@@ -225,13 +225,23 @@ def _setup_callbacks(self) -> None:
225225self ._update_layers
226226 )
227227
228+ @property
229+ def _valid_layer_selection (self )-> bool :
230+ """
231+ Return `True` if layer selection is valid.
232+ """
233+ return self .n_selected_layers in self .n_layers_input and all (
234+ isinstance (layer ,self .input_layer_types )for layer in self .layers
235+ )
236+
228237def _update_layers (self ,event :napari .utils .events .Event )-> None :
229238"""
230239 Update the ``layers`` attribute with currently selected layers and re-draw.
231240 """
232241self .layers = list (self .viewer .layers .selection )
233242self .layers = sorted (self .layers ,key = lambda layer :layer .name )
234- self .on_update_layers ()
243+ if self ._valid_layer_selection :
244+ self .on_update_layers ()
235245self ._draw ()
236246
237247def _draw (self )-> None :
@@ -244,10 +254,7 @@ def _draw(self) -> None:
244254with mplstyle .context (self .napari_theme_style_sheet ):
245255# everything should be done in the style context
246256self .clear ()
247- if self .n_selected_layers in self .n_layers_input and all (
248- isinstance (layer ,self .input_layer_types )
249- for layer in self .layers
250- ):
257+ if self ._valid_layer_selection :
251258self .draw ()
252259self .canvas .draw ()# type: ignore[no-untyped-call]
253260
@@ -270,6 +277,7 @@ def on_update_layers(self) -> None:
270277 Called when the selected layers are updated.
271278
272279 This is a no-op, and is intended for derived classes to override.
280+ It is only called if a selected layer is one of the input layer types.
273281 """
274282
275283