Restructured widget initialization order

changes/2942.misc.rst

@@ -0,0 +1 @@
+The initialization process for widgets has been internally restructured to avoid unnecessary style reapplications.

changes/2942.removal.rst

@@ -0,0 +1 @@
+Widgets now create and return their implementations via a ``_create()`` method. A user-created custom widget that inherits from an existing Toga widget and uses its same implementation will require no changes; any user-created widgets that need to specify their own implementation should do so in ``_create()`` and return it. Existing user code inheriting from Widget that assigns its implementation before calling ``super().__init__()`` will continue to function, but give a RuntimeWarning; unfortunately, this change breaks any existing code that doesn't create its implementation until afterward. Such usage will now raise an exception.

cocoa/src/toga_cocoa/widgets/base.py

@@ -9,12 +9,10 @@ class Widget:
     def __init__(self, interface):
         super().__init__()
         self.interface = interface
-        self.interface._impl = self
         self._container = None
         self.constraints = None
         self.native = None
         self.create()
-        self.interface.style.reapply()
 
     @abstractmethod
     def create(self): ...

cocoa/src/toga_cocoa/widgets/button.py

@@ -32,7 +32,6 @@ def create(self):
         self._icon = None
 
         self.native.buttonType = NSMomentaryPushInButton
-        self._set_button_style()
 
         self.native.target = self.native
         self.native.action = SEL("onPress:")

core/src/toga/style/applicator.py

@@ -1,16 +1,34 @@
 from __future__ import annotations
 
+import warnings
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
     from toga.widgets.base import Widget
 
+# Make sure deprecation warnings are shown by default
+warnings.filterwarnings("default", category=DeprecationWarning)
+
 
 class TogaApplicator:
     """Apply styles to a Toga widget."""
 
-    def __init__(self, widget: Widget):
-        self.widget = widget
+    def __init__(self, widget: None = None):
+        if widget is not None:
+            warnings.warn(
+                "Widget parameter is deprecated. Applicator will be given a reference "
+                "to its widget when it is assigned as that widget's applicator.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
+
+    @property
+    def widget(self) -> Widget:
+        """The widget to which this applicator is assigned.
+
+        Syntactic sugar over the node attribute set by Travertino.
+        """
+        return self.node
 
     def refresh(self) -> None:
         # print("RE-EVALUATE LAYOUT", self.widget)

core/src/toga/widgets/activityindicator.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import Literal
+from typing import Any, Literal
 
 from .base import StyleT, Widget
 
@@ -22,11 +22,12 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        self._impl = self.factory.ActivityIndicator(interface=self)
-
         if running:
             self.start()
 
+    def _create(self) -> Any:
+        return self.factory.ActivityIndicator(interface=self)
+
     @property
     def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?

core/src/toga/widgets/base.py

@@ -2,6 +2,7 @@
 
 from builtins import id as identifier
 from typing import TYPE_CHECKING, Any, TypeVar
+from warnings import warn
 
 from travertino.declaration import BaseStyle
 from travertino.node import Node
@@ -33,18 +34,70 @@ def __init__(
         :param style: A style object. If no style is provided, a default style
             will be applied to the widget.
         """
-        super().__init__(
-            style=style if style else Pack(),
-            applicator=TogaApplicator(self),
-        )
+        super().__init__(style=style if style is not None else Pack())
 
         self._id = str(id if id else identifier(self))
         self._window: Window | None = None
         self._app: App | None = None
-        self._impl: Any = None
 
+        # Get factory and assign implementation
         self.factory = get_platform_factory()
 
+        ###########################################
+        # Backwards compatibility for Toga <= 0.4.8
+        ###########################################
+
+        # Just in case we're working with a third-party widget created before
+        # the _create() mechanism was added, which has already defined its
+        # implementation. We still want to call _create(), to issue the warning and
+        # inform users about where they should be creating the implementation, but if
+        # there already is one, we don't want to do the assignment and thus replace it
+        # with None.
+
+        impl = self._create()
+
+        if not hasattr(self, "_impl"):
+            self._impl = impl
+
+        #############################
+        # End backwards compatibility
+        #############################
+
+        self.applicator = TogaApplicator()
+
+        ##############################################
+        # Backwards compatibility for Travertino 0.3.0
+        ##############################################
+
+        # The below if block will execute when using Travertino 0.3.0. For future
+        # versions of Travertino, these assignments (and the reapply) will already have
+        # been handled "automatically" by assigning the applicator above; in that case,
+        # we want to avoid doing a second, redundant style reapplication.
+
+        # This whole section can be removed as soon as there's a newer version of
+        # Travertino to set as Toga's minimum requirement.
+
+        if not hasattr(self.applicator, "node"):  # pragma: no cover
+            self.applicator.node = self
+            self.style._applicator = self.applicator
+            self.style.reapply()
+
+        #############################
+        # End backwards compatibility
+        #############################
+
+    def _create(self) -> Any:
+        """Create a platform-specific implementation of this widget.
+
+        A subclass of Widget should redefine this method to return its implementation.
+        """
+        warn(
+            "Widgets should create and return their implementation in ._create(). This "
+            "will be an exception in a future version.",
+            RuntimeWarning,
+            stacklevel=2,
+        )
+
     def __repr__(self) -> str:
         return f"<{self.__class__.__name__}:0x{identifier(self):x}>"
 

core/src/toga/widgets/box.py

@@ -24,14 +24,14 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        # Create a platform specific implementation of a Box
-        self._impl = self.factory.Box(interface=self)
-
         # Children need to be added *after* the impl has been created.
         self._children: list[Widget] = []
         if children is not None:
             self.add(*children)
 
