Avoid duplicate draws in FRBMark

CHANGES.rst

@@ -1,3 +1,8 @@
+0.8 (unreleased)
+================
+
+* Avoid updating FRBMark multiple times when adding a new layer. [#248]
+
 0.7 (2021-07-02)
 ================
 

glue_jupyter/bqplot/image/frb_mark.py

@@ -35,7 +35,10 @@ def __init__(self, viewer, array_maker):
         self.viewer.figure.axes[1].scale.observe(self.debounced_update, 'min')
         self.viewer.figure.axes[1].scale.observe(self.debounced_update, 'max')
 
-        self.update()
+        # NOTE: we deliberately don't call .update() here because when FRBImage
+        # is created for the main composite image layer the composite arrays
+        # haven't been set up yet, and for subset layers the layer gets force
+        # updated anyway when the layers are added to the viewer.
 
     @debounced(method=True)
     def debounced_update(self, *args, **kwargs):

glue_jupyter/bqplot/image/layer_artist.py

@@ -135,7 +135,7 @@ def _update_image(self, force=False, **kwargs):
             self._update_contour_lines()
 
     def _update_image_data(self, *args, **kwargs):
-        super()._update_image_data(*args, **kwargs)
+        self.composite_image.invalidate_cache()
         # if the image data change, the contour lines are invalid
         self._contour_line_cache.clear()
         self._update_contour_lines()