Dart on keyword

on 在 mixin 中的使用

abstract class Animal {
  // properties
  String name;
  double speed;

  // constructor
  Animal(this.name, this.speed);

  // abstract method
  void run();
}

// mixin CanRun is only used by class that extends Animal
// 其实就是把 mixin CanRun 依附于 Animal 上面(只有在有Animal的时候才能 with Canrun)
mixin CanRun on Animal {
  // implementation of abstract method
  @override
  void run() => print('$name is Running at speed $speed');
}

class Dog extends Animal with CanRun {
  // constructor
  Dog(String name, double speed) : super(name, speed);
}

void main() {
  var dog = Dog('My Dog', 25);
  dog.run();
}

// Not Possible
// class Bird with Animal { } 

on 在捕获异常中的使用

Future<void> main() async {
  AnimalAction dog = Dog('sDog');
  dog.crow();

  /// 直接申明了一个变量，用来存储
  dynamic res;
  try {
    res = 1 / 0;
  } on Exception catch (e) {
    print(e); // 不执行
  } finally {
    print(res);
  }

  try {
    throw Exception('my err');
  } on Exception {
    stderr.write('null err');
    await stderr.flush();
    await stderr.close();
  }
}