image doesn't display on the app even though it was in assets folder

Question

I added the image file on assets folder. but when I run the app, image doesn't display at all. and there isn't any error message. I don't know the reason why I cannot see the picture I added. (of course, it isn't the white picture. It isthe normal picture.) Please let me know the reason.

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Title'),
      ),
      body: Center(
        child: GestureDetector(
          onTap: (){
            Navigator.push(
                context,
                MaterialPageRoute(builder: (context) => HeroDetailPage()),
            );
          },
          child: Hero(
            tag: 'image',
            child: Image.asset(
                'assets/sample.jpg',
              width: 100,
              height: 100,
            ),
          ),
        ),
      ),
    );
  }
}

class HeroDetailPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Hero Detail'),
      ),
      body: Hero(
        tag: 'image',
        child: Image.asset('assets/sample.jpg'),
      ),
    );
  }
}

Answer 1

Please check: https://flutter.dev/docs/development/ui/assets-and-images first.

There you can find this:

Flutter uses the pubspec.yaml file, located at the root of your project, to identify assets required by an app.

Here is an example:

flutter:
  assets:
    - assets/my_icon.png
    - assets/background.png

To include all assets under a directory, specify the directory name with the / character at the end:

flutter:
  assets:
    - directory/
    - directory/subdirectory/