Merge pull request #4828 from Textualize/mutate-bind

mutate via data bind

Author: willmcgugan

docs/guide/reactivity.md

@@ -410,7 +410,8 @@ Note the call to `mutate_reactive`. Without it, the display would not update whe
 
 ## Data binding
 
-Reactive attributes from one widget may be *bound* (connected) to another widget, so that changes to a single reactive will automatically update another widget (potentially more than one).
+Reactive attributes may be *bound* (connected) to attributes on child widgets, so that changes to the parent are automatically reflected in the children.
+This can simplify working with compound widgets where the value of an attribute might be used in multiple places.
 
 To bind reactive attributes, call [data_bind][textual.dom.DOMNode.data_bind] on a widget.
 This method accepts reactives (as class attributes) in positional arguments or keyword arguments.

src/textual/dom.py

@@ -41,7 +41,7 @@
 from .css.styles import RenderStyles, Styles
 from .css.tokenize import IDENTIFIER
 from .message_pump import MessagePump
-from .reactive import Reactive, ReactiveError, _watch
+from .reactive import Reactive, ReactiveError, _Mutated, _watch
 from .timer import Timer
 from .walk import walk_breadth_first, walk_depth_first
 from .worker_manager import WorkerManager
@@ -253,6 +253,10 @@ def mutate_reactive(self, reactive: Reactive[ReactiveType]) -> None:
         this method after your reactive is updated. This will ensure that all the reactive _superpowers_
         work.
 
+        !!! note
+
+            This method will cause watchers to be called, even if the value hasn't changed.
+
         Args:
             reactive: A reactive property (use the class scope syntax, i.e. `MyClass.my_reactive`).
         """
@@ -279,6 +283,7 @@ def compose(self) -> ComposeResult:
                 yield WorldClock("Asia/Tokyo").data_bind(WorldClockApp.time)
             ```
 
+
         Raises:
             ReactiveError: If the data wasn't bound.
 
@@ -334,7 +339,8 @@ def setter(value: object) -> None:
                     """Set bound data."""
                     _rich_traceback_omit = True
                     Reactive._initialize_object(self)
-                    setattr(self, variable_name, value)
+                    # Wrap the value in `_Mutated` so the setter knows to invoke watchers etc.
+                    setattr(self, variable_name, _Mutated(value))
 
                 return setter
 

src/textual/reactive.py

@@ -42,6 +42,13 @@
 ReactableType = TypeVar("ReactableType", bound="DOMNode")
 
 
+class _Mutated:
+    """A wrapper to indicate a value was mutated."""
+
+    def __init__(self, value: Any) -> None:
+        self.value = value
+
+
 class ReactiveError(Exception):
     """Base class for reactive errors."""
 
@@ -273,6 +280,10 @@ def _set(self, obj: Reactable, value: ReactiveType, always: bool = False) -> Non
                 f"Node is missing data; Check you are calling super().__init__(...) in the {obj.__class__.__name__}() constructor, before setting reactives."
             )
 
+        if isinstance(value, _Mutated):
+            value = value.value
+            always = True
+
         self._initialize_reactive(obj, self.name)
 
         if hasattr(obj, self.compute_name):

tests/test_reactive.py

@@ -783,3 +783,44 @@ def compose(self) -> ComposeResult:
         widget.mutate_reactive(TestWidget.names)
         # Watcher should be invoked
         assert watched_names == [[], ["Paul"], ["Jessica"]]
+
+
+async def test_mutate_reactive_data_bind() -> None:
+    """https://github.com/Textualize/textual/issues/4825"""
+
+    # Record mutations to TestWidget.messages
+    widget_messages: list[list[str]] = []
+
+    class TestWidget(Widget):
+        messages: reactive[list[str]] = reactive(list, init=False)
+
+        def watch_messages(self, names: list[str]) -> None:
+            widget_messages.append(names.copy())
+
+    class TestApp(App):
+        messages: reactive[list[str]] = reactive(list, init=False)
+
+        def compose(self) -> ComposeResult:
+            yield TestWidget().data_bind(TestApp.messages)
+
+    app = TestApp()
+    async with app.run_test():
+        test_widget = app.query_one(TestWidget)
+        assert widget_messages == [[]]
+        assert test_widget.messages == []
+
+        # Should be the same instance
+        assert app.messages is test_widget.messages
+
+        # Mutate app
+        app.messages.append("foo")
+        # Mutations aren't detected
+        assert widget_messages == [[]]
+        assert app.messages == ["foo"]
+        assert test_widget.messages == ["foo"]
+        # Explicitly mutate app reactive
+        app.mutate_reactive(TestApp.messages)
+        # Mutating app, will also invoke watchers on any data binds
+        assert widget_messages == [[], ["foo"]]
+        assert app.messages == ["foo"]
+        assert test_widget.messages == ["foo"]