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

material_3_demo/lib/src/one_two_transition.dart

@@ -0,0 +1,62 @@
+// 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/material.dart';
+
+import 'animations.dart';
+import 'constants.dart';
+
+class OneTwoTransition extends StatefulWidget {
+  const OneTwoTransition({
+    super.key,
+    required this.animation,
+    required this.one,
+    required this.two,
+  });
+
+  final Animation<double> animation;
+  final Widget one;
+  final Widget two;
+
+  @override
+  State<OneTwoTransition> createState() => _OneTwoTransitionState();
+}
+
+class _OneTwoTransitionState extends State<OneTwoTransition> {
+  late final Animation<Offset> offsetAnimation;
+  late final Animation<double> widthAnimation;
+
+  @override
+  void initState() {
+    super.initState();
+
+    offsetAnimation = Tween<Offset>(
+      begin: const Offset(1, 0),
+      end: Offset.zero,
+    ).animate(OffsetAnimation(widget.animation));
+
+    widthAnimation = Tween<double>(
+      begin: 0,
+      end: mediumWidthBreakpoint,
+    ).animate(SizeAnimation(widget.animation));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      children: <Widget>[
+        Flexible(flex: mediumWidthBreakpoint.toInt(), child: widget.one),
+        if (widthAnimation.value.toInt() > 0) ...[
+          Flexible(
+            flex: widthAnimation.value.toInt(),
+            child: FractionalTranslation(
+              translation: offsetAnimation.value,
+              child: widget.two,
+            ),
+          ),
+        ],
+      ],
+    );
+  }
+}