参数‘firebaseFirestore’因其类型不能有‘null’值，但flutter中隐含的默认值为‘null’

Question

我声明了FirebaseFirestore 和FireBaseAuth 类型的两个变量，现在我面临一个空安全错误。我在我的类中使用了一个构造函数，并将它们作为参数传递，得到一个错误 - The parameter 'firebaseFirestore' can't have a value of 'null' because of its type, but the implicit default value is 'null' in flutter 和 The parameter 'firebaseAuth' can't have a value of 'null' because of its type, but the implicit default value is 'null' in flutter。

构成我的问题的代码部分是-

import 'package:flutter_instagram_clone_final/repositories/repositories.dart';
import 'package:firebase_auth/firebase_auth.dart' as auth;

class AuthRepository extends BaseAuthRepository {

  final FirebaseFirestore _firebaseFirestore;
  final auth.FirebaseAuth _firebaseAuth;

  AuthRepository({
    FirebaseFirestore firebaseFirestore,
    auth.FirebaseAuth firebaseAuth,
  })  : _firebaseFirestore = firebaseFirestore ?? FirebaseFirestore.instance,
        _firebaseAuth = firebaseAuth ?? auth.FirebaseAuth.instance;

然后我尝试用实例初始化它，比如 -

    FirebaseFirestore firebaseFirestore = FirebaseFirestore.instance,

但它给了我一个错误 - The default value of an optional parameter must be constant.

Answer 1

为了保证您永远不会看到带有不可为空的空参数 类型，类型检查器要求所有可选参数都具有 可空类型或默认值。

Understanding null safety | Dart

您需要将firebaseFirestore 和firebaseAuth 设为可空，因为它们是可选的，并且它们的默认值不是常量。

您可以通过在类型后面添加? 来做到这一点。

像这样：

    FirebaseFirestore? firebaseFirestore,
    auth.FirebaseAuth? firebaseAuth,

所以你的构造函数可以更新为：

    AuthRepository({
        FirebaseFirestore? firebaseFirestore,
        auth.FirebaseAuth? firebaseAuth,
      })  : _firebaseFirestore = firebaseFirestore ?? FirebaseFirestore.instance,
            _firebaseAuth = firebaseAuth ?? auth.FirebaseAuth.instance;