持久化存储架构：SQL

dart

ListenableBuilder(
  listenable: widget.viewModel,
  builder: (context, child) {
    return ListView.builder(
      itemCount: widget.viewModel.todos.length,
      itemBuilder: (context, index) {
        final todo = widget.viewModel.todos[index];
        return ListTile(
          title: Text(todo.task),
          trailing: IconButton(
            icon: const Icon(Icons.delete),
            onPressed: () => widget.viewModel.delete.execute(todo.id),
          ),
        );
      },
    );
  },
)

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dart

List<Todo> _todos = [];
​
List<Todo> get todos => _todos;
​
Future<Result<void>> _load() async {
  try {
    final result = await _todoRepository.fetchTodos();
    switch (result) {
      case Ok<List<Todo>>():
        _todos = result.value;
        return Result.ok(null);
      case Error():
        return Result.error(result.error);
    }
  } on Exception catch (e) {
    return Result.error(e);
  } finally {
    notifyListeners();
  }
}

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dart

FilledButton.icon(
  onPressed: () =>
      widget.viewModel.add.execute(_controller.text),
  label: const Text('Add'),
  icon: const Icon(Icons.add),
)

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dart

Future<Result<void>> _add(String task) async {
  try {
    final result = await _todoRepository.createTodo(task);
    switch (result) {
      case Ok<Todo>():
        _todos.add(result.value);
        return Result.ok(null);
      case Error():
        return Result.error(result.error);
    }
  } on Exception catch (e) {
    return Result.error(e);
  } finally {
    notifyListeners();
  }
}

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dart

void _onAdd() {
  // Clear the text field when the add command completes.
  if (widget.viewModel.add.completed) {
    widget.viewModel.add.clearResult();
    _controller.clear();
  }
}

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dart

IconButton(
  icon: const Icon(Icons.delete),
  onPressed: () => widget.viewModel.delete.execute(todo.id),
)

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dart

Future<Result<void>> _delete(int id) async {
  try {
    final result = await _todoRepository.deleteTodo(id);
    switch (result) {
      case Ok<void>():
        _todos.removeWhere((todo) => todo.id == id);
        return Result.ok(null);
      case Error():
        return Result.error(result.error);
    }
  } on Exception catch (e) {
    return Result.error(e);
  } finally {
    notifyListeners();
  }
}

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dart

@freezed
abstract class Todo with _$Todo {
  const factory Todo({
    /// The unique identifier of the Todo item.
    required int id,
​
    /// The task description of the Todo item.
    required String task,
  }) = _Todo;
}

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dart

class TodoRepository {
  TodoRepository({required DatabaseService database}) : _database = database;
​
  final DatabaseService _database;
​
  Future<Result<List<Todo>>> fetchTodos() async {
    if (!_database.isOpen()) {
      await _database.open();
    }
    return _database.getAll();
  }
​
  Future<Result<Todo>> createTodo(String task) async {
    if (!_database.isOpen()) {
      await _database.open();
    }
    return _database.insert(task);
  }
​
  Future<Result<void>> deleteTodo(int id) async {
    if (!_database.isOpen()) {
      await _database.open();
    }
    return _database.delete(id);
  }
}

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dart

static const String _todoTableName = 'todo';
static const String _idColumnName = '_id';
static const String _taskColumnName = 'task';

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dart

Future<void> open() async {
  _database = await databaseFactory.openDatabase(
    join(await databaseFactory.getDatabasesPath(), 'app_database.db'),
    options: OpenDatabaseOptions(
      onCreate: (db, version) {
        return db.execute(
          'CREATE TABLE $_todoTableName($_idColumnName INTEGER PRIMARY KEY AUTOINCREMENT, $_taskColumnName TEXT)',
        );
      },
      version: 1,
    ),
  );
}

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dart

Future<Result<Todo>> insert(String task) async {
  try {
    final id = await _database!.insert(_todoTableName, {
      _taskColumnName: task,
    });
    return Result.ok(Todo(id: id, task: task));
  } on Exception catch (e) {
    return Result.error(e);
  }
}

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dart

Future<Result<List<Todo>>> getAll() async {
  try {
    final entries = await _database!.query(
      _todoTableName,
      columns: [_idColumnName, _taskColumnName],
    );
    final list = entries
        .map(
          (element) => Todo(
            id: element[_idColumnName] as int,
            task: element[_taskColumnName] as String,
          ),
        )
        .toList();
    return Result.ok(list);
  } on Exception catch (e) {
    return Result.error(e);
  }
}

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dart

Future<Result<void>> delete(int id) async {
  try {
    final rowsDeleted = await _database!.delete(
      _todoTableName,
      where: '$_idColumnName = ?',
      whereArgs: [id],
    );
    if (rowsDeleted == 0) {
      return Result.error(Exception('No todo found with id $id'));
    }
    return Result.ok(null);
  } on Exception catch (e) {
    return Result.error(e);
  }
}

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dart

void main() {
  late DatabaseService databaseService;
  if (kIsWeb) {
    throw UnsupportedError('Platform not supported');
  } else if (Platform.isLinux || Platform.isWindows || Platform.isMacOS) {
    // Initialize FFI SQLite
    sqfliteFfiInit();
    databaseService = DatabaseService(databaseFactory: databaseFactoryFfi);
  } else {
    // Use default native SQLite
    databaseService = DatabaseService(databaseFactory: databaseFactory);
  }
​
  runApp(
    MainApp(
      // ···
      todoRepository: TodoRepository(database: databaseService),
    ),
  );
}

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dart

TodoListScreen(
  viewModel: TodoListViewModel(todoRepository: widget.todoRepository),
)

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