add a Window.closed attribute

core/src/toga/window.py

@@ -115,6 +115,7 @@ def __init__(
         self._app = None
         self._content = None
         self._is_full_screen = False
+        self._closed = False
 
         self._resizable = resizable
         self._closable = closable
@@ -157,6 +158,15 @@ def app(self, app: App) -> None:
         if self.content:
             self.content.app = app
 
+    @property
+    def closed(self) -> bool:
+        """Whether the window was closed."""
+        return self._closed
+
+    def _check_closed(self) -> None:
+        if self.closed:
+            raise RuntimeError("Window was already closed")
+
     @property
     def _default_title(self) -> str:
         return "Toga"
@@ -316,10 +326,13 @@ def close(self) -> None:
         and unconditionally closed.
 
         Once a window has been closed, it *cannot* be reused. The behavior of any method
-        on a :class:`~toga.Window` instance after it has been closed is undefined.
+        or property on a :class:`~toga.Window` instance after it has been closed is
+        undefined, except for :attr:`closed` which can be used to check if the window
+        was closed.
         """
         self.app.windows -= self
         self._impl.close()
+        self._closed = True
 
     ############################################################
     # Dialogs

core/tests/test_window.py

@@ -260,6 +260,7 @@ def test_close_direct(window, app):
     window.close()
 
     # Window has been closed, but the close handler has *not* been invoked.
+    assert window.closed
     assert window.app == app
     assert window not in app.windows
     assert_action_performed(window, "close")
@@ -276,6 +277,7 @@ def test_close_no_handler(window, app):
     window._impl.simulate_close()
 
     # Window has been closed, and is no longer in the app's list of windows.
+    assert window.closed
     assert window.app == app
     assert window not in app.windows
     assert_action_performed(window, "close")
@@ -294,6 +296,7 @@ def test_close_sucessful_handler(window, app):
     window._impl.simulate_close()
 
     # Window has been closed, and is no longer in the app's list of windows.
+    assert window.closed
     assert window.app == app
     assert window not in app.windows
     assert_action_performed(window, "close")
@@ -313,6 +316,7 @@ def test_close_rejected_handler(window, app):
     window._impl.simulate_close()
 
     # Window has *not* been closed
+    assert not window.closed
     assert window.app == app
     assert window in app.windows
     assert_action_not_performed(window, "close")