diff --git a/skimage/viewer/widgets/core.py b/skimage/viewer/widgets/core.py
index 02d29545..e2086b4f 100644
--- a/skimage/viewer/widgets/core.py
+++ b/skimage/viewer/widgets/core.py
@@ -16,6 +16,8 @@ from PyQt4 import QtCore
 from skimage.io._plugins.q_color_mixer import IntelligentSlider
 
 
+#TODO: Add WidgetBase class (requires reimplementation of IntelligentSlider).
+
 class Slider(IntelligentSlider):
     """Slider widget.