List<>

первая версия

// Some code
class TaskApp extends StatefulWidget {
  const TaskApp({super.key});

  @override
  State<TaskApp> createState() => _TaskAppState();
}

class _TaskAppState extends State<TaskApp> {
  TextEditingController taskController = TextEditingController();
  TextEditingController editingController = TextEditingController();

  final _formKey = GlobalKey<FormState>();

  // CRUD - create - read - update - delete

  // =,
  // bool ? первый вариант : второй вариант
  // методы -

  List<String> tasks = ['Learn flutter', 'Hello', 'Say'];

  void back() {
    Navigator.pop(context);
  }

  void create() {
    if (_formKey.currentState!.validate()) {
      setState(() {
        tasks.add(taskController.text);
        taskController.clear();
        back();
      });
    }
  }

  void delete({required int index}) {
    setState(() {
      tasks.removeAt(index);
      back();
    });
  }

  void update ({required int index}){
    setState(() {
      tasks[index] = editingController.text;
      editingController.clear();
      back();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: tasks.isEmpty
          ? Center(
              child: Text(
                'Пока задач нету ',
                style: TextStyle(
                  fontSize: 30,
                ),
              ),
            )
          : ListView.builder(
              itemCount: tasks.length,
              itemBuilder: (context, index) {
                return ListTile(
                  leading: Text(
                    '${index}:',
                    style: TextStyle(fontSize: 25),
                  ),
                  title: Text(
                    tasks[index],
                    style: TextStyle(fontSize: 25),
                  ),

                  trailing: Row(
                    mainAxisSize: MainAxisSize.min,
                    children: [
                      IconButton(
                        onPressed: () {
                          showDialog(
                            context: context,
                            builder: (context) {
                              return AlertDialog(
                                title: Text('Вы действительно хотите удалить?'),
                                content: Icon(
                                  Icons.cancel_outlined,
                                  color: Colors.red,
                                  size: 40,
                                ),
                                actions: [
                                  TextButton(
                                    onPressed: () {
                                      back();
                                    },
                                    child: Text('Отмена'),
                                  ),
                                  TextButton(
                                    onPressed: () {
                                      delete(index: index);
                                    },
                                    child: Text('Уадить'),
                                  ),
                                ],
                              );
                            },
                          );
                        },
                        icon: Icon(
                          Icons.delete,
                        ),
                      ),
                      IconButton(
                        onPressed: () {
                          showDialog(
                            context: context,
                            builder: (context) {
                              editingController.text = tasks[index];
                              return Form(
                                key: _formKey,
                                child: AlertDialog(
                                  title: Text('Обновить'),
                                  content: TextFormField(
                                    controller: editingController,
                                    validator: (value) {
                                      if (value!.isEmpty) {
                                        return 'Не может быть паустым';
                                      }
                                      return null;
                                    },
                                    decoration: InputDecoration(
                                      border: OutlineInputBorder(),
                                      hintText: 'напишите задачу',
                                    ),
                                  ),
                                  actions: [
                                    TextButton(
                                      onPressed: () {
                                        back();
                                      },
                                      child: Text('Отмена'),
                                    ),
                                    TextButton(
                                      onPressed: () {
                                        update(index: index);
                                      },
                                      child: Text('Обновить'),
                                    ),
                                  ],
                                ),
                              );
                            },
                          );
                        },
                        icon: Icon(Icons.edit),
                      ),
                    ],
                  ),
                );
              },
            ),

      floatingActionButton: FloatingActionButton(
        onPressed: () {
          showDialog(
            context: context,
            builder: (context) {
              return Form(
                key: _formKey,
                child: AlertDialog(
                  title: Text('Создать задачу'),
                  content: TextFormField(
                    controller: taskController,
                    validator: (value) {
                      if (value!.isEmpty) {
                        return 'Не может быть паустым';
                      }
                      return null;
                    },
                    decoration: InputDecoration(
                      border: OutlineInputBorder(),
                      hintText: 'напишите задачу',
                    ),
                  ),
                  actions: [
                    TextButton(
                      onPressed: () {
                        back();
                      },
                      child: Text('Отмена'),
                    ),
                    TextButton(
                      onPressed: () {
                        create();
                      },
                      child: Text('Добавить'),
                    ),
                  ],
                ),
              );
            },
          );
        },
        child: Text('Создать'),
      ),
    );
  }
}

Вторая версия

// Some code
class TaskApp extends StatefulWidget {
  const TaskApp({super.key});

