Flutter 如何使 dart 函数变得通用

遗憾的是，不可能在DART中(或至少在Fighter中)以通用的方式实现这一点.类的静态成员不是泛型函数可用的类型参数的一部分，因此无法在泛型类型上调用工厂构造函数或静态方法.实现此功能的最直接方法是将解析回调传递给函数:

Future<List<T>> read<T>(T Function(Map<String, dynamic>) parser) async {
  final snapshot = await _collection.get();
  final data = <User>[];

  for (final document in snapshot.docs) {
    final json = document.data() as Map<String, dynamic>;
    final object = parser(json);
    data.add(object);
  }

  return data;
}

final result = await manager.read<User>(User.fromJson);
final result = await manager.read<Address>(Address.fromJson);
final result = await manager.read<Contacts>(Contacts.fromJson);

还有一种稍微更方便的方法，尽管它需要一些设置并且不能动态使用.不是将解析方法传递给read函数，而是保留一个具有已知解析函数的类型查找表.然后，您可以使用泛型类型作为键，直接在read函数中使用它们:

typedef T _Parser<T>(Map<String, dynamic> map);
final Map<Type, _Parser> _parsers = {
  User: User.fromJson,
  Address: Address.fromJson,
  Contacts: Contacts.fromJson,
};

class FirebaseManager implements DataSourceRepository {
  // ...
  
  @override
  Future<List<T>> read<T>() async {
    final parser = _parsers[T];
    if (parser == null) {
      throw ArgumentError('Unsupported parsable type $T');
    }
    
    final snapshot = await _collection.get();
    final data = <T>[];

    for (final document in snapshot.docs) {
      final json = document.data() as Map<String, dynamic>;
      final object = parser(json) as T;
      data.add(object);
    }

    return data;
  }
}

final result = await manager.read<User>();
final result = await manager.read<Address>();
final result = await manager.read<Contacts>();