Flutter test - Force rebuild of a widget

Question

I'm trying to write tests to my flutter widgets.

Let's say I have this stateful widget:

class A extends StatefulWidget {
  const A();
  
  @override
  AState createState() => AState();
}

class AState extends State<A> {
  bool value = false;

  @override
  Widget build(BuildContext context) {
    final newValue = getNewValue(); // When the widget is rebuilt, I'm getting a value which can be `false` or `true`. I just want the widget to perfom actions if it changed;
    if (value != newValue) {
      setState(() {
        value = newValue;
      });
      WidgetsBinding.instance.addPostFrameCallback((_) async {
        if (mounted) {
          // Do something, like show a snackbar for example
        }
      });
    }
    return const SizedBox.shrink();
  }
}

When the widget rebuilds, it looks if the value from getNewValue changed and rebuilds itself and then performs some actions with WidgetsBinding.instance.addPostFrameCallback. (I only want the widget to perform this action when it rebuilds, and not subscribing to getNewValue is wanted in my case).

Anyway, I wrote a test for this and where I pump the widget

// Mock `getNewValue` here so it returns `true`
const widget = A();
await tester.pumpWidget(widget);
await tester.pump();
// Verify the action after the `WidgetsBinding.instance.addPostFrameCallback` as been done

which works fine.

But now I want to force the widget to rebuild to test its behavior.

This is how I managed to do it (I call setState):

// Mock `getNewValue` here so it returns `false` (change since last build)
tester.state<AState>(find.byType(A)).setState(() {});
await tester.pump();
await tester.pump();
// Verify the action after the `WidgetsBinding.instance.addPostFrameCallback` as been done

And this works fine too.

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However, calling the setState raises the warning

The member 'setState' can only be used within instance members of subclasses of 'package:flutter/src/widgets/framework.dart'.dart(invalid_use_of_protected_member)

and this trick won't work for a StatelessWidget

---

I was wondering what was the proper way to force a rebuild of a widget in a flutter test?