Make EventLoopHelper expect an IGUI instance (#953)

* add a failing test

* make gui trait an Instance(IGUI)

* Update pyface/ui/qt4/util/tests/test_event_loop_helper.py

Co-authored-by: Poruri Sai Rahul <rporuri@enthought.com>

Co-authored-by: Poruri Sai Rahul <rporuri@enthought.com>

pyface/ui/qt4/util/event_loop_helper.py

@@ -12,7 +12,7 @@
 import contextlib
 import threading
 
-from pyface.gui import GUI
+from pyface.i_gui import IGUI
 from pyface.qt import QtCore, QtGui
 from traits.api import HasStrictTraits, Instance
 
@@ -39,7 +39,7 @@ class EventLoopHelper(HasStrictTraits):
 
     qt_app = Instance(QtGui.QApplication)
 
-    gui = Instance(GUI)
+    gui = Instance(IGUI)
 
     def event_loop_with_timeout(self, repeat=2, timeout=10.0):
         """Helper function to send all posted events to the event queue and

pyface/ui/qt4/util/tests/test_event_loop_helper.py

@@ -0,0 +1,28 @@
+# Copyright (c) 2005-2021, Enthought Inc.
+# All rights reserved.
+#
+# This software is provided without warranty under the terms of the BSD
+# license included in LICENSE.txt and may be redistributed only
+# under the conditions described in the aforementioned license.  The license
+# is also available online at http://www.enthought.com/licenses/BSD.txt
+# Thanks for using Enthought open source!
+import unittest
+
+from traits.api import HasTraits, provides
+
+from pyface.i_gui import IGUI
+from pyface.ui.qt4.util.event_loop_helper import EventLoopHelper
+
+
+@provides(IGUI)
+class DummyGUI(HasTraits):
+    pass
+
+
+class TestEventLoopHelper(unittest.TestCase):
+
+    def test_gui_trait_expects_IGUI_interface(self):
+        # Trivial test where we simply set the trait
+        # and the test passes because no errors are raised.
+        event_loop_helper = EventLoopHelper()
+        event_loop_helper.gui = DummyGUI()