2

我想知道是否可以检查我使用哪个构造函数在 dart 的 if 语句中创建创建的元素。

我想做的一个简单的例子：

class Employee {
  int id;
  String name;
  String title;

  Employee.id(this.id);

  Employee.name(this.name);

  Employee.title(this.title);
}

现在我的代码中有一个 if 语句，并想检查我是否使用了构造函数 Employee.id。在这种情况下，我会做一些事情，就像这样：

Employee e = new Employee.id(1)

//check if e was created with Employee.id constructur
if (e == Emploee.id) { 
   print(e.id)
} else {
   print("no id")
}

有没有办法做到这一点？谢谢您的回答。

4

2 回答 2

2

您可以Union type使用freezed包使您的类 a 并使用如下所示的折叠方法来查看使用了什么构造函数：

 import 'package:freezed_annotation/freezed_annotation.dart';

part 'tst.freezed.dart';

@freezed
abstract class Employee with _$Employee {
  const factory Employee.id(int id) = IdEmployee;

  const factory Employee.name(String name) = NameEmployee;

  const factory Employee.title(String title) = TitleEmployee;
}

void main() {
  Employee employee1 = Employee.id(0);
  Employee employee2 = Employee.name('some name');
  Employee employee3 = Employee.title('some title');

  employee1.when(
    id: (int id) => print('created using id contsrutor and id= $id'),
    name: (String name) => print('created using name const and name = $name'),
    title: (String title)=>print('created using title const and title = $title'),
  );//prints the first statement

  employee2.when(
    id: (int id) => print('created using id contsrutor and id= $id'),
    name: (String name) => print('created using name const and name = $name'),
    title: (String title)=>print('created using title const and title = $title'),
  );//prints the second statement

  employee3.when(
    id: (int id) => print('created using id contsrutor and id= $id'),
    name: (String name) => print('created using name const and name = $name'),
    title: (String title)=>print('created using title const and title = $title'),
  );//prints the third statement


  print(employee1 is IdEmployee);
  print(employee1 is NameEmployee);
}

输出将是：

created using id contsrutor and id= 0
created using name const and name = some name
created using title const and title = some title
true
false
于 2020-05-02T14:19:54.153 回答
1

您可以为您定义私有枚举属性来设置这样的私有信息，并稍后使用函数打印它。也不要忘记用 . 标记你的构造函数factory。

enum _ConstructorType {
  Identifier,
  Name,
  Title,
}

class Employee {
  int id;
  String name;
  String title;
  _ConstructorType _constructorType;

  factory Employee.id(id) {
    return Employee._privateConstructor(_ConstructorType.Identifier, id: id);
  }

  factory Employee.name(name) {
    return Employee._privateConstructor(_ConstructorType.Name, name: name);
  }

  factory Employee.title(title) {
    return Employee._privateConstructor(_ConstructorType.Title, title: title);
  }

  Employee._privateConstructor(this._constructorType,
      {this.id, this.name, this.title});

  String constructorDescription() {
    return this._constructorType.toString();
  }
}

如果您需要此信息不是作为字符串，而是作为枚举，您可以随时删除其上的下划线，并将此信息公开以供您在课堂外使用。

于 2020-05-02T14:17:43.383 回答