Added more examples in animations (#341)

animations/lib/src/basics/10_animated_switcher.dart

@@ -0,0 +1,72 @@
+// Copyright 2020 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 'dart:math';
+
+import 'package:flutter/material.dart';
+
+Color generateColor() => Color(0xFFFFFFFF & Random().nextInt(0xFFFFFFFF));
+
+Widget generateContainer(int keyCount) => Container(
+      key: ValueKey<int>(keyCount),
+      height: Random().nextDouble() * 200,
+      width: Random().nextDouble() * 200,
+      decoration: BoxDecoration(
+        color: generateColor(),
+        borderRadius: BorderRadius.circular(Random().nextDouble() * 100),
+        border: Border.all(
+          color: generateColor(),
+          width: Random().nextDouble() * 5,
+        ),
+      ),
+    );
+
+class AnimatedSwitcherDemo extends StatefulWidget {
+  static String routeName = '/basics/10_animated_switcher';
+
+  _AnimatedSwitcherDemoState createState() => _AnimatedSwitcherDemoState();
+}
+
+class _AnimatedSwitcherDemoState extends State<AnimatedSwitcherDemo> {
+  Widget container;
+  int keyCount;
+
+  void initState() {
+    super.initState();
+    keyCount = 0;
+    container = generateContainer(keyCount);
+  }
+
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        actions: [
+          MaterialButton(
+            onPressed: () => setState(
+              () => container = generateContainer(++keyCount),
+            ),
+            child: Text(
+              "Change Widget",
+              style: TextStyle(
+                  color: Theme.of(context).buttonTheme.colorScheme.onPrimary),
+            ),
+          ),
+        ],
+      ),
+      body: Center(
+        // AnimatedSwitcher Widget is used to switch between different widgets
+        // with a given transition. You can change the transitions by using
+        // transitionBuilder property.
+        child: AnimatedSwitcher(
+          duration: Duration(seconds: 1),
+          child: container,
+          transitionBuilder: (child, animation) => ScaleTransition(
+            child: child,
+            scale: animation,
+          ),
+        ),
+      ),
+    );
+  }
+}