Only update layers when selection is valid

Author: dstansby

docs/changelog.rst

@@ -13,6 +13,12 @@ napari-matplotlib now adheres to `SPEC 0 <https://scientific-python.org/specs/sp
 - Pinned the maximum napari version to ``< 0.5``.
   Version 3.0 of ``napari-matplotlib`` will introduce support for ``napari`` version 0.5.
 
+Bug fixes
+~~~~~~~~~
+- Only trigger layer update code paths if the layer selection is valid for the current
+  widget. This prevents errors when e.g., a labels layer is selected when a widget
+  that does not support a labels layer is open.
+
 2.0.1
 -----
 Bug fixes

src/napari_matplotlib/base.py

@@ -224,13 +224,23 @@ def _setup_callbacks(self) -> None:
             self._update_layers
         )
 
+    @property
+    def _valid_layer_selection(self) -> bool:
+        """
+        Return `True` if layer selection is valid.
+        """
+        return self.n_selected_layers in self.n_layers_input and all(
+            isinstance(layer, self.input_layer_types) for layer in self.layers
+        )
+
     def _update_layers(self, event: napari.utils.events.Event) -> None:
         """
         Update the ``layers`` attribute with currently selected layers and re-draw.
         """
         self.layers = list(self.viewer.layers.selection)
         self.layers = sorted(self.layers, key=lambda layer: layer.name)
-        self.on_update_layers()
+        if self._valid_layer_selection:
+            self.on_update_layers()
         self._draw()
 
     def _draw(self) -> None:
@@ -243,10 +253,7 @@ def _draw(self) -> None:
         with mplstyle.context(self.napari_theme_style_sheet):
             # everything should be done in the style context
             self.clear()
-            if self.n_selected_layers in self.n_layers_input and all(
-                isinstance(layer, self.input_layer_types)
-                for layer in self.layers
-            ):
+            if self._valid_layer_selection:
                 self.draw()
             self.canvas.draw()  # type: ignore[no-untyped-call]
 
@@ -269,6 +276,7 @@ def on_update_layers(self) -> None:
         Called when the selected layers are updated.
 
         This is a no-op, and is intended for derived classes to override.
+        It is only called if a selected layer is one of the input layer types.
         """