add Shape selection event

[OnionKiller]

🚀 Feature

selected event for shapes in Shape layer

Motivation

When selecting a shape (Poligon, Line, etc.) it is uesful if there is an event which can capture the fact this happend.

Pitch

I am trying to implement a segmentation visualization where the generated segments are displayed as Polygon shapes in a Layer.
I would like to create a visualization, which displays specific information of a single Polygon, selected by the user.

Alternatives

Mouse click event, especially the doubleclick event can be captured, and by finding if the ursor position is inside a Shape, te event can be possibly deduced, but it is not ideal.

Additional context

The request was mentioned here originally.
I'll try to implement this feature, @brisvag could you please help where to start?

[brisvag]

Hi @OnionKiller, thanks for giving it a go! There are a few ways to go about it.

The "best" way (=harder lift and requires more discussion) would be refactor the Shapes layer to use our Selection object just like Points already do (see

napari/napari/layers/points/points.py

Line 543 in 36bb0d3

self._selected_data: Selection[int] = Selection()

and all the lines there that refer to self._selected_data). This way you'd contribute to unifying our layers and get "for free" the Shapes.selection.events to connect to.

The quicker way is to instead add an event to here:

napari/napari/layers/shapes/shapes.py

Lines 489 to 500 in 36bb0d3

	self.events.add(
	edge_width=Event,
	edge_color=Event,
	face_color=Event,
	properties=Event,
	current_edge_color=Event,
	current_face_color=Event,
	current_properties=Event,
	highlight=Event,
	features=Event,
	feature_defaults=Event,
	)

and fire it whenever selected_data is changed. In fact, this makes me notice that you can technically use the highlight event as a stand-in for a selection event, now that I think about it... it won't work when the layer invisible maybe, but that probably won't matter.