dart中解析对象（不支持的操作：无法添加到固定长度列表）

我有一个用户对象，当用户登录/注册时，该对象保存到云 Firestore 数据库中。 因此，当用户登录时，将从数据库中检索用户对象，并且一切正常，直到我尝试对列表“usersProject”执行“添加”操作：

// Add the new project ID to the user's project list
user.userProjectsIDs.add(projectID);

所以我得到了例外Unhandled Exception: Unsupported operation: Cannot add to a fixed-length list我相信问题在于将用户从 json 转换为对象时，因为当用户注册时，对象会转换为 json 并存储在数据库中，并且用户将在转换之前使用该对象自动登录。

void createUser(String email, String password, String username, String name, String birthDate) async {
try {
  // Check first if username is taken
  bool usernameIsTaken = await UserProfileCollection()
      .checkIfUsernameIsTaken(username.toLowerCase().trim());
  if (usernameIsTaken) throw FormatException("Username is taken");

  // Create the user in the Authentication first
  final firebaseUser = await _auth.createUserWithEmailAndPassword(
      email: email.trim(), password: password.trim());

  // Encrypting the password
  String hashedPassword = Password.hash(password.trim(), new PBKDF2());

  // Create new list of project for the user
  List<String> userProjects = new List<String>();

  // Create new list of friends for the user
  List<String> friends = new List<String>();

  // Creating user object and assigning the parameters
  User _user = new User(
    userID: firebaseUser.uid,
    userName: username.toLowerCase().trim(),
    email: email.trim(),
    password: hashedPassword,
    name: name,
    birthDate: birthDate.trim(),
    userAvatar: '',
    userProjectsIDs: userProjects,
    friendsIDs: friends,
  );

  // Create a new user in the fire store database
  await UserProfileCollection().createNewUser(_user);
  
  // Assigning the user controller to the 'user' object
    Get.find<UserController>().user = _user;
    Get.back();

} catch (e) {
  print(e.toString());
}}

当用户注销后，再登录并尝试对用户对象进行操作时，就会出现一些属性（List类型）无法使用的问题。 此代码创建项目并添加projectID到用户列表

  Future<void> createNewProject(String projectName, User user) async {

String projectID = Uuid().v1(); // Project ID, UuiD is package that generates random ID

// Add the creator of the project to the members list and assign him as admin
var member = Member(
  memberUID: user.userID,
  isAdmin: true,
);
List<Member> membersList = new List();
membersList.add(member);

// Save his ID in the membersUIDs list
List <String> membersIDs = new List();
membersIDs.add(user.userID);

// Create chat for the new project
var chat = Chat(chatID: projectID);

// Create the project object
var newProject = Project(
  projectID: projectID,
  projectName: projectName,
  image: '',
  joiningLink: '$projectID',
  isJoiningLinkEnabled: true,
  pinnedMessage: '',
  chat: chat,
  members: membersList,
  membersIDs: membersIDs,
);


// Add the new project ID to the user's project list
user.userProjectsIDs.add(projectID);

try {
  // Convert the project object to be a JSON
  var jsonUser = user.toJson();

  // Send the user JSON data to the fire base
  await Firestore.instance
      .collection('userProfile')
      .document(user.userID)
      .setData(jsonUser);

  // Convert the project object to be a JSON
  var jsonProject = newProject.toJson();

  // Send the project JSON data to the fire base
  return await Firestore.instance
      .collection('projects')
      .document(projectID)
      .setData(jsonProject);
} catch (e) {
  print(e);
}}

这里是异常发生的地方仅当用户注销然后登录时但他第一次报名的时候也不会有例外。

 // Add the new project ID to the user's project list
user.userProjectsIDs.add(projectID);

签到功能

void signIn(String email, String password) async {
try {
  // Signing in
  FirebaseUser firebaseUser = await _auth.signInWithEmailAndPassword(email: email.trim(), password: password.trim());

  // Getting user document form firebase
  DocumentSnapshot userDoc = await UserProfileCollection().getUser(firebaseUser.uid);
 

  // Converting the json data to user object and assign the user object to the controller
  Get.find<UserController>().user = User.fromJson(userDoc.data);
  print(Get.find<UserController>().user.userName);

} catch (e) {
  print(e.toString());
}}

我认为问题是由User.fromJson为什么它使 firestore 中的数组不可修改？

用户等级

class User {
  String userID;
  String userName;
  String email;
  String password;
  String name;
  String birthDate;
  String userAvatar;
  List<String> userProjectsIDs;
  List<String> friendsIDs;

  User(
      {this.userID,
      this.userName,
      this.email,
      this.password,
      this.name,
      this.birthDate,
      this.userAvatar,
      this.userProjectsIDs,
      this.friendsIDs});

  User.fromJson(Map<String, dynamic> json) {
    userID = json['userID'];
    userName = json['userName'];
    email = json['email'];
    password = json['password'];
    name = json['name'];
    birthDate = json['birthDate'];
    userAvatar = json['UserAvatar'];
    userProjectsIDs = json['userProjectsIDs'].cast<String>();
    friendsIDs = json['friendsIDs'].cast<String>();
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['userID'] = this.userID;
    data['userName'] = this.userName;
    data['email'] = this.email;
    data['password'] = this.password;
    data['name'] = this.name;
    data['birthDate'] = this.birthDate;
    data['UserAvatar'] = this.userAvatar;
    data['userProjectsIDs'] = this.userProjectsIDs;
    data['friendsIDs'] = this.friendsIDs;
    return data;
  }
}

只需添加可增长的参数..

如果 [growable] 为 false（默认值），则列表是长度为零的固定长度列表。如果 [growable] 为 true，则列表是可增长的并且相当于 []。

final growableList = List.empty(growable: true);