Update ex9-4.md

ch09/ex9-4.md

@@ -6,46 +6,46 @@
 ```dart
 import 'package:flutter/material.dart';
 
-void main() => runApp(const MyApp());
+void main() {
+  runApp(const MyApp());
+}
 
 class MyApp extends StatelessWidget {
   const MyApp({Key? key}) : super(key: key);
-
-  static const String _title = 'Example';
-
   @override
   Widget build(BuildContext context) {
-    return const MaterialApp(
-      title: _title,
-      home: MyStatelessWidget(),
+    const title = 'Expanded Widget';
+    return MaterialApp(
+      title: title,
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text(title),
+        ),
+        body: const MyExpandedWidget(),
+      ),
     );
   }
 }
 
-class MyStatelessWidget extends StatelessWidget {
-  const MyStatelessWidget({Key? key}) : super(key: key);
-
+class MyExpandedWidget extends StatelessWidget {
+  const MyExpandedWidget();
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(title: const Text('Placeholder Example')),
-      body: _buildPlaceholderWidget(),
-    );
-  }
-
-  Widget _buildPlaceholderWidget() {
     return Column(
-      children: const <Widget> [
-        Placeholder(fallbackHeight: 400, strokeWidth: 10, color: Colors.red),
+      children: const [
+        Expanded(
+          child: Placeholder(
+              fallbackHeight: 400, strokeWidth: 10, color: Colors.red),
+        ),
         Expanded(
-          child: Text("Expanded Text")
+          child: Placeholder(
+              fallbackHeight: 400, strokeWidth: 10, color: Colors.white),
         ),
-        Placeholder(fallbackHeight: 200, strokeWidth: 5, color: Colors.green),
         Expanded(
-          child: Text("Expanded Text")
+          child: Placeholder(
+              fallbackHeight: 400, strokeWidth: 10, color: Colors.blue),
         ),
-        Placeholder(fallbackHeight: 100, strokeWidth: 1, color: Colors.blue),
-      ]
+      ],
     );
   }
 }