Why build method isn't defined inside StatefulWidget?

Question

I'm currently learning Flutter. I tried to deep dive into Flutter Widget life-cycle, and I wonder why StatefulWidget are written like this :

class Example extends StatefulWidget {
  @override
  _ExampleState createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  // initState
  // setState
  // ...
  @override
  Widget build(BuildContext build) {
    ...
  }
}

but not :

class Example extends StatefulWidget {
  // initState
  // setState
  // ...
  @override
  Widget build(BuildContext build) {
    ...
  }
}

I think the latter makes the source simple. But I don't know why they're using the former style ?

Answer 1

The reason why StatefulWidget uses a separate State class and not having build method inside its body is because all fields inside a Widget are immutable, and this includes all its sub-classes.

You might have noticed that StatelessWidget has its build and other associated methods defined inside it, but that was possible due to the nature of StatelessWidget which is rendered completely using the provided info, and doesn't expect any future change in its State.

In the case of StatefulWidget, State information occasionally change (or expected to change) during the course of the app, thus this information isn't suitable for storage in a final field (build) to satisfy Widget class conditions (all fields are immutable). That's why State class is introduced. You just have to override the createState function to attach your defined State to your StatefulWidget, and let all that change happens in a separate class.