Extend @on decorator to filter matchable attributes (#2498)

* Add tests for #2484.

* Implement @on extension.

[skip ci]
Related issues: #2484.

* Changelog.

* Add missing @on test.

* Remove debug prints.

* Document changes.

* Update tests.

Test now fully works, as of #2490.

* Cache parsed selectors.

* Streamline exit condition.

* Fix typing.

* More succint wording.

* Document 'on' kwargs.

* Update src/textual/_on.py

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

* Update docs/guide/events.md

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

* Change 'on' API.

* Remove example code.

* Address feedback.

* Update src/textual/_on.py

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

* Address review feedback.

* Fix #2499.

* don't require control to be manually specified

* update docstring

* deleted words

---------

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `always_update` as an optional argument for `reactive.var`
 - Made Binding description default to empty string, which is equivalent to show=False https://github.com/Textualize/textual/pull/2501
 - Modified Message to allow it to be used as a dataclass https://github.com/Textualize/textual/pull/2501
+- Decorator `@on` accepts arbitrary `**kwargs` to apply selectors to attributes of the message https://github.com/Textualize/textual/pull/2498
 
 ### Added
 
@@ -34,6 +35,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `TreeNode.is_root` https://github.com/Textualize/textual/pull/2510
 - Added `TreeNode.remove_children` https://github.com/Textualize/textual/pull/2510
 - Added `TreeNode.remove` https://github.com/Textualize/textual/pull/2510
+- Added classvar `Message.ALLOW_SELECTOR_MATCH` https://github.com/Textualize/textual/pull/2498
+- Added `ALLOW_SELECTOR_MATCH` to all built-in messages associated with widgets https://github.com/Textualize/textual/pull/2498
 - Markdown document sub-widgets now reference the container document
 - Table of contents of a markdown document now references the document
 - Added the `control` property to messages

docs/guide/events.md

@@ -218,10 +218,23 @@ Messages from builtin controls will have this attribute, but you may need to add
 
 !!! note
 
-    If multiple decorated handlers match the `control`, then they will *all* be called in the order they are defined.
+    If multiple decorated handlers match the message, then they will *all* be called in the order they are defined.
 
     The naming convention handler will be called *after* any decorated handlers.
 
+#### Applying CSS selectors to arbitrary attributes
+
+The `on` decorator also accepts selectors as keyword arguments that may be used to match other attributes in a Message, provided those attributes are in [`Message.ALLOW_SELECTOR_MATCH`][textual.message.Message.ALLOW_SELECTOR_MATCH].
+
+The snippet below shows how to match the message [`TabbedContent.TabActivated`][textual.widgets.TabbedContent.TabActivated] only when the tab with id `home` was activated:
+
+```py
+@on(TabbedContent.TabActivated, tab="#home")
+def home_tab(self) -> None:
+    self.log("Switched back to home tab.")
+    ...
+```
+
 ### Handler arguments
 
 Message handler methods can be written with or without a positional argument. If you add a positional argument, Textual will call the handler with the event object. The following handler (taken from `custom01.py` above) contains a `message` parameter. The body of the code makes use of the message to set a preset color.
@@ -231,6 +244,14 @@ Message handler methods can be written with or without a positional argument. If
         self.screen.styles.animate("background", message.color, duration=0.5)
 ```
 
+A similar handler can be written using the decorator `on`:
+
+```python
+    @on(ColorButton.Selected)
+    def animate_background_color(self, message: ColorButton.Selected) -> None:
+        self.screen.styles.animate("background", message.color, duration=0.5)
+```
+
 If the body of your handler doesn't require any information in the message you can omit it from the method signature. If we just want to play a bell noise when the button is clicked, we could write our handler like this:
 
 ```python

src/textual/_on.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from typing import Callable, TypeVar
 
+from .css.model import SelectorSet
 from .css.parse import parse_selectors
 from .css.tokenizer import TokenError
 from .message import Message
@@ -13,39 +14,71 @@ class OnDecoratorError(Exception):
     """Errors related to the `on` decorator.
 
     Typically raised at import time as an early warning system.
-
     """
 
 
+class OnNoWidget(Exception):
+    """A selector was applied to an attribute that isn't a widget."""
+
+
 def on(
-    message_type: type[Message], selector: str | None = None
+    message_type: type[Message], selector: str | None = None, **kwargs: str
 ) -> Callable[[DecoratedType], DecoratedType]:
-    """Decorator to declare method is a message handler.
+    """Decorator to declare that the method is a message handler.
+
+    The decorator accepts an optional CSS selector that will be matched against a widget exposed by
+    a `control` attribute on the message.
 
