Simplify Placeholder implementation.

Instead of creating a private widget that is the only child of 'Placeholder', that was inheriting from 'Container', simplify everything. 'Placeholder' now inherits directly from 'Widget' and it saves a dictionary with its renderables per variant, instead of deferring that to the child '_PlaceholderLabel'.

src/textual/widgets/_placeholder.py

@@ -3,7 +3,6 @@ from __future__ import annotations
 from itertools import cycle
 
 from .. import events
-from ..containers import Container
 from ..css._error_tools import friendly_list
 from ..reactive import Reactive, reactive
 from ..widget import Widget, RenderResult
@@ -36,19 +35,10 @@ _LOREM_IPSUM_PLACEHOLDER_TEXT = "Lorem ipsum dolor sit amet, consectetur adipisc
 
 
 class InvalidPlaceholderVariant(Exception):
-    pass
+    """Raised when an invalid Placeholder variant is set."""
 
 
-class _PlaceholderLabel(Widget):
-    def __init__(self, content, classes) -> None:
-        super().__init__(classes=classes)
-        self._content = content
-
-    def render(self) -> RenderResult:
-        return self._content
-
-
-class Placeholder(Container):
+class Placeholder(Widget):
     """A simple placeholder widget to use before you build your custom widgets.
 
     This placeholder has a couple of variants that show different data.
@@ -63,44 +53,24 @@ class Placeholder(Container):
 
     DEFAULT_CSS = """
     Placeholder {
-        align: center middle;
+        content-align: center middle;
         overflow: hidden;
+        color: $text;
     }
 
     Placeholder.-text {
         padding: 1;
     }
-
-    _PlaceholderLabel {
-        height: auto;
-        color: $text;
-    }
-
-    Placeholder > _PlaceholderLabel {
-        content-align: center middle;
-    }
-
-    Placeholder.-default > _PlaceholderLabel.-size,
-    Placeholder.-default > _PlaceholderLabel.-text,
-    Placeholder.-size    > _PlaceholderLabel.-default,
-    Placeholder.-size    > _PlaceholderLabel.-text,
-    Placeholder.-text    > _PlaceholderLabel.-default,
-    Placeholder.-text    > _PlaceholderLabel.-size {
-        display: none;
-    }
-
-    Placeholder.-default > _PlaceholderLabel.-default,
-    Placeholder.-size    > _PlaceholderLabel.-size,
-    Placeholder.-text    > _PlaceholderLabel.-text {
-        display: block;
-    }
     """
+
     # Consecutive placeholders get assigned consecutive colors.
     _COLORS = cycle(_PLACEHOLDER_BACKGROUND_COLORS)
     _SIZE_RENDER_TEMPLATE = "[b]{} x {}[/b]"
 
     variant: Reactive[PlaceholderVariant] = reactive("default")
 
+    _renderables: dict[PlaceholderVariant, RenderResult]
+
     @classmethod
     def reset_color_cycle(cls) -> None:
         """Reset the placeholder background color cycle."""
@@ -128,27 +98,14 @@ class Placeholder(Container):
             classes (str | None, optional): A space separated string with the CSS classes
                 of the placeholder, if any. Defaults to None.
         """
-        # Create and cache labels for all the variants.
-        self._default_label = _PlaceholderLabel(
-            label if label else f"#{id}" if id else "Placeholder",
-            "-default",
-        )
-        self._size_label = _PlaceholderLabel(
-            "",
-            "-size",
-        )
-        self._text_label = _PlaceholderLabel(
-            "\n\n".join(_LOREM_IPSUM_PLACEHOLDER_TEXT for _ in range(5)),
-            "-text",
-        )
-        super().__init__(
-            self._default_label,
-            self._size_label,
-            self._text_label,
-            name=name,
-            id=id,
-            classes=classes,
-        )
+        # Create and cache renderables for all the variants.
+        self._renderables = {
+            "default": label if label else f"#{id}" if id else "Placeholder",
+            "size": "",
+            "text": "\n\n".join(_LOREM_IPSUM_PLACEHOLDER_TEXT for _ in range(5)),
+        }
+
+        super().__init__(name=name, id=id, classes=classes)
 
         self.styles.background = f"{next(Placeholder._COLORS)} 50%"
 
@@ -158,6 +115,9 @@ class Placeholder(Container):
         while next(self._variants_cycle) != self.variant:
             pass
 
+    def render(self) -> RenderResult:
+        return self._renderables[self.variant]
+
     def cycle_variant(self) -> None:
         """Get the next variant in the cycle."""
         self.variant = next(self._variants_cycle)
@@ -183,6 +143,6 @@ class Placeholder(Container):
 
     def on_resize(self, event: events.Resize) -> None:
         """Update the placeholder "size" variant with the new placeholder size."""
-        self._size_label._content = self._SIZE_RENDER_TEMPLATE.format(*self.size)
+        self._renderables["size"] = self._SIZE_RENDER_TEMPLATE.format(*self.size)
         if self.variant == "size":
-            self._size_label.refresh(layout=True)
+            self.refresh(layout=True)