小编Mik*_*orn的帖子

我正在使用下拉按钮字段并收到此错误：

A value of type 'Object?' can't be assigned to a variable of type 'String'.
Try changing the type of the variable, or casting the right-hand type to 'String'.dart(invalid_assignment)

代码 ：

    class SettingsForm extends StatefulWidget {
  @override
  _SettingsFormState createState() => _SettingsFormState();
}

class _SettingsFormState extends State<SettingsForm> {
  final _formKey = GlobalKey<FormState>();
  final List<String> sugars = ['0', '1', '2', '3', '4'];
  final List<int> strengths = [100, 200, 300, 400, 500, 600, 700, 800, 900];

  // form values
   String? _currentName;
   String? _currentSugars; …

changenotifierprovider我在我的 flutter 项目中使用时遇到错误。

错误：

“MyUser”不符合绑定的“ChangeNotifier？” 类型参数“T”。尝试使用“ChangeNotifier?”或其子类的类型。

班级：

class SettingsForm extends StatefulWidget {
  static Widget getWidget() {
    return new Provider(
      create: (_) => MyUser(uid: ''),
      child: ChangeNotifierProvider(  <-- here **ChangeNotifierProvider**
          create: (BuildContext context) => MyUser(uid: ''),
          builder: (_, _) => SettingsForm()),
      )
    );
  }

运行 flutter 项目时出现此 gradle 错误：

我正在使用 github 上的代码来查看 flutter pdf 视图，并使用 firebase 来查看它。

Launching lib\main.dart on AOSP on IA Emulator in debug mode...
/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/path_provider_windows-0.0.4+3/lib/src/path_provider_windows_real.dart:126:25: Error: The getter 'addressOf' isn't defined for the class 'GUID'.
 - 'GUID' is from 'package:win32/src/structs.dart' ('/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/win32-1.7.4+1/lib/src/structs.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named 'addressOf'.
          knownFolderID.addressOf, KF_FLAG_DEFAULT, NULL, pathPtrPtr);

                        ^^^^^^^^^

      ^
/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/ffi-0.1.3/lib/src/utf16.dart:16:7: Error: Struct 'Utf16' is empty. Empty structs are undefined behavior.

class Utf16 extends Struct …

关于 flutter firebase 应用程序构建。Stream 函数有一条红线。目前使用 2 个文件 auth.dart 和 user.dart

我的代码 auth.dart 文件：

import 'package:coffee_dan/models/user.dart';
import 'package:firebase_auth/firebase_auth.dart';

class AuthService {
  final FirebaseAuth _auth = FirebaseAuth.instance;

  MyUser Function(User? event)? get userFromFirebaseUser => null;

  MyUser? _userFromFirebaseUser(User user) {
    // ignore: unnecessary_null_comparison
    return user!= null ? MyUser(uid: user.uid) : null;
  }

  Stream<MyUser> get user {
    return _auth.authStateChanges().map(_userFromFirebaseUser);  Red Line in here **_userFromFirebaseUser**
  }

  Future signInAnon() async {
    try {
      UserCredential result = await _auth.signInAnonymously();
      User? user = result.user;
      return _userFromFirebaseUser(user!);
    } catch (e) …