diff --git a/docs/examples/widgets/custom.py b/docs/examples/widgets/custom.py
index b1fdbe8ec..7be00ed8a 100644
--- a/docs/examples/widgets/custom.py
+++ b/docs/examples/widgets/custom.py
@@ -1,6 +1,6 @@
+from rich.console import RenderableType
 from rich.panel import Panel
 
-from textual import events
 from textual.app import App
 from textual.reactive import Reactive
 from textual.widget import Widget
@@ -8,22 +8,22 @@ from textual.widget import Widget
 
 class Hover(Widget):
 
-    mouse_over: Reactive[bool] = Reactive(False)
+    mouse_over = Reactive(False)
 
-    def render(self) -> Panel:
+    def render(self) -> RenderableType:
         return Panel("Hello [b]World[/b]", style=("on red" if self.mouse_over else ""))
 
-    async def on_enter(self, event: events.Enter) -> None:
+    def on_enter(self) -> None:
         self.mouse_over = True
 
-    async def on_leave(self, event: events.Leave) -> None:
+    def on_leave(self) -> None:
         self.mouse_over = False
 
 
 class HoverApp(App):
     """Demonstrates smooth animation"""
 
-    async def on_mount(self, event: events.Mount) -> None:
+    async def on_mount(self) -> None:
         """Build layout here."""
 
         hovers = (Hover() for _ in range(10))
diff --git a/src/textual/_callback.py b/src/textual/_callback.py
new file mode 100644
index 000000000..de001e09c
--- /dev/null
+++ b/src/textual/_callback.py
@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+from functools import lru_cache
+
+from inspect import signature, isawaitable
+from typing import Any, Callable
+
+
+@lru_cache(maxsize=2048)
+def count_parameters(func: Callable) -> int:
+    """Count the number of parameters in a callable"""
+    return len(signature(func).parameters)
+
+
+async def invoke(callback: Callable, *params: object) -> Any:
+    """Invoke a callback with an arbitrary number of parameters.
+
+    Args:
+        callback (Callable): [description]
+
+    Returns:
+        Any: [description]
+    """
+    parameter_count = count_parameters(callback)
+
+    result = callback(*params[:parameter_count])
+    if isawaitable(result):
+        await result
diff --git a/src/textual/message_pump.py b/src/textual/message_pump.py
index 5653885c8..16a06669b 100644
--- a/src/textual/message_pump.py
+++ b/src/textual/message_pump.py
@@ -12,6 +12,7 @@ from rich.traceback import Traceback
 from . import events
 from . import log
 from ._timer import Timer, TimerCallback
+from ._callback import invoke
 from ._context import active_app
 from .message import Message
 from .reactive import Reactive
@@ -241,7 +242,7 @@ class MessagePump:
 
         for method in self._get_dispatch_methods(f"on_{event.name}", event):
             log(event, ">>>", self, verbosity=event.verbosity)
-            await method(event)
+            await invoke(method, event)
 
         if event.bubble and self._parent and not event._stop_propagation:
             if event.sender != self._parent and self.is_parent_active:
@@ -308,9 +309,7 @@ class MessagePump:
         event.stop()
         if event.callback is not None:
             try:
-                callback_result = event.callback()
-                if inspect.isawaitable(callback_result):
-                    await callback_result
+                await invoke(event.callback)
             except Exception as error:
                 raise CallbackError(
                     f"unable to run callback {event.callback!r}; {error}"
diff --git a/src/textual/reactive.py b/src/textual/reactive.py
index 7ae603207..e3a9cf6ba 100644
--- a/src/textual/reactive.py
+++ b/src/textual/reactive.py
@@ -16,6 +16,7 @@ from typing import (
 from . import log
 from . import events
 
+from ._callback import count_parameters
 from ._types import MessageTarget
 
 if TYPE_CHECKING:
@@ -28,10 +29,6 @@ if TYPE_CHECKING:
 ReactiveType = TypeVar("ReactiveType")
 
 
-def count_params(func: Callable) -> int:
-    return len(inspect.signature(func).parameters)
-
-
 class Reactive(Generic[ReactiveType]):
     """Reactive descriptor."""
 
@@ -92,7 +89,7 @@ class Reactive(Generic[ReactiveType]):
             obj: Reactable, watch_function: Callable, old_value: Any, value: Any
         ) -> None:
             _rich_traceback_guard = True
-            if count_params(watch_function) == 2:
+            if count_parameters(watch_function) == 2:
                 await watch_function(old_value, value)
             else:
                 await watch_function(value)