  @override
  State<TaskApp> createState() => _TaskAppState();
}


class _TaskAppState extends State<TaskApp> {
  TextEditingController taskController = TextEditingController();
  TextEditingController editingController = TextEditingController();

  final _formKey = GlobalKey<FormState>();

  // CRUD - create - read - update - delete

  // =,
  // bool ? первый вариант : второй вариант
  // методы -

  List<String> tasks = ['Learn flutter', 'Hello', 'Say'];

  void back() {
    Navigator.pop(context);
  }

  void create() {
    if (_formKey.currentState!.validate()) {
      setState(() {
        tasks.add(taskController.text);
        taskController.clear();
        back();
      });
    }
  }

  void delete({required int index}) {
    setState(() {
      tasks.removeAt(index);
      back();
    });
  }

  void update({required int index}) {
    setState(() {
      tasks[index] = editingController.text;
      editingController.clear();
      back();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: tasks.isEmpty
          ? Center(
              child: Text(
                'Пока задач нету ',
                style: TextStyle(
                  fontSize: 30,
                ),
              ),
            )
          : ListView.builder(
              itemCount: tasks.length,
              itemBuilder: (context, index) {
                return ListTile(
                  leading: Text(
                    '${index}:',
                    style: TextStyle(fontSize: 25),
                  ),
                  title: Text(
                    tasks[index],
                    style: TextStyle(fontSize: 25),
                  ),

                  trailing: Row(
                    mainAxisSize: MainAxisSize.min,
                    children: [
                      IconButton(
                        onPressed: () {
                          showDialog(
                            context: context,
                            builder: (context) {
                              return AlertDialogWidget(
                                title: 'удалить',
                                text: 'Удалить',
                                isShow: false,
                                onPressed: (){
                                  delete(index: index);
                                },
                              );
                            },
                          );
                        },
                        icon: Icon(
                          Icons.delete,
                        ),
                      ),
                      IconButton(
                        onPressed: () {
                          showDialog(
                            context: context,
                            builder: (context) {
                              editingController.text = tasks[index];
                              return Form(
                                key: _formKey,
                                child: AlertDialogWidget(
                                  title: 'Обновить задачу',
                                  controller: editingController,
                                  hinText: 'Обновите задачу',
                                  text: 'Обновить',
                                  onPressed: () {
                                    update(index: index);
                                  },
                                ),
                              );
                            },
                          );
                        },
                        icon: Icon(Icons.edit),
                      ),
                    ],
                  ),
                );
              },
            ),

      floatingActionButton: FloatingActionButton(
        onPressed: () {
          showDialog(
            context: context,
            builder: (context) {
              return Form(
                key: _formKey,
                child: AlertDialogWidget(
                  title: 'Создать Задачу',
                  controller: taskController,
                  hinText: 'напишите задачу',
                  text: 'Добавить',
                  onPressed: () {
                    create();
                  },
                ),
              );
            },
          );
        },
        child: Text('Создать'),
      ),
    );
  }
}


class AlertDialogWidget extends StatelessWidget {
  final String title;
  final TextEditingController? controller;
  final String? hinText;
  final String text;
  final Function() onPressed;
  final bool isShow;

  const AlertDialogWidget({
    super.key,
    required this.title,
     this.controller,
     this.hinText,
    required this.text,
    required this.onPressed,
    this.isShow = true,
  });

  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: Text('Создать задачу'),
      content: isShow
          ? TextFormField(
              controller: controller,
              validator: (value) {
                if (value!.isEmpty) {
                  return 'Не может быть паустым';
                }
                return null;
              },
              decoration: InputDecoration(
                border: OutlineInputBorder(),
                hintText: hinText,
              ),
            )
          : Icon(
              Icons.cancel_outlined,
              color: Colors.red,
              size: 45,
            ),
      actions: [
        TextButton(
          onPressed: () {
            Navigator.pop(context);
          },
          child: Text('Отмена'),
        ),
        TextButton(
          onPressed: onPressed,
          child: Text(text),
        ),
      ],
    );
  }
}


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

  @override
  State<Buttons> createState() => _ButtonsState();
}

class _ButtonsState extends State<Buttons> {

  void nextPage(){
    Navigator.push(context, MaterialPageRoute(builder: (context) => TaskApp()));
  }


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [

          ElevatedButton(onPressed: (){

            nextPage();

          }, child: Text(''))

        ],
      ),
    );
  }
}