Added on_resize handler on toga.Window

android/src/toga_android/window.py

@@ -26,6 +26,7 @@ def onGlobalLayout(self):
         """
         native_parent = self.window.native_content.getParent()
         self.window.resize_content(native_parent.getWidth(), native_parent.getHeight())
+        self.window.interface.on_resize()
 
 
 class Window(Container):

changes/2304.feature.rst

@@ -0,0 +1 @@
+Toga Windows now supports calling user functions on resize events.

cocoa/src/toga_cocoa/window.py

@@ -40,6 +40,7 @@ def windowDidResize_(self, notification) -> None:
         if self.interface.content:
             # Set the window to the new size
             self.interface.content.refresh()
+            self.impl.interface.on_resize()
 
     ######################################################################
     # Toolbar delegate methods

core/src/toga/window.py

@@ -90,6 +90,17 @@ def __call__(self, window: Window, **kwargs: Any) -> bool:
         ...
 
 
+class OnResizeHandler(Protocol):
+    def __call__(self, window: Window, **kwargs: Any) -> None:
+        """A handler to invoke when a window resizes.
+
+        :param window: The window instance that resizes.
+        :param kwargs: Ensures compatibility with additional arguments introduced in
+            future ver
+        """
+        ...
+
+
 T = TypeVar("T")
 
 
@@ -126,6 +137,7 @@ def __init__(
         resizable: bool = True,
         closable: bool = True,
         minimizable: bool = True,
+        on_resize: OnResizeHandler | None = None,
         on_close: OnCloseHandler | None = None,
         resizeable=None,  # DEPRECATED
         closeable=None,  # DEPRECATED
@@ -197,6 +209,7 @@ def __init__(
         self._toolbar = CommandSet(on_change=self._impl.create_toolbar, app=self._app)
 
         self.on_close = on_close
+        self.on_resize = on_resize
 
     def __lt__(self, other) -> bool:
         return self.id < other.id
@@ -468,6 +481,15 @@ def as_image(self, format: type[ImageT] = Image) -> ImageT:
     # Window events
     ######################################################################
 
+    @property
+    def on_resize(self) -> OnResizeHandler:
+        """The handler to invoke when the window resizes."""
+        return self._on_resize
+
+    @on_resize.setter
+    def on_resize(self, handler):
+        self._on_resize = wrapped_handler(self, handler)
+
     @property
     def on_close(self) -> OnCloseHandler:
         """The handler to invoke if the user attempts to close the window."""

core/tests/test_window.py

@@ -354,6 +354,19 @@ def test_as_image(window):
     assert image.size == (318, 346)
 
 
+def test_on_resize(window):
+    assert window.on_resize._raw is None
+
+    on_resize_handler = Mock()
+    window.on_resize = on_resize_handler
+
+    assert window.on_resize._raw == on_resize_handler
+
+    window._impl.simulate_on_resize()
+
+    on_resize_handler.assert_called_once_with(window)
+
+
 def test_screen(window, app):
     """A window can be moved to a different screen."""
     # Cannot actually change window.screen, so just check

dummy/src/toga_dummy/window.py

@@ -110,3 +110,6 @@ def simulate_close(self):
     def get_current_screen(self):
         # `window.screen` will return `Secondary Screen`
         return ScreenImpl(native=("Secondary Screen", (-1366, -768), (1366, 768)))
+
+    def simulate_on_resize(self):
+        self.interface.on_resize()

gtk/src/toga_gtk/window.py

@@ -17,6 +17,7 @@ def __init__(self, interface, title, position, size):
         self.create()
         self.native._impl = self
 
+        self.native.connect("configure-event", self.gtk_configure_event)
         self.native.connect("delete-event", self.gtk_delete_event)
 
         self.native.set_default_size(size[0], size[1])
@@ -57,6 +58,9 @@ def create(self):
     # Native event handlers
     ######################################################################
 
+    def gtk_configure_event(self, widget, data):
+        self.interface.on_resize()
+
     def gtk_delete_event(self, widget, data):
         if self._is_closing:
             should_close = True

iOS/src/toga_iOS/app.py

@@ -50,6 +50,8 @@ def application_didChangeStatusBarOrientation_(
         """This callback is invoked when rotating the device from landscape to portrait
         and vice versa."""
         App.app.interface.main_window.content.refresh()
+        for window in App.app.interface.windows:
+            window.on_resize()
 
 
 class App:

testbed/tests/test_window.py

@@ -520,6 +520,28 @@ async def test_screen(second_window, second_window_probe):
             )
 
 
+async def test_on_resize(main_window, main_window_probe):
+    if toga.platform.current_platform in {"android", "iOS", "web"}:
+        pytest.xfail("Window.on_resize is non functional on current platform.")
+    main_window_on_resize_handler = Mock()
+    main_window.on_resize = main_window_on_resize_handler
+
+    main_window.show()
+    await main_window_probe.wait_for_window("Main window has been shown")
+
+    initial_size = main_window.size
+
+    main_window.size = (200, 150)
+    await main_window_probe.wait_for_window("Main window has been resized")
+    assert main_window.size == (200, 150)
+
+    main_window_on_resize_handler.assert_called_with(main_window)
+
+    main_window.size = initial_size
+    await main_window_probe.wait_for_window("Main window has been resized")
+    assert main_window.size == initial_size
+
+
 async def test_as_image(main_window, main_window_probe):
     """The window can be captured as a screenshot"""
 

textual/src/toga_textual/window.py

@@ -112,6 +112,7 @@ def on_mount(self) -> None:
 
     def on_resize(self, event) -> None:
         self.interface.content.refresh()
+        self.interface.on_resize()
 
 
 class Window:

winforms/src/toga_winforms/window.py

@@ -47,6 +47,7 @@ def __init__(self, interface, title, position, size):
 
     def winforms_Resize(self, sender, event):
         self.resize_content()
+        self.interface.on_resize()
 
     def winforms_FormClosing(self, sender, event):
         # If the app is exiting, or a manual close has been requested, don't get