+    def _create(self):
+        return self.factory.Box(interface=self)
+
     @property
     def enabled(self) -> bool:
         """Is the widget currently enabled? i.e., can the user interact with the widget?

core/src/toga/widgets/button.py

@@ -44,9 +44,6 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        # Create a platform specific implementation of a Button
-        self._impl = self.factory.Button(interface=self)
-
         # Set a dummy handler before installing the actual on_press, because we do not
         # want on_press triggered by the initial value being set
         self.on_press = None
@@ -63,6 +60,9 @@ def __init__(
         self.on_press = on_press
         self.enabled = enabled
 
+    def _create(self) -> Any:
+        return self.factory.Button(interface=self)
+
     @property
     def text(self) -> str:
         """The text displayed on the button.

core/src/toga/widgets/canvas.py

@@ -1239,14 +1239,10 @@ def __init__(
         :param on_alt_release: Initial :any:`on_alt_release` handler.
         :param on_alt_drag: Initial :any:`on_alt_drag` handler.
         """
-
         super().__init__(id=id, style=style)
 
         self._context = Context(canvas=self)
 
-        # Create a platform specific implementation of Canvas
-        self._impl = self.factory.Canvas(interface=self)
-
         # Set all the properties
         self.on_resize = on_resize
         self.on_press = on_press
@@ -1257,6 +1253,9 @@ def __init__(
         self.on_alt_release = on_alt_release
         self.on_alt_drag = on_alt_drag
 
+    def _create(self) -> Any:
+        return self.factory.Canvas(interface=self)
+
     @property
     def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?

core/src/toga/widgets/dateinput.py

@@ -51,16 +51,16 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        # Create a platform specific implementation of a DateInput
-        self._impl = self.factory.DateInput(interface=self)
-
         self.on_change = None
         self.min = min
         self.max = max
 
         self.value = value
         self.on_change = on_change
 
+    def _create(self) -> Any:
+        return self.factory.DateInput(interface=self)
+
     @property
     def value(self) -> datetime.date:
         """The currently selected date. A value of ``None`` will be converted into

core/src/toga/widgets/detailedlist.py

@@ -85,6 +85,16 @@ def __init__(
         :param on_refresh: Initial :any:`on_refresh` handler.
         :param on_delete: **DEPRECATED**; use ``on_primary_action``.
         """
+        # Prime the attributes and handlers that need to exist when the widget is
+        # created.
+        self._accessors = accessors
+        self._missing_value = missing_value
+        self._primary_action = primary_action
+        self._secondary_action = secondary_action
+        self.on_select = None
+
+        self._data: SourceT | ListSource = None
+
         super().__init__(id=id, style=style)
 
         ######################################################################
@@ -104,24 +114,15 @@ def __init__(
         # End backwards compatibility.
         ######################################################################
 
-        # Prime the attributes and handlers that need to exist when the
-        # widget is created.
-        self._accessors = accessors
-        self._missing_value = missing_value
-        self._primary_action = primary_action
-        self._secondary_action = secondary_action
-        self.on_select = None
-
-        self._data: SourceT | ListSource = None
-
-        self._impl = self.factory.DetailedList(interface=self)
-
         self.data = data
         self.on_primary_action = on_primary_action
         self.on_secondary_action = on_secondary_action
         self.on_refresh = on_refresh
         self.on_select = on_select
 
+    def _create(self) -> Any:
+        return self.factory.DetailedList(interface=self)
+
     @property
     def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?

core/src/toga/widgets/divider.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import Literal
+from typing import Any, Literal
 
 from toga.constants import Direction
 
@@ -29,10 +29,11 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        # Create a platform specific implementation of a Divider
-        self._impl = self.factory.Divider(interface=self)
         self.direction = direction
 
+    def _create(self) -> Any:
+        return self.factory.Divider(interface=self)
+
     @property
     def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?

core/src/toga/widgets/imageview.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Literal
+from typing import TYPE_CHECKING, Any, Literal
 
 from travertino.size import at_least
 
@@ -83,12 +83,16 @@ def __init__(
         :param style: A style object. If no style is provided, a default style will be
             applied to the widget.
         """
-        super().__init__(id=id, style=style)
         # Prime the image attribute
         self._image = None
-        self._impl = self.factory.ImageView(interface=self)
+
+        super().__init__(id=id, style=style)
+
         self.image = image
 
+    def _create(self) -> Any:
+        return self.factory.ImageView(interface=self)
+
     @property
     def enabled(self) -> Literal[True]:
         """Is the widget currently enabled? i.e., can the user interact with the widget?

core/src/toga/widgets/label.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from typing import Any
+
 from .base import StyleT, Widget
 
 
@@ -19,11 +21,11 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        # Create a platform specific implementation of a Label
-        self._impl = self.factory.Label(interface=self)
-
         self.text = text
 
+    def _create(self) -> Any:
+        return self.factory.Label(interface=self)
+
     def focus(self) -> None:
         """No-op; Label cannot accept input focus."""
         pass

core/src/toga/widgets/mapview.py

@@ -155,8 +155,6 @@ def __init__(
         """
         super().__init__(id=id, style=style)
 
-        self._impl: Any = self.factory.MapView(interface=self)
-
         self._pins = MapPinSet(self, pins)
 
         if location:
@@ -169,6 +167,9 @@ def __init__(
 
         self.on_select = on_select
 
+    def _create(self) -> Any:
+        return self.factory.MapView(interface=self)
+
     @property
     def location(self) -> toga.LatLng:
         """The latitude/longitude where the map is centered.