Added watch method

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added Shift+scroll wheel and ctrl+scroll wheel to scroll horizontally
 - Added `Tree.action_toggle_node` to toggle a node without selecting, and bound it to <kbd>Space</kbd> https://github.com/Textualize/textual/issues/1433
 - Added `Tree.reset` to fully reset a `Tree` https://github.com/Textualize/textual/issues/1437
+- Added DOMNode.watch method https://github.com/Textualize/textual/pull/1750
 
 ### Changed
 

src/textual/demo.py

@@ -15,7 +15,7 @@ from rich.text import Text
 from textual.app import App, ComposeResult
 from textual.binding import Binding
 from textual.containers import Container, Horizontal
-from textual.reactive import reactive, watch
+from textual.reactive import reactive
 from textual.widgets import (
     Button,
     Checkbox,
@@ -203,7 +203,7 @@ class DarkSwitch(Horizontal):
         yield Static("Dark mode toggle", classes="label")
 
     def on_mount(self) -> None:
-        watch(self.app, "dark", self.on_dark_change, init=False)
+        self.watch(self.app, "dark", self.on_dark_change, init=False)
 
     def on_dark_change(self, dark: bool) -> None:
         self.query_one(Checkbox).value = self.app.dark

src/textual/dom.py

@@ -4,6 +4,7 @@ import re
 from inspect import getfile
 from typing import (
     TYPE_CHECKING,
+    Callable,
     ClassVar,
     Iterable,
     Iterator,
@@ -31,7 +32,7 @@ from .css.parse import parse_declarations
 from .css.styles import RenderStyles, Styles
 from .css.tokenize import IDENTIFIER
 from .message_pump import MessagePump
-from .reactive import Reactive
+from .reactive import Reactive, _watch
 from .timer import Timer
 from .walk import walk_breadth_first, walk_depth_first
 
@@ -647,6 +648,19 @@ class DOMNode(MessagePump):
         """
         return [child for child in self.children if child.display]
 
+    def watch(
+        self, obj: DOMNode, attribute_name: str, callback: Callable, init: bool = True
+    ) -> None:
+        """Watches for modifications to reactive attributes on another object.
+
+        Args:
+            obj: Object containing attribute to watch.
+            attribute_name: Attribute to watch.
+            callback: A callback to run when attribute changes.
+            init: Check watchers on first call.
+        """
+        _watch(self, obj, attribute_name, callback, init=init)
+
     def get_pseudo_classes(self) -> Iterable[str]:
         """Get any pseudo classes applicable to this Node, e.g. hover, focus.
 

src/textual/message_pump.py

@@ -126,7 +126,6 @@ class MessagePump(metaclass=MessagePumpMeta):
     @property
     def is_attached(self) -> bool:
         """Check the node is attached to the app via the DOM."""
-
         from .app import App
 
         node = self

src/textual/reactive.py

@@ -325,10 +325,12 @@ class var(Reactive[ReactiveType]):
         )
 
 
-def watch(
+def _watch(
+    node: DOMNode,
     obj: Reactable,
     attribute_name: str,
     callback: Callable[[Any], object],
+    *,
     init: bool = True,
 ) -> None:
     """Watch a reactive variable on an object.
@@ -346,7 +348,7 @@ def watch(
     watcher_list = watchers.setdefault(attribute_name, [])
     if callback in watcher_list:
         return
-    watcher_list.append((obj, callback))
+    watcher_list.append((node, callback))
     if init:
         current_value = getattr(obj, attribute_name, None)
         Reactive._check_watchers(obj, attribute_name, current_value)

src/textual/widgets/_footer.py

@@ -8,7 +8,7 @@ from rich.console import RenderableType
 from rich.text import Text
 
 from .. import events
-from ..reactive import Reactive, watch
+from ..reactive import Reactive
 from ..widget import Widget
 
 
@@ -66,7 +66,7 @@ class Footer(Widget):
         self.refresh()
 
     def on_mount(self) -> None:
-        watch(self.screen, "focused", self._focus_changed)
+        self.watch(self.screen, "focused", self._focus_changed)
 
     def _focus_changed(self, focused: Widget | None) -> None:
         self._key_text = None

src/textual/widgets/_header.py

@@ -5,7 +5,7 @@ from datetime import datetime
 from rich.text import Text
 
 from ..widget import Widget
-from ..reactive import Reactive, watch
+from ..reactive import Reactive
 
 
 class HeaderIcon(Widget):
@@ -133,5 +133,5 @@ class Header(Widget):
         def set_sub_title(sub_title: str) -> None:
             self.query_one(HeaderTitle).sub_text = sub_title
 
-        watch(self.app, "title", set_title)
-        watch(self.app, "sub_title", set_sub_title)
+        self.watch(self.app, "title", set_title)
+        self.watch(self.app, "sub_title", set_sub_title)