Reconnect the 'modified' signal after setting a new ListBox.body (#474)

Ensure that the cached canvas is marked invalidated when a newly set ListBox.body is modified at runtime.

Fixes #428

2 files changed, 24 insertions, 8 deletions

diff --git a/urwid/listbox.py b/urwid/listbox.py
index fd4b117..e49edaa 100644
--- a/urwid/listbox.py
+++ b/urwid/listbox.py
@@ -27,7 +27,7 @@ from urwid.canvas import SolidCanvas, CanvasCombine
 from urwid.widget import Widget, nocache_widget_render_instance, BOX, GIVEN
 from urwid.decoration import calculate_top_bottom_filler, normalize_valign
 from urwid import signals
-from urwid.signals import connect_signal
+from urwid.signals import connect_signal, disconnect_signal
 from urwid.monitored_list import MonitoredList, MonitoredFocusList
 from urwid.container import WidgetContainerMixin
 from urwid.command_map import (CURSOR_UP, CURSOR_DOWN,
@@ -250,13 +250,6 @@ class ListBox(Widget, WidgetContainerMixin):
         :type body: ListWalker
         """
         self._set_body(body)
-        try:
-            connect_signal(self._body, "modified", self._invalidate)
-        except NameError:
-            # our list walker has no modified signal so we must not
-            # cache our canvases because we don't know when our
-            # content has changed
-            self.render = nocache_widget_render_instance(self)
 
         # offset_rows is the number of rows between the top of the view
         # and the top of the focused item
@@ -281,10 +274,22 @@ class ListBox(Widget, WidgetContainerMixin):
         return self._body
 
     def _set_body(self, body):
+        try:
+            disconnect_signal(self._body, "modified", self._invalidate)
+        except AttributeError:
+            # _body is not yet assigned
+            pass
         if getattr(body, 'get_focus', None):
             self._body = body
         else:
             self._body = SimpleListWalker(body)
+        try:
+            connect_signal(self._body, "modified", self._invalidate)
+        except NameError:
+            # our list walker has no modified signal so we must not
+            # cache our canvases because we don't know when our
+            # content has changed
+            self.render = nocache_widget_render_instance(self)
         self._invalidate()
 
     body = property(_get_body, _set_body, doc="""
diff --git a/urwid/tests/test_listbox.py b/urwid/tests/test_listbox.py
index adeb8d5..9d2eddb 100644
--- a/urwid/tests/test_listbox.py
+++ b/urwid/tests/test_listbox.py
@@ -802,3 +802,14 @@ class ZeroHeightContentsTest(unittest.TestCase):
         lb.keypress((40,10), 'up')
         self.assertEqual(lb.get_focus()[1], 1)
 
+
+class ListBoxSetBodyTest(unittest.TestCase):
+    def test_signal_connected(self):
+        lb = urwid.ListBox([])
+        lb.body = urwid.SimpleListWalker([])
+        self.assertEqual(
+                lb.body._urwid_signals['modified'][0][1],
+                lb._invalidate,
+                "outdated canvas cache reuse "
+                "after ListWalker's contents modified"
+                )