SizeTransition sample code (#63833)

packages/flutter/lib/src/widgets/transitions.dart

@@ -436,6 +436,51 @@ class RotationTransition extends AnimatedWidget {
 /// animated by a [CurvedAnimation] set to [Curves.fastOutSlowIn]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/size_transition.mp4}
 ///
+/// {@tool dartpad --template=stateful_widget_material_ticker}
+///
+/// This code defines a widget that uses [SizeTransition] to change the size
+/// of [FlutterLogo] continually. It is built with a [Scaffold]
+/// where the internal widget has space to change its size.
+///
+/// ```dart
+/// AnimationController _controller;
+/// Animation<double> _animation;
+///
+/// @override
+/// void initState() {
+///   super.initState();
+///   _controller = AnimationController(
+///     duration: const Duration(seconds: 3),
+///     vsync: this,
+///   )..repeat();
+///   _animation = CurvedAnimation(
+///     parent: _controller,
+///     curve: Curves.fastOutSlowIn,
+///   );
+/// }
+///
+/// @override
+/// void dispose() {
+///   super.dispose();
+///   _controller.dispose();
+/// }
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: SizeTransition(
+///       sizeFactor: _animation,
+///       axis: Axis.horizontal,
+///       axisAlignment: -1,
+///       child: Center(
+///           child: FlutterLogo(size: 200.0),
+///       ),
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [AnimatedCrossFade], for a widget that automatically animates between