feat(collapsible): make title a reactive attribute

src/textual/widgets/_collapsible.py

@@ -44,6 +44,7 @@ class CollapsibleTitle(Widget, can_focus=True):
     """
 
     collapsed = reactive(True)
+    label = reactive("Toggle")
 
     def __init__(
         self,
@@ -83,6 +84,7 @@ class Collapsible(Widget):
     """A collapsible container."""
 
     collapsed = reactive(True)
+    title = reactive("Toggle")
 
     DEFAULT_CSS = """
     Collapsible {
@@ -169,14 +171,15 @@ def __init__(
             classes: The CSS classes of the collapsible.
             disabled: Whether the collapsible is disabled or not.
         """
+        super().__init__(name=name, id=id, classes=classes, disabled=disabled)
+        self.title = title
         self._title = CollapsibleTitle(
             label=title,
             collapsed_symbol=collapsed_symbol,
             expanded_symbol=expanded_symbol,
             collapsed=collapsed,
         )
         self._contents_list: list[Widget] = list(children)
-        super().__init__(name=name, id=id, classes=classes, disabled=disabled)
         self.collapsed = collapsed
 
     def _on_collapsible_title_toggle(self, event: CollapsibleTitle.Toggle) -> None:
@@ -214,3 +217,8 @@ def compose_add_child(self, widget: Widget) -> None:
             widget: A Widget to add.
         """
         self._contents_list.append(widget)
+
+    def _watch_title(self, title: str) -> None:
+        if not self.is_mounted:
+            return
+        self._title.label = title

tests/test_collapsible.py

@@ -189,3 +189,15 @@ def on_collapsible_collapsed(self) -> None:
 
         assert pilot.app.query_one(Collapsible).collapsed
         assert len(hits) == 1
+
+
+async def test_collapsible_title_reactive_change():
+    class CollapsibleApp(App[None]):
+        def compose(self) -> ComposeResult:
+            yield Collapsible(title="Old title")
+
+    async with CollapsibleApp().run_test() as pilot:
+        collapsible = pilot.app.query_one(Collapsible)
+        assert get_title(collapsible).label == "Old title"
+        collapsible.title = "New title"
+        assert get_title(collapsible).label == "New title"