Mixins: Sharing Behavior Across Class Trees

Mixins: Sharing Behavior Across Class Trees

Suppose several kinds of animals can all swim. You could write a swim method in every animal class, but that means copying the same code again and again. A mixin solves this. A mixin is a small bundle of methods and fields that you can add to many classes without making them related by inheritance.

In Dart, you define a mixin with the mixin keyword, and you add it to a class with the with keyword.

mixin Swimmer {
  void swim() => print('Paddling through the water!');
}

mixin Flyer {
  void fly() => print('Up, up, and away!');
}

class Duck with Swimmer, Flyer {}

class Fish with Swimmer {}

void main() {
  final duck = Duck();
  duck.swim();
  duck.fly();
  final fish = Fish();
  fish.swim();
}

Here Duck can both swim and fly, while Fish only swims. Neither class had to copy any code.

Mixins versus inheritance

Inheritance (the extends keyword) gives a class everything from one parent. It answers the question "what is this?" A Duck is a Bird.

Mixins answer a different question: "what can this do?" A duck can swim, a fish can swim, but ducks and fish live on different branches of the class tree. Mixins let abilities cut across those branches.

Think of it this way:

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Show diagram source

classDiagram
    class Animal
    class Bird
    class Fish
    class Duck
    Animal <|-- Bird
    Animal <|-- Fish
    Bird <|-- Duck
    Swimmer ..> Fish
    Swimmer ..> Duck
    Flyer ..> Duck

A few helpful details

• A class can use several mixins: class Duck with Swimmer, Flyer. If two mixins define the same method, the one listed last wins.
• A mixin can depend on other members. Writing mixin Swimmer on Animal says the mixin may only be added to classes that extend Animal, so it can safely call Animal's methods.
• Mixins can also use extends-free helper state, like fields and getters, just like a normal class.

mixin Greeter {
  String get name;
  void greet() => print('Hi, I am $name.');
}

class Robot with Greeter {
  @override
  String get name => 'Rover';
}

void main() => Robot().greet();

When to reach for a mixin

• The behavior is a small, reusable ability (Comparable, logging, formatting).
• The classes that need it do not share a sensible parent.
• You want to combine several abilities freely.

If two classes are truly the same kind of thing, use inheritance. If they merely share a job, use a mixin.

callout tip Mixins in Dart appear everywhere in real projects and frameworks, especially for UI code. Recognizing the with keyword is the key reading skill; writing your own small mixin is the key writing skill. endcallout

exercise Q: In class Penguin with Swimmer, Flyer {}, could you swap the order of the mixins and change the program's behavior? A: Not here, because Swimmer and Flyer define different methods with no overlap. Order only matters when two mixins supply the same member name; then the mixin listed last wins. endexercise

estimate minutes: 20 endestimate