Merge pull request #1014 from Textualize/reactive-always-update

Reactive `always_update`

Author: willmcgugan

.gitignore

@@ -116,3 +116,6 @@ venv.bak/
 
 # Snapshot testing report output directory
 tests/snapshot_tests/output
+
+# Sandbox folder - convenient place for us to develop small test apps without leaving the repo
+sandbox/

docs/guide/reactivity.md

@@ -165,7 +165,11 @@ If you click the buttons in the above example it will show the current count. Wh
 
 ## Watch methods
 
-Watch methods are another superpower. Textual will call watch methods when reactive attributes are modified. Watch methods begin with `watch_` followed by the name of the attribute. If the watch method accepts a positional argument, it will be called with the new assigned value. If the watch method accepts *two* positional arguments, it will be called with both the *old* value and the *new* value.
+Watch methods are another superpower.
+Textual will call watch methods when reactive attributes are modified.
+Watch methods begin with `watch_` followed by the name of the attribute.
+If the watch method accepts a positional argument, it will be called with the new assigned value.
+If the watch method accepts *two* positional arguments, it will be called with both the *old* value and the *new* value.
 
 The following app will display any color you type in to the input. Try it with a valid color in Textual CSS. For example `"darkorchid"` or `"#52de44"`.
 
@@ -192,6 +196,12 @@ The following app will display any color you type in to the input. Try it with a
 
 The color is parsed in `on_input_submitted` and assigned to `self.color`. Because `color` is reactive, Textual also calls `watch_color` with the old and new values.
 
+### When are watch methods called?
+
+Textual only calls watch methods if the value of a reactive attribute _changes_.
+If the newly assigned value is the same as the previous value, the watch method is not called.
+You can override this behaviour by passing `always_update=True` to `reactive`.
+
 ## Compute methods
 
 Compute methods are the final superpower offered by the `reactive` descriptor. Textual runs compute methods to calculate the value of a reactive attribute. Compute methods begin with `compute_` followed by the name of the reactive value.

src/textual/reactive.py

@@ -5,7 +5,6 @@
 from typing import TYPE_CHECKING, Any, Callable, Generic, Type, TypeVar, Union
 from weakref import WeakSet
 
-
 from . import events
 from ._callback import count_parameters, invoke
 from ._types import MessageTarget
@@ -16,7 +15,6 @@
 
     Reactable = Union[Widget, App]
 
-
 ReactiveType = TypeVar("ReactiveType")
 
 
@@ -37,7 +35,7 @@ class Reactive(Generic[ReactiveType]):
         layout (bool, optional): Perform a layout on change. Defaults to False.
         repaint (bool, optional): Perform a repaint on change. Defaults to True.
         init (bool, optional): Call watchers on initialize (post mount). Defaults to False.
-
+        always_update(bool, optional): Call watchers even when the new value equals the old value. Defaults to False.
     """
 
     def __init__(
@@ -47,11 +45,13 @@ def __init__(
         layout: bool = False,
         repaint: bool = True,
         init: bool = False,
+        always_update: bool = False,
     ) -> None:
         self._default = default
         self._layout = layout
         self._repaint = repaint
         self._init = init
+        self._always_update = always_update
 
     @classmethod
     def init(
@@ -60,18 +60,25 @@ def init(
         *,
         layout: bool = False,
         repaint: bool = True,
+        always_update: bool = False,
     ) -> Reactive:
         """A reactive variable that calls watchers and compute on initialize (post mount).
 
         Args:
             default (ReactiveType | Callable[[], ReactiveType]): A default value or callable that returns a default.
             layout (bool, optional): Perform a layout on change. Defaults to False.
             repaint (bool, optional): Perform a repaint on change. Defaults to True.
-
+            always_update(bool, optional): Call watchers even when the new value equals the old value. Defaults to False.
         Returns:
             Reactive: A Reactive instance which calls watchers or initialize.
         """
-        return cls(default, layout=layout, repaint=repaint, init=True)
+        return cls(
+            default,
+            layout=layout,
+            repaint=repaint,
+            init=True,
+            always_update=always_update,
+        )
 
     @classmethod
     def var(
@@ -153,7 +160,7 @@ def __set__(self, obj: Reactable, value: ReactiveType) -> None:
         if callable(validate_function) and not first_set:
             value = validate_function(value)
         # If the value has changed, or this is the first time setting the value
-        if current_value != value or first_set:
+        if current_value != value or first_set or self._always_update:
             # Set the first set flag to False
             setattr(obj, f"__first_set_{self.internal_name}", False)
             # Store the internal value
@@ -259,7 +266,7 @@ class reactive(Reactive[ReactiveType]):
         layout (bool, optional): Perform a layout on change. Defaults to False.
         repaint (bool, optional): Perform a repaint on change. Defaults to True.
         init (bool, optional): Call watchers on initialize (post mount). Defaults to True.
-
+        always_update(bool, optional): Call watchers even when the new value equals the old value. Defaults to False.
     """
 
     def __init__(
@@ -269,8 +276,15 @@ def __init__(
         layout: bool = False,
         repaint: bool = True,
         init: bool = True,
+        always_update: bool = False,
     ) -> None:
-        super().__init__(default, layout=layout, repaint=repaint, init=init)
+        super().__init__(
+            default,
+            layout=layout,
+            repaint=repaint,
+            init=init,
+            always_update=always_update,
+        )
 
 
 class var(Reactive[ReactiveType]):