ChangeNotifier模型-如何使用fromMap()？

Question

我有一个扩展ChangeNotifier的类，如下所示

import 'package:flutter/foundation.dart';

class Person extends ChangeNotifier {
  int _age = 0;

  int get age => _age;

  set age(int value) {
    _age = value;
    notifyListeners();
  }

  factory Person.fromMap(Map<String, dynamic> obj) => Person(
        age: obj["Age"],
      );
}

但在添加fromMap方法时出现此错误

The class 'Person' doesn't have a default constructor.
Try using one of the named constructors defined in 'Person'.

你知道怎么纠正这个问题吗？

我确实试过了

class Person extends ChangeNotifier {

  Person();

  int _age = 0;

  int get age => _age;

  set age(int value) {
    _age = value;
    notifyListeners();
  }

  factory Person.fromMap(Map<String, dynamic> obj) => Person(
        age: obj["Age"],
      );
}

但后来却被告知

The named parameter 'age' isn't defined.
Try correcting the name to an existing named parameter's name, or defining a named parameter with the name 'age'

Answer 1

您必须创建一个以年龄为参数的命名构造函数。

class Person extends ChangeNotifier {

  int _age = 0;

  Person({int age}) : this._age = age;

  // Rest of your code
}

您可以在Dart Language Tour此处阅读有关构造函数的更多信息