How a const widget works with its children?-CodePudding

Let's say I have this HomePage widget:

class HomePage extends StatelessWidget {
  const HomePage({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Scaffold(),
    );
  }
}

I am able to use a const constructor at the root (MaterialApp widget) because all children do have const constructors too.

If I add the AppBar widget, I will have to remove the const constructor from the root, because AppBar does not have a const constructor.

class HomePage extends StatelessWidget {
  const HomePage({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp( // no const anymore
      home: Scaffold(
        appBar: AppBar(...),
      ),
    );
  }
}

But why am I able to use the HomePage widget with a const constructor ? See code below:

class App extends StatelessWidget {
  const App({super.key});

  @override
  Widget build(BuildContext context) {
    return const HomePage(); // using a const constructor
  }
}

I thought it would be impossible because of the AppBar child widget.

CodePudding user response：

Because the HomePage class is immutable. The build function implementation doesn't affect it.

The const keyword can be used when the object is immutable. A Compiler allocates the same portion of memory for all objects.

For more info, reference the Using constructors: Dart doc