Add App.dom_ready_signal

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+- Added `App.dom_ready_signal`. This signal is published one time when the App first finishes loading. Its purpose is to provide a simple way for widget libraries to do work only when the DOM is ready, since they cannot use the `Ready` event sent to the App class.
+
 ### Fixed
 
 - Fixed `VERTICAL_BREAKPOINTS` doesn't work https://github.com/Textualize/textual/pull/5785

src/textual/app.py

@@ -791,6 +791,16 @@ def __init__(
         perform work after the app has resumed.
         """
 
+        self.dom_ready_signal: Signal[App] = Signal(self, "dom-ready")
+        """Signal that is published when the app is finished loading and the
+        DOM is ready. Note that this signal is only sent once when the app first loads.
+
+        This is intended for third-party widget libraries that need to perform 
+        actions requiring a fully loaded DOM (such as querying for widgets) but 
+        cannot override the App class's `on_ready` method. Subscribers must be 
+        DOMNode instances (such as widgets or other DOM components).
+        """
+
         self.set_class(self.current_theme.dark, "-dark-mode")
         self.set_class(not self.current_theme.dark, "-light-mode")
 

src/textual/screen.py

@@ -1269,6 +1269,7 @@ def _refresh_layout(self, size: Size | None = None, scroll: bool = False) -> Non
         else:
             self.app.post_message(events.Ready())
             self.app._dom_ready = True
+            self.call_next(self.app.dom_ready_signal.publish, self.app)
 
     async def _on_update(self, message: messages.Update) -> None:
         message.stop()