Dart tour: 11、Mixin

mixin 关键字在 Dart 2.1 中才被引用支持。早期版本中的代码通常使用 abstract class 代替。

Mixin 是一种在多重继承中复用某个类中代码的方法模式。使用 with 关键字并在其后跟上 Mixin 类的名字来使用 Mixin 模式。

class Musician extends Performer with Musical {

}

class Maestro extends Person
    with Musical, Aggressive, Demented {
  Maestro(String maestroName) {
    name = maestroName;
    canConduct = true;
  }
}

想要创建一个 Mixin，需要创建继承自 Object 且不声明构造函数的类。除非你想让该类与普通的类一样可以被正常地使用，否则请使用关键字 mixin 替代 class。

mixin Musical {

  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

可以使用关键字 on 来指定哪些类可以使用该 Mixin 类，比如有 Mixin 类 A，但是 A 只能被 B 类使用，则可以这样定义 A。

mixin Musical {

  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

mixin Musical {

  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

class Musician {

}
mixin MusicalPerformer on Musician {

  String get title {
    return "musician";
  }

  void playMusic() {
    print("play music");
  }
}
class SingerDancer extends Musician with MusicalPerformer {
  // ...
}

// Error: 'Object' doesn't implement 'Musician' so it can't be used with 'MusicalPerformer'.
class Dancer with MusicalPerformer {

}

void main() {
  var sd = SingerDancer();
  sd.playMusic();   // OK
  print(sd.title);  // musician

  var dancer = Dancer();
  dancer.playMusic(); // Error
}