     Example:
         ```python
+        # Handle the press of buttons with ID "#quit".
         @on(Button.Pressed, "#quit")
         def quit_button(self) -> None:
             self.app.quit()
         ```
 
+    Keyword arguments can be used to match additional selectors for attributes
+    listed in [`ALLOW_SELECTOR_MATCH`][textual.message.Message.ALLOW_SELECTOR_MATCH].
+
+    Example:
+        ```python
+        # Handle the activation of the tab "#home" within the `TabbedContent` "#tabs".
+        @on(TabbedContent.TabActivated, "#tabs", tab="#home")
+        def switch_to_home(self) -> None:
+            self.log("Switching back to the home tab.")
+            ...
+        ```
+
     Args:
         message_type: The message type (i.e. the class).
         selector: An optional [selector](/guide/CSS#selectors). If supplied, the handler will only be called if `selector`
             matches the widget from the `control` attribute of the message.
+        **kwargs: Additional selectors for other attributes of the message.
     """
 
-    if selector is not None and not hasattr(message_type, "control"):
+    selectors: dict[str, str] = {}
-        raise OnDecoratorError(
-            "The 'selector' argument requires a message class with a 'control' attribute (such as events from controls)."
-        )
-
     if selector is not None:
-        try:
+        selectors["control"] = selector
-            parse_selectors(selector)
+    if kwargs:
-        except TokenError as error:
+        selectors.update(kwargs)
+
+    parsed_selectors: dict[str, tuple[SelectorSet, ...]] = {}
+    for attribute, css_selector in selectors.items():
+        if attribute == "control":
+            if message_type.control is None:
+                raise OnDecoratorError(
+                    "The message class must have a 'control' to match with the on decorator"
+                )
+        elif attribute not in message_type.ALLOW_SELECTOR_MATCH:
             raise OnDecoratorError(
-                f"Unable to parse selector {selector!r}; check for syntax errors"
+                f"The attribute {attribute!r} can't be matched; have you added it to "
+                + f"{message_type.__name__}.ALLOW_SELECTOR_MATCH?"
+            )
+        try:
+            parsed_selectors[attribute] = parse_selectors(css_selector)
+        except TokenError:
+            raise OnDecoratorError(
+                f"Unable to parse selector {css_selector!r} for {attribute}; check for syntax errors"
             ) from None
 
     def decorator(method: DecoratedType) -> DecoratedType:
@@ -53,7 +86,7 @@ def on(
 
         if not hasattr(method, "_textual_on"):
             setattr(method, "_textual_on", [])
-        getattr(method, "_textual_on").append((message_type, selector))
+        getattr(method, "_textual_on").append((message_type, parsed_selectors))
 
         return method
 

src/textual/message.py

@@ -16,6 +16,7 @@ from .case import camel_to_snake
 
 if TYPE_CHECKING:
     from .message_pump import MessagePump
+    from .widget import Widget
 
 
 @rich.repr.auto
@@ -32,10 +33,16 @@ class Message:
         "_prevent",
     ]
 
+    ALLOW_SELECTOR_MATCH: ClassVar[set[str]] = set()
+    """Additional attributes that can be used with the [`on` decorator][textual.on].
+
+    These attributes must be widgets.
+    """
     bubble: ClassVar[bool] = True  # Message will bubble to parent
     verbose: ClassVar[bool] = False  # Message is verbose
     no_dispatch: ClassVar[bool] = False  # Message may not be handled by client code
     namespace: ClassVar[str] = ""  # Namespace to disambiguate messages
+    control: Widget | None = None
 
     def __init__(self) -> None:
         self.__post_init__()

src/textual/message_pump.py

@@ -22,11 +22,11 @@ from ._context import (
     active_message_pump,
     prevent_message_types_stack,
 )
+from ._on import OnNoWidget
 from ._time import time
 from ._types import CallbackType
 from .case import camel_to_snake
 from .css.match import match
-from .css.parse import parse_selectors
 from .errors import DuplicateKeyHandlers
 from .events import Event
 from .message import Message
@@ -35,6 +35,7 @@ from .timer import Timer, TimerCallback
 
 if TYPE_CHECKING:
     from .app import App
+    from .css.model import SelectorSet
 
 
 class CallbackError(Exception):
@@ -60,15 +61,15 @@ class _MessagePumpMeta(type):
         namespace = camel_to_snake(name)
         isclass = inspect.isclass
         handlers: dict[
-            type[Message], list[tuple[Callable, str | None]]
+            type[Message], list[tuple[Callable, dict[str, tuple[SelectorSet, ...]]]]
         ] = class_dict.get("_decorated_handlers", {})
 
         class_dict["_decorated_handlers"] = handlers
 
         for value in class_dict.values():
             if callable(value) and hasattr(value, "_textual_on"):
-                for message_type, selector in getattr(value, "_textual_on"):
+                for message_type, selectors in getattr(value, "_textual_on"):
-                    handlers.setdefault(message_type, []).append((value, selector))
+                    handlers.setdefault(message_type, []).append((value, selectors))
             if isclass(value) and issubclass(value, Message):
                 if "namespace" not in value.__dict__:
                     value.namespace = namespace
@@ -563,14 +564,23 @@ class MessagePump(metaclass=_MessagePumpMeta):
             decorated_handlers = cls.__dict__.get("_decorated_handlers")
             if decorated_handlers is not None:
                 handlers = decorated_handlers.get(type(message), [])
-                for method, selector in handlers:
+                from .widget import Widget
-                    if selector is None:
+
+                for method, selectors in handlers:
+                    if not selectors:
                         yield cls, method.__get__(self, cls)
                     else:
-                        selector_sets = parse_selectors(selector)
+                        if not message._sender:
-                        if message._sender is not None and match(
+                            continue
-                            selector_sets, message.control
+                        for attribute, selector in selectors.items():
-                        ):
+                            node = getattr(message, attribute)
+                            if not isinstance(node, Widget):
+                                raise OnNoWidget(
+                                    f"on decorator can't match against {attribute!r} as it is not a widget."
+                                )
+                            if not match(selector, node):
+                                break
+                        else:
                             yield cls, method.__get__(self, cls)
 
             # Fall back to the naming convention

src/textual/widgets/_list_view.py

@@ -46,6 +46,9 @@ class ListView(VerticalScroll, can_focus=True, can_focus_children=False):
         or in a parent widget in the DOM.
         """
 
+        ALLOW_SELECTOR_MATCH = {"item"}
+        """Additional message attributes that can be used with the [`on` decorator][textual.on]."""
+
         def __init__(self, list_view: ListView, item: ListItem | None) -> None:
             super().__init__()
             self.list_view: ListView = list_view
@@ -69,6 +72,9 @@ class ListView(VerticalScroll, can_focus=True, can_focus_children=False):
         a parent widget in the DOM.
         """
 
+        ALLOW_SELECTOR_MATCH = {"item"}
+        """Additional message attributes that can be used with the [`on` decorator][textual.on]."""
+
         def __init__(self, list_view: ListView, item: ListItem) -> None:
             super().__init__()
             self.list_view: ListView = list_view

src/textual/widgets/_radio_set.py

@@ -78,6 +78,9 @@ class RadioSet(Container, can_focus=True, can_focus_children=False):
         This message can be handled using an `on_radio_set_changed` method.
         """
 
+        ALLOW_SELECTOR_MATCH = {"pressed"}
+        """Additional message attributes that can be used with the [`on` decorator][textual.on]."""
+
         def __init__(self, radio_set: RadioSet, pressed: RadioButton) -> None:
             """Initialise the message.
 

src/textual/widgets/_tabbed_content.py

@@ -88,6 +88,9 @@ class TabbedContent(Widget):
     class TabActivated(Message):
         """Posted when the active tab changes."""
 
+        ALLOW_SELECTOR_MATCH = {"tab"}
+        """Additional message attributes that can be used with the [`on` decorator][textual.on]."""
+
         def __init__(self, tabbed_content: TabbedContent, tab: Tab) -> None:
             """Initialize message.
 

src/textual/widgets/_tabs.py

@@ -178,6 +178,9 @@ class Tabs(Widget, can_focus=True):
     class TabActivated(Message):
         """Sent when a new tab is activated."""
 
+        ALLOW_SELECTOR_MATCH = {"tab"}
+        """Additional message attributes that can be used with the [`on` decorator][textual.on]."""
+
         tabs: Tabs
         """The tabs widget containing the tab."""
         tab: Tab

tests/test_on.py

@@ -5,7 +5,7 @@ from textual._on import OnDecoratorError
 from textual.app import App, ComposeResult
 from textual.message import Message
 from textual.widget import Widget
-from textual.widgets import Button
+from textual.widgets import Button, TabbedContent, TabPane
 
 
 async def test_on_button_pressed() -> None:
@@ -102,3 +102,44 @@ def test_on_no_control() -> None:
         @on(CustomMessage, "#foo")
         def foo():
             pass
+
+
+def test_on_attribute_not_listed() -> None:
+    """Check `on` checks if the attribute is in ALLOW_SELECTOR_MATCH."""
+
+    class CustomMessage(Message):
+        pass
+
+    with pytest.raises(OnDecoratorError):
+
+        @on(CustomMessage, foo="bar")
+        def foo():
+            pass
+
+
+async def test_on_arbitrary_attributes() -> None:
+    log: list[str] = []
+
+    class OnArbitraryAttributesApp(App[None]):
+        def compose(self) -> ComposeResult:
+            with TabbedContent():
+                yield TabPane("One", id="one")
+                yield TabPane("Two", id="two")
+                yield TabPane("Three", id="three")
+
+        def on_mount(self) -> None:
+            self.query_one(TabbedContent).add_class("tabs")
+
+        @on(TabbedContent.TabActivated, tab="#one")
+        def one(self) -> None:
+            log.append("one")
+
+        @on(TabbedContent.TabActivated, ".tabs", tab="#two")
+        def two(self) -> None:
+            log.append("two")
+
+    app = OnArbitraryAttributesApp()
+    async with app.run_test() as pilot:
+        await pilot.press("tab", "right", "right")
+
+    assert log == ["one", "two"]