6 Mixins

Mixins are an interesting feature of Dart that you might not be familiar with, even if you know other programming languages. They’re a way to reuse methods or variables among otherwise unrelated classes.

Note

For you Swift developers, Dart mixins work like protocol extensions.

Before showing you what mixins look like, you’ll first take a look at why you need them.

Problems With Extending and Implementing

Think back to the Animal examples again. Say you’ve got a bunch of birds, so you’re carefully planning an abstract class to represent them. Here’s what you come up with:

abstract class Bird {
  void fly();
  void layEggs();
}

“It’s looking good!” you think. “I’m getting the hang of this.” So you try it out on Robin:

class Robin extends Bird {
  @override
  void fly() {
    print('Swoosh swoosh');
  }

  @override
  void layEggs() {
    print('Plop plop');
  }
}

“Perfect!” You smile contentedly at your handiwork.

Then you hear a sound behind you.

“Munch, munch. Glide, glide. Plop, plop. I’m a platypus.”

Oh. Right. The platypus.

Here’s the code you wrote for Platypus back in Chapter 3, “Inheritance”:

abstract class Animal {
  bool isAlive = true;
  void eat();
  void move();

  @override
  String toString() {
    return "I'm a $runtimeType";
  }
}

class Platypus extends Animal {
  @override
  void eat() {
    print('Munch munch');
  }

  @override
  void move() {
    print('Glide glide');
  }

  void layEggs() {
    print('Plop plop');
  }
}

Your layEggs code for Robin is exactly the same as it is for Platypus. That means you’re duplicating code, which violates the DRY principle. If there are any future changes to layEggs, you’ll have to remember to change both instances. Consider your options:

1. Platypus can’t extend Bird or Robin, because platypi can’t fly.
2. Birds probably shouldn’t extend Platypus, because who knows when you’re going to add the stingWithVenomSpur method?
3. You could create an EggLayer class and have Bird and Platypus both extend that. But then what about flying? Make a Flyer class, too? Dart only allows you to extend one class, so that won’t work.
4. You could have birds implement EggLayer and Flyer while Platypus implements only EggLayer. But then you’re back to code duplication since implementing requires you to supply the implementation code for every class.

The solution? Mixins!

Mixing in Code

To make a mixin, you take whatever concrete code you want to share with different classes, and package it in its own special mixin class.

Write the following two mixins:

mixin EggLayer {
  void layEggs() {
    print('Plop plop');
  }
}

mixin Flyer {
  void fly() {
    print('Swoosh swoosh');
  }
}

The mixin keyword indicates that these classes can only be used as mixins. You can also use a normal class as a mixin as long as that class doesn’t extend another non-Object class. So if you wanted to use EggLayer as a normal class, then just replace the mixin keyword with class or abstract class.

Now refactor Robin as follows, using the with keyword to identify the mixins:

class Robin extends Bird with EggLayer, Flyer {}

There are two mixins, so you separate them with a comma. Since those two mixins contain all the code that Bird needs, the class body is now empty.

Refactor Platypus as well:

class Platypus extends Animal with EggLayer {
  @override
  void eat() {
    print('Munch munch');
  }

  @override
  void move() {
    print('Glide glide');
  }
}

The layEggs logic has moved to the mixin. Now both Robin and Platypus share the code that the EggLayer mixin contains. Just to make sure it works, run the following code:

final platypus = Platypus();
final robin = Robin();
platypus.layEggs();
robin.layEggs();

Four plops, and all is well.

Challenges

Before moving on, here