Estados de carga, error y vacio en Flutter: ejercicio resuelto

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import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(home: ExercisePage()));
}

class ExercisePage extends StatefulWidget {
  const ExercisePage({super.key});

  @override
  State<ExercisePage> createState() => _ExercisePageState();
}

class _ExercisePageState extends State<ExercisePage> {
  bool loading = false;
  String message = 'Estado inicial';

  Future<void> runExercise() async {
    setState(() {
      loading = true;
      message = 'Procesando...';
    });

    await Future<void>.delayed(const Duration(milliseconds: 700));

    if (!mounted) return;
    setState(() {
      loading = false;
      message = 'Ejercicio completado';
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Flutter ejercicio resuelto')),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            Text(message),
            const SizedBox(height: 12),
            ElevatedButton(
              onPressed: loading ? null : runExercise,
              child: Text(loading ? 'Cargando...' : 'Ejecutar'),
            ),
          ],
        ),
      ),
    );
  }
}