the hood, the provider just calls and returns the current_viewer:

def current_viewer() -> Optional[Viewer]:
    """Return the currently active napari viewer."""
    try:
        from napari._qt.qt_main_window import _QtMainWindow
    except ImportError:
        return None
    else:
        return _QtMainWindow.current_viewer()

This means even though the Viewer can exist in headless mode, the Viewer provider only works when qt and the window are present.

Do we envisage a world where a plugin (or other API user) might want the headless viewer injected if the qt one is unavailable e.g. chaining commands, computing stuff on layers before showing the viewer, idk. If we think that's a valid use case, we should consider adding a provider for the headless viewer, with a lower weighting than the one we already have.

[Czaki]

I think that we should promote using ViewerModel in headless mode.

[lucyleeow]

But we never instantiate a ViewerModel in napari? What do you mean @Czaki ?

[lucyleeow]

Or do you mean headless users should always instantiate ViewerModel and we thus should never inject viewer in headless mode?

[Czaki]

For current usage we provide active viewer (last focused). How will you determine which viewer use?

[lucyleeow]

True. So no non qt viewer injection then?

[Czaki]

I think than in headless mode, user should provide own injection.

[DragaDoncila]

I'm on board with disambiguating the use of napari.Viewer as both headless and GUI modes in favour of using ViewerModel in headless mode and napari.Viewer in GUI mode. In that case there should never be any need for plugins/users to need napari to perform injection for them, I think? However, I do think we need a way for a ViewerModel to be "upgraded" to a full Viewer somehow - maybe via an overloaded constructor so like Viewer(my_model) or maybe like viewer = napari.Viewer.from_model(my_model)?