dart - 为抽象类中的方法提供默认实现

Question

在下面的示例中，我希望 sum getter 返回 8，但这是一个编译错误。

Class 'B' has no instance getter 'sum'.

根据规范:

Using an abstract class instead of an interface has important advantages. An abstract class can provide default implementations; it can also provide static methods, obviating the need for service classes such as Collections or Lists, whose entire purpose is to group utilities related to a given type.

提供添加 x 和 y 的 sum 的默认实现的正确方法是什么？

abstract class A {
  int get x;
  int get y;
  int get sum => x+y;
}

class B implements A {
  int get x => 3;
  int get y => 5;
}

main() {
  B b = new B();
  print(b.x);
  print(b.sum); // Not working, why not 8?
}

Answer 1

你必须做 B延长 A而不是实现。

abstract class A {
  int get x;
  int get y;
  int get sum => x+y;
}

class B extends A {
  int get x => 3;
  int get y => 5;
}

main() {
  B b = new B();
  print(b.x);
  print(b.sum); // displays 8
}

或者，如果您不想使用扩展，因为您的类可能已经扩展了另一个类，您可以使用 mixins :

abstract class M {
  int get x;
  int get y;
  int get sum => x+y;
}

class A {
  String s = "s";
}
class B extends A with M {
  int get x => 3;
  int get y => 5;
}

main() {
  B b = new B();
  print(b.s);
  print(b.x);
  print(b.sum); // displays 8
}