android - Dart Flutter "Non-nullable instance field ' _isSigningIn' 必须初始化。”错误

Question

我正在尝试使用 Dart Flutter 进行 Google Auth 操作。
我制作了与我观看的视频中完全相同的代码。虽然它没有在视频中给出错误，但它给了我一个错误。
我的代码:

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:google_sign_in/google_sign_in.dart';

class GoogleSignInProvider extends ChangeNotifier {
  final googleSignIn = GoogleSignIn();
  bool _isSigningIn;

  GoogleSignInProvider() {
    _isSigningIn = false;
  }

  bool get isSigningIn => _isSigningIn;

  set isSigningIn(bool isSigningIn) {
    _isSigningIn = isSigningIn;
    notifyListeners();
  }

  Future login() async {
    isSigningIn = true;

    final user = await googleSignIn.signIn();
    if (user == null) {
      isSigningIn = false;
      return;
    } else {
      final googleAuth = await user.authentication;

      final credential = GoogleAuthProvider.credential(
        accessToken: googleAuth.accessToken,
        idToken: googleAuth.idToken,
      );

      await FirebaseAuth.instance.signInWithCredential(credential);

      isSigningIn = false;
    }
  }

  void logout() async {
    await googleSignIn.disconnect();
    FirebaseAuth.instance.signOut();
  }
}

错误:

GoogleSignInProvider GoogleSignInProvider()
package:todolist/google_sign_in.dart

Non-nullable instance field '_isSigningIn' must be initialized.
Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.

问题是什么？我该如何解决？我提前感谢您的帮助。

Answer 1

因为您的值不可为空，所以它需要一个默认值

bool _isSigningIn; //non-nullable variable should be inialized
bool? _isSigningIn; //nullable variable and does not requires initialization

如果你让你的变量可以为空，你可以让它保持未初始化，在其他情况下，你应该在定义它时初始化它

bool _isSigningIn = false;

或者在你的构造函数里面有一个值，像这样:

GoogleSignInProvider(this._isSigningIn);

还有一个选项，你也可以给一个默认值:

将变量放入 [] 并给出默认值，它将是位置参数，但不是必需的

请参见下面的示例:

class GoogleSignInProvider extends ChangeNotifier {
  final googleSignIn = GoogleSignIn();
  bool _isSigningIn;
  
  GoogleSignInProvider([this._isSigningIn = false]);
  
}