mainexplain

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(

        primarySwatch: Colors.blue,
      ),
      home:  const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatelessWidget {
   const MyHomePage({Key? key, this.title}) : super(key: key);

   final String? title;
   @override
   Widget build(BuildContext context) {
      return Scaffold(
         appBar: AppBar(title: Text(title!), ),
         body: const Center(child: Text( 'Hello World',)),
      );
   }
}

1. Import Flutter packages:

   import 'package:flutter/material.dart';

   This line imports the necessary Flutter packages, including material.dart, which provides widgets for building material design applications.

2. Define the main function and runApp:

   void main() {
     runApp(const MyApp());
   }

   The main function is the entry point of the application. It calls runApp to start the Flutter application and render the MyApp widget.

3. MyApp Class:

   class MyApp extends StatelessWidget {
     const MyApp({Key? key}) : super(key: key);

   This class represents the top-level widget of your application. It is a stateless widget, so it doesn't have mutable state. It defines the overall structure of the app.

   • title: The title of the app is set to "Flutter Demo."

   • theme: The theme is set to use the primary color swatch of blue.

   • home: The MyHomePage widget is set as the home screen of the app with the title "Flutter Demo Home Page."

4. MyHomePage Class:

   class MyHomePage extends StatelessWidget {
     const MyHomePage({Key? key, this.title}) : super(key: key);

   This class represents the main content of the app, which is a simple page with an app bar and a "Hello World" message in the center. It also inherits from StatelessWidget.

   • title: The title for the app bar is passed as a parameter and displayed in the app bar.

   • The build method creates a Scaffold widget, which provides the basic material design structure for the screen.

   • The appBar property sets an AppBar with the title set to the provided title.

   • The body property sets the main content to be a centered Text widget with the text "Hello World."

     UML

     Widget Tree

Copy code
+-----------------------------------+
|             StatelessWidget         |
|           MyApp                   |
+-----------------------------------+
| - title: String                   |
+-----------------------------------+
| + build(BuildContext): Widget       |
+-----------------------------------+
          |
          |
          V
+-----------------------------------+
|             StatelessWidget         |
|           MyHomePage              |
+-----------------------------------+
| - title: String                   |
+-----------------------------------+
| + build(BuildContext): Widget       |
+-----------------------------------+