refactor: split app wrapper from focused example code (single-file apps)

Convert 20 more single-file apps to the validated pattern: lib/main.dart
holds only the runApp + MaterialApp bootstrap, while a new lib/example.dart
holds the full screen (Scaffold, AppBar) and the focused example code.

Apps: enabling_splash_screen, getx_counter_app, image_from_network,
infinite_list, load_local_image, load_local_json, persist_key_value,
push_notifications, sliver_app_bar_example, stateless_widgets, tic_tac_toe,
tip_calculator, using_alert_dialog, using_edittext, using_expansionpanel,
using_http_get, using_interactiveviewer, using_snackbar, using_stepper,
using_theme

using_http_get/lib/example.dart

@@ -0,0 +1,89 @@
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:http/http.dart' as http;
+
+// Example: fetch JSON data over HTTP GET and display the results in a list.
+class Example extends StatefulWidget {
+  const Example({super.key});
+
+  @override
+  State<Example> createState() => _ExampleState();
+}
+
+class _ExampleState extends State<Example> {
+  final String url = "https://swapi.dev/api/people";
+  List? data;
+
+  // Function to get the JSON data
+  Future<String> getJSONData() async {
+    var response = await http.get(
+        // Encode the url
+        Uri.parse(url),
+        // Only accept JSON response
+        headers: {"Accept": "application/json"});
+
+    // Logs the response body to the console
+    print(response.body);
+
+    // To modify the state of the app, use this method
+    setState(() {
+      // Get the JSON data
+      var dataConvertedToJSON = json.decode(response.body);
+      try {
+        if (dataConvertedToJSON.statusCode == 200) {
+          // Extract the required part and assign it to the global variable named data
+          data = dataConvertedToJSON['results'];
+        }
+      } catch (e) {
+        print(dataConvertedToJSON.statusCode);
+      }
+    });
+
+    return "Successfull";
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text("Retrieve JSON Data via HTTP GET"),
+      ),
+      // Create a Listview and load the data when available
+      body: ListView.builder(
+          itemCount: data == null ? 0 : data!.length,
+          itemBuilder: (BuildContext context, int index) {
+            return Container(
+              child: Center(
+                  child: Column(
+                // Stretch the cards in horizontal axis
+                crossAxisAlignment: CrossAxisAlignment.stretch,
+                children: <Widget>[
+                  Card(
+                    child: Container(
+                      padding: const EdgeInsets.all(15.0),
+                      child: Text(
+                        // Read the name field value and set it in the Text widget
+                        data![index]['name'],
+                        // set some style to text
+                        style: const TextStyle(
+                            fontSize: 20.0, color: Colors.lightBlueAccent),
+                      ),
+                    ),
+                  )
+                ],
+              )),
+            );
+          }),
+    );
+  }
+
+  @override
+  void initState() {
+    super.initState();
+
+    // Call the getJSONData() method when the app initializes
+    getJSONData();
+  }
+}

using_http_get/lib/main.dart

@@ -1,95 +1,19 @@
 import 'package:flutter/material.dart';
-import 'dart:async';
-import 'package:http/http.dart' as http;
-import 'dart:convert';
+
+import 'example.dart';
 
 void main() {
-  runApp(MaterialApp(
-    home: MyGetHttpData(),
-  ));
+  runApp(const MyApp());
 }
 
-// Create a stateful widget
-class MyGetHttpData extends StatefulWidget {
-  const MyGetHttpData({super.key});
-
-  @override
-  MyGetHttpDataState createState() => MyGetHttpDataState();
-}
-
-// Create the state for our stateful widget
-class MyGetHttpDataState extends State<MyGetHttpData> {
-  final String url = "https://swapi.dev/api/people";
-  List? data;
-
-  // Function to get the JSON data
-  Future<String> getJSONData() async {
-    var response = await http.get(
-        // Encode the url
-        Uri.parse(url),
-        // Only accept JSON response
-        headers: {"Accept": "application/json"});
-
-    // Logs the response body to the console
-    print(response.body);
-
-    // To modify the state of the app, use this method
-    setState(() {
-      // Get the JSON data
-      var dataConvertedToJSON = json.decode(response.body);
-      try {
-        if (dataConvertedToJSON.statusCode == 200) {
-          // Extract the required part and assign it to the global variable named data
-          data = dataConvertedToJSON['results'];
-        }
-      } catch (e) {
-        print(dataConvertedToJSON.statusCode);
-      }
-    });
-
-    return "Successfull";
-  }
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
-      appBar: AppBar(
-        title: Text("Retrieve JSON Data via HTTP GET"),
-      ),
-      // Create a Listview and load the data when available
-      body: ListView.builder(
-          itemCount: data == null ? 0 : data!.length,
-          itemBuilder: (BuildContext context, int index) {
-            return Container(
-              child: Center(
-                  child: Column(
-                // Stretch the cards in horizontal axis
-                crossAxisAlignment: CrossAxisAlignment.stretch,
-                children: <Widget>[
-                  Card(
-                    child: Container(
-                      padding: const EdgeInsets.all(15.0),
-                      child: Text(
-                        // Read the name field value and set it in the Text widget
-                        data![index]['name'],
-                        // set some style to text
-                        style: TextStyle(
-                            fontSize: 20.0, color: Colors.lightBlueAccent),
-                      ),
-                    ),
-                  )
-                ],
-              )),
-            );
-          }),
+    return MaterialApp(
+      title: "Retrieve JSON Data via HTTP GET",
+      home: const Example(),
     );
   }
-
-  @override
-  void initState() {
-    super.initState();
-
-    // Call the getJSONData() method when the app initializes
-    getJSONData();
-  }
 }