问题描述

error • The parameter 'name' can't have a value of 'null' because of its type, but the implicit default value is 'null' at lib\models\chat_model.dart:7:19 • (missing_default_value_for_parameter)
error • The parameter 'message' can't have a value of 'null' because of its type, but the implicit default value is 'null' at lib\models\chat_model.dart:7:30 • (missing_default_value_for_parameter)
error • The parameter 'time' can't have a value of 'null' because of its type, but the implicit default value is 'null' at lib\models\chat_model.dart:7:44 • (missing_default_value_for_parameter)
error • The parameter 'avatarUrl' can't have a value of 'null' because of its type, but the implicit default value is 'null' at lib\models\chat_model.dart:7:55 • (missing_default_value_for_parameter)

如图：

报错的代码：

class ChatModel {
  final String name;
  final String message;
  final String time;
  final String avatarUrl;

  ChatModel({this.name, this.message, this.time, this.avatarUrl});
}

报错是说参数不能为null，但是可能存在隐形的赋值为null的可能。代码修改成如下

class ChatModel {
  final String? name;
  final String? message;
  final String? time;
  final String? avatarUrl;

  ChatModel({this.name, this.message, this.time, this.avatarUrl});
}

改成这样后会报其的错误：

error • The argument type 'String?' can't be assigned to the parameter type 'String' at lib\pages\chat_screen.dart:25:53 • (argument_type_not_assignable)
error • The argument type 'String?' can't be assigned to the parameter type 'String' at lib\pages\chat_screen.dart:31:23 • (argument_type_not_assignable)
error • The argument type 'String?' can't be assigned to the parameter type 'String' at lib\pages\chat_screen.dart:35:23 • (argument_type_not_assignable)
error • The argument type 'String?' can't be assigned to the parameter type 'String' at lib\pages\chat_screen.dart:43:21 • (argument_type_not_assignable)

原因分析

报错是说参数不能为null，但是可能存在隐形的赋值为null的可能。这些参数都是在构造函数中调用。说明这些参数是必须的就可以了。

解决措施

构造函数添加required。代码如下：

class ChatModel {
  final String name;
  final String message;
  final String time;
  final String avatarUrl;

  ChatModel({required this.name, required this.message,required  this.time, required this.avatarUrl});
}

另外最新的Dart 强制null safety。定义一个变量时暂时没有赋值，记得加上late告知Dart 这个变量稍后会赋值。不然会报“Non-nullable instance field 'controller' must be initialized” 错误。 eg

late CameraController controller;