[material_3_demo] Refactor application code into multiple small libraries (#2581)

material_3_demo/lib/src/rail_transition.dart

@@ -0,0 +1,64 @@
+// Copyright 2021 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/widgets.dart';
+
+import 'animations.dart';
+
+class RailTransition extends StatefulWidget {
+  const RailTransition({
+    super.key,
+    required this.animation,
+    required this.backgroundColor,
+    required this.child,
+  });
+
+  final Animation<double> animation;
+  final Widget child;
+  final Color backgroundColor;
+
+  @override
+  State<RailTransition> createState() => _RailTransition();
+}
+
+class _RailTransition extends State<RailTransition> {
+  late Animation<Offset> offsetAnimation;
+  late Animation<double> widthAnimation;
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+
+    // The animations are only rebuilt by this method when the text
+    // direction changes because this widget only depends on Directionality.
+    final bool ltr = Directionality.of(context) == TextDirection.ltr;
+
+    widthAnimation = Tween<double>(
+      begin: 0,
+      end: 1,
+    ).animate(SizeAnimation(widget.animation));
+
+    offsetAnimation = Tween<Offset>(
+      begin: ltr ? const Offset(-1, 0) : const Offset(1, 0),
+      end: Offset.zero,
+    ).animate(OffsetAnimation(widget.animation));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return ClipRect(
+      child: DecoratedBox(
+        decoration: BoxDecoration(color: widget.backgroundColor),
+        child: Align(
+          alignment: Alignment.topLeft,
+          widthFactor: widthAnimation.value,
+          child: FractionalTranslation(
+            translation: offsetAnimation.value,
+            child: widget.child,
+          ),
+        ),
+      ),
+    );
+  }
+}