Understanding Dart private class [duplicate]

Question

In Flutter we commonly have something like this:

class MyStatefulWidget extends StatefulWidget {
  @override
  _MyState createState() => _MyState();
}

class _MyState extends State<MyStatefulWidget> {
  void doSomething() => print('hi');

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

So _MyState is declared with a _, which makes it library private.

So how come the Flutter render engine can use _MySate if it's sopposed to be private?

It's funny because I can access doSomething() from other files, but if I make it _doSomething(), I can't access it anymore...So how come I can access a public method from a private class, but I can't access a private method from a private class?

Answer 1

While _MyState is private, StatefulWidget and State are not.

The framework doesn't manipulate _MyState, it manipulates these lower layer that he has access to, with a well-defined prototype.

This basically sums up into:

StatefulWidget widget;
State foo = widget.createState();

foo.initState();
final newWidget = foo.build(this);
...