prevent implementation

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added horizontal rule to Markdown https://github.com/Textualize/textual/pull/1832
 - Added `Widget.disabled` https://github.com/Textualize/textual/pull/1785
 - Added `DOMNode.notify_style_update` to replace `messages.StylesUpdated` message https://github.com/Textualize/textual/pull/1861
+- Added `MessagePump.prevent` context manager to temporarily suppress a given message type
 
 ### Changed
 

src/textual/events.py

@@ -29,9 +29,13 @@ class Event(Message):
 @rich.repr.auto
 class Callback(Event, bubble=False, verbose=True):
     def __init__(
-        self, sender: MessageTarget, callback: Callable[[], Awaitable[None]]
+        self,
+        sender: MessageTarget,
+        callback: Callable[[], Awaitable[None]],
+        prevent: set[type[Message]] | None = None,
     ) -> None:
         self.callback = callback
+        self.prevent = frozenset(prevent) if prevent else None
         super().__init__(sender)
 
     def __rich_repr__(self) -> rich.repr.Result:

src/textual/message_pump.py

@@ -10,8 +10,9 @@ from __future__ import annotations
 import asyncio
 import inspect
 from asyncio import CancelledError, Queue, QueueEmpty, Task
+from contextlib import contextmanager
 from functools import partial
-from typing import TYPE_CHECKING, Any, Awaitable, Callable, Iterable
+from typing import TYPE_CHECKING, Any, Awaitable, Callable, Generator, Iterable
 from weakref import WeakSet
 
 from . import Logger, events, log, messages
@@ -19,6 +20,7 @@ from ._asyncio import create_task
 from ._callback import invoke
 from ._context import NoActiveAppError, active_app, active_message_pump
 from ._time import time
+from ._types import CallbackType
 from .case import camel_to_snake
 from .errors import DuplicateKeyHandlers
 from .events import Event
@@ -77,6 +79,7 @@ class MessagePump(metaclass=MessagePumpMeta):
         self._max_idle: float | None = None
         self._mounted_event = asyncio.Event()
         self._next_callbacks: list[CallbackType] = []
+        self._prevent_events: list[set[type[Message]]] = []
 
     @property
     def task(self) -> Task:
@@ -149,6 +152,9 @@ class MessagePump(metaclass=MessagePumpMeta):
         self._parent = None
 
     def check_message_enabled(self, message: Message) -> bool:
+        message_type = type(message)
+        if self._prevent_events and message_type in self._prevent_events[-1]:
+            return False
         return type(message) not in self._disabled_messages
 
     def disable_messages(self, *messages: type[Message]) -> None:
@@ -527,6 +533,27 @@ class MessagePump(metaclass=MessagePumpMeta):
         if self._message_queue.empty():
             self.post_message_no_wait(messages.Prompt(sender=self))
 
+    @contextmanager
+    def prevent(self, *message_types: type[Message]) -> Generator[None, None, None]:
+        """A context manager to *temporarily* prevent the given message types from being posted.
+
+        Example:
+
+            input = self.query_one(Input)
+            with self.prevent(Input.Changed):
+                input.value = "foo"
+
+
+        """
+        if self._prevent_events:
+            self._prevent_events.append(self._prevent_events[-1].union(message_types))
+        else:
+            self._prevent_events.append(set(message_types))
+        try:
+            yield
+        finally:
+            self._prevent_events.pop()
+
     async def post_message(self, message: Message) -> bool:
         """Post a message or an event to this message pump.
 
@@ -594,7 +621,11 @@ class MessagePump(metaclass=MessagePumpMeta):
         return self.post_message_no_wait(message)
 
     async def on_callback(self, event: events.Callback) -> None:
-        await invoke(event.callback)
+        if event.prevent:
+            with self.prevent(*event.prevent):
+                await invoke(event.callback)
+        else:
+            await invoke(event.callback)
 
     # TODO: Does dispatch_key belong on message pump?
     async def dispatch_key(self, event: events.Key) -> bool:

src/textual/reactive.py

@@ -235,7 +235,9 @@ class Reactive(Generic[ReactiveType]):
                 # Result is awaitable, so we need to await it within an async context
                 obj.post_message_no_wait(
                     events.Callback(
-                        sender=obj, callback=partial(await_watcher, watch_result)
+                        sender=obj,
+                        callback=partial(await_watcher, watch_result),
+                        prevent=obj._prevent_events[0] if obj._prevent_events else None,
                     )
                 )
 

tests/test_message_pump.py

@@ -1,8 +1,10 @@
 import pytest
 
+from textual.app import App, ComposeResult
 from textual.errors import DuplicateKeyHandlers
 from textual.events import Key
 from textual.widget import Widget
+from textual.widgets import Input
 
 
 class ValidWidget(Widget):
@@ -54,3 +56,34 @@ async def test_dispatch_key_raises_when_conflicting_handler_aliases():
     with pytest.raises(DuplicateKeyHandlers):
         await widget.dispatch_key(Key(widget, key="tab", character="\t"))
     assert widget.called_by == widget.key_tab
+
+
+class PreventTestApp(App):
+    def __init__(self) -> None:
+        self.input_changed_events = []
+        super().__init__()
+
+    def compose(self) -> ComposeResult:
+        yield Input()
+
+    def on_input_changed(self, event: Input.Changed) -> None:
+        self.input_changed_events.append(event)
+
+
+async def test_prevent() -> None:
+    app = PreventTestApp()
+
+    async with app.run_test() as pilot:
+        assert not app.input_changed_events
+        input = app.query_one(Input)
+        input.value = "foo"
+        await pilot.pause()
+        assert len(app.input_changed_events) == 1
+        assert app.input_changed_events[0].value == "foo"
+
+        with input.prevent(Input.Changed):
+            input.value = "bar"
+
+        await pilot.pause()
+        assert len(app.input_changed_events) == 1
+        assert app.input_changed_events[0].value == "foo"