Always prefer 'Tango' icon theme (#250)

The legacy behavior was to attempt to detect if an existing theme was
already set which supplied some of the needed icons, and if not, use the
Tango theme specifically. If that didn't work either, revert the change
to the original theme (or lack thereof).

The current behavior changes the theme only if no theme is set, and only
if tango_icons_vendor is installed.

This change makes qt_gui always prefer the 'Tango' theme, and always
prefer the version supplied by tango_icons_vendor (if available). If
'Tango' is not available, fall back to the system theme.

Signed-off-by: Scott K Logan <logans@cottsay.net>
Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>
Co-authored-by: Chris Lalancette <clalancette@openrobotics.org>

qt_gui/src/qt_gui/main.py

@@ -34,7 +34,6 @@
 
 from argparse import ArgumentParser, SUPPRESS
 import os
-import platform
 import signal
 import sys
 
@@ -193,20 +192,19 @@ def _add_reload_paths(self, reload_importer):
 
     def _set_theme_if_necessary(self):
         from python_qt_binding.QtGui import QIcon
-        # if themeName is defined we are on Linux
-        # otherwise try to use the them provided by tango_icons_vendor
-        if not QIcon.themeName():
-            package_path = has_resource('packages', 'tango_icons_vendor')
-            if package_path:
-                icon_paths = QIcon.themeSearchPaths()
-                icon_paths.append(os.path.join(
-                    package_path, 'share', 'tango_icons_vendor',
-                    'resource', 'icons', 'Tango'))
-                QIcon.setThemeSearchPaths(icon_paths)
-                QIcon.setThemeName('scalable')
-            elif platform.system() != 'Linux':
-                print("The 'tango_icons_vendor' package was not found - icons "
-                      'will not work', file=sys.stderr)
+        # Always prefer tango_icons_vendor
+        package_path = has_resource('packages', 'tango_icons_vendor')
+        if package_path:
+            icon_paths = QIcon.themeSearchPaths()
+            icon_paths = [
+                os.path.join(package_path, 'share', 'tango_icons_vendor', 'resource', 'icons')
+            ] + icon_paths
+            QIcon.setThemeSearchPaths(icon_paths)
+        # Use Tango if possible, fall back to system default
+        original_theme = QIcon.themeName()
+        QIcon.setThemeName('Tango')
+        if QIcon.fromTheme('document-save').isNull():
+            QIcon.setThemeName(original_theme)
 
     def create_application(self, argv):
         from python_qt_binding.QtCore import Qt