如何在 Dart/Flutter 中扩展 class

Question

I have class A:

class A{
    String title;
    String content;
    IconData iconData;
    Function onTab;
    A({this.title, this.content, this.iconData, this.onTab});
}

How can i create class B that extends class A with additional variable like following:

class B extends A{
    bool read;
    B({this.read});
}

Tried with this but not working

let o = new B(
          title: "New notification",
          iconData: Icons.notifications,
          content: "Lorem ipsum doro si maet 100",
          read: false,
          onTab: (context) => {

          });

Answer 1

You have to define the constructor on the child class.

class B extends A {
  bool read;
  B({title, content, iconData, onTab, this.read}) : super(title: title, content: content, iconData: iconData, onTab: onTab);
}

Answer 2

You can inherit from or extend a class using the extends keyword. This allows you share properties and methods between classes that are similar, but not exactly the same. Also, it allows different subtypes to share a common runtime type so that static analysis doesn't fail. (More on this below); The classic example is using different types of animals.

class Animal {
  Animal(this.name, this.age);
  
  int age;
  String name;

  void talk() {
    print('grrrr');
  }
}

class Cat extends Animal {
  // use the 'super' keyword to interact with 
  // the super class of Cat
  Cat(String name, int age) : super(name, age);
  
  void talk() {
    print('meow');
  }
  
}


class Dog extends Animal {
  // use the 'super' keyword to interact with 
  // the super class of Cat
  Dog(String name, int age) : super(name, age);
  
  void talk() {
    print('bark');
  }
  
}

void main() {
  var cat = Cat("Phoebe",1);
  var dog = Dog("Cowboy", 2);
  
  dog.talk();
  cat.talk();
}

Answer 3

Just to update for 2023, since Dart 2.17 we have Super Initializers - described in detail by Michael Thomsen here

You no longer need to make the explicit call to super.

Example:

class B extends A {
    bool read;
    B({super.title, super.content, super.iconData, super.onTab, this.read});
}