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;
