Reactivity improvements

src/textual/app.py

@@ -414,6 +414,10 @@ class App(Generic[ReturnType], DOMNode):
         """ReturnType | None: The return type of the app."""
         return self._return_value
 
+    def _post_mount(self):
+        """Called after the object has been mounted."""
+        Reactive._initialize_object(self)
+
     def animate(
         self,
         attribute: str,

src/textual/cli/previews/easing.py

@@ -5,7 +5,7 @@ from textual._easing import EASING
 from textual.app import App, ComposeResult
 from textual.cli.previews.borders import TEXT
 from textual.containers import Container, Horizontal, Vertical
-from textual.reactive import Reactive
+from textual.reactive import reactive, var
 from textual.scrollbar import ScrollBarRender
 from textual.widget import Widget
 from textual.widgets import Button, Footer, Label, Input
@@ -23,8 +23,8 @@ class EasingButtons(Widget):
 
 
 class Bar(Widget):
-    position = Reactive.init(START_POSITION)
-    animation_running = Reactive(False)
+    position = reactive(START_POSITION)
+    animation_running = reactive(False)
 
     DEFAULT_CSS = """
 
@@ -53,8 +53,8 @@ class Bar(Widget):
 
 
 class EasingApp(App):
-    position = Reactive.init(START_POSITION)
-    duration = Reactive.var(1.0)
+    position = reactive(START_POSITION)
+    duration = var(1.0)
 
     def on_load(self):
         self.bind(

src/textual/message_pump.py

@@ -358,7 +358,10 @@ class MessagePump(metaclass=MessagePumpMeta):
         finally:
             # This is critical, mount may be waiting
             self._mounted_event.set()
-        Reactive._initialize_object(self)
+        self._post_mount()
+
+    def _post_mount(self):
+        """Called after the object has been mounted."""
 
     async def _process_messages_loop(self) -> None:
         """Process messages until the queue is closed."""

src/textual/reactive.py

@@ -7,6 +7,7 @@ from typing import (
     Any,
     Awaitable,
     Callable,
+    ClassVar,
     Generic,
     Type,
     TypeVar,
@@ -16,7 +17,7 @@ from typing import (
 import rich.repr
 
 from . import events
-from ._callback import count_parameters, invoke
+from ._callback import count_parameters
 from ._types import MessageTarget
 
 if TYPE_CHECKING:
@@ -28,15 +29,6 @@ if TYPE_CHECKING:
 ReactiveType = TypeVar("ReactiveType")
 
 
-class _NotSet:
-    pass
-
-
-_NOT_SET = _NotSet()
-
-T = TypeVar("T")
-
-
 @rich.repr.auto
 class Reactive(Generic[ReactiveType]):
     """Reactive descriptor.
@@ -50,7 +42,7 @@ class Reactive(Generic[ReactiveType]):
         compute: Run compute methods when attribute is changed. Defaults to True.
     """
 
-    _reactives: TypeVar[dict[str, object]] = {}
+    _reactives: ClassVar[dict[str, object]] = {}
 
     def __init__(
         self,
@@ -77,37 +69,6 @@ class Reactive(Generic[ReactiveType]):
         yield "always_update", self._always_update
         yield "compute", self._run_compute
 
-    @classmethod
-    def init(
-        cls,
-        default: ReactiveType | Callable[[], ReactiveType],
-        *,
-        layout: bool = False,
-        repaint: bool = True,
-        always_update: bool = False,
-        compute: bool = True,
-    ) -> Reactive:
-        """A reactive variable that calls watchers and compute on initialize (post mount).
-
-        Args:
-            default: A default value or callable that returns a default.
-            layout: Perform a layout on change. Defaults to False.
-            repaint: Perform a repaint on change. Defaults to True.
-            always_update: Call watchers even when the new value equals the old value. Defaults to False.
-            compute: Run compute methods when attribute is changed. Defaults to True.
-
-        Returns:
-            A Reactive instance which calls watchers or initialize.
-        """
-        return cls(
-            default,
-            layout=layout,
-            repaint=repaint,
-            init=True,
-            always_update=always_update,
-            compute=compute,
-        )
-
     @classmethod
     def var(
         cls,

src/textual/widget.py

@@ -360,6 +360,10 @@ class Widget(DOMNode):
     def offset(self, offset: Offset) -> None:
         self.styles.offset = ScalarOffset.from_offset(offset)
 
+    def _post_mount(self):
+        """Called after the object has been mounted."""
+        Reactive._initialize_object(self)
+
     ExpectType = TypeVar("ExpectType", bound="Widget")
 
     @overload

src/textual/widgets/_button.py

@@ -11,7 +11,7 @@ from rich.text import Text, TextType
 from .. import events
 from ..css._error_tools import friendly_list
 from ..message import Message
-from ..reactive import Reactive
+from ..reactive import reactive
 from ..widgets import Static
 
 
@@ -151,13 +151,13 @@ class Button(Static, can_focus=True):
     ACTIVE_EFFECT_DURATION = 0.3
     """When buttons are clicked they get the `-active` class for this duration (in seconds)"""
 
-    label: Reactive[RenderableType] = Reactive("")
+    label: reactive[RenderableType] = reactive("")
     """The text label that appears within the button."""
 
-    variant = Reactive.init("default")
+    variant = reactive("default")
     """The variant name for the button."""
 
-    disabled = Reactive(False)
+    disabled = reactive(False)
     """The disabled state of the button; `True` if disabled, `False` if not."""
 
     class Pressed(Message, bubble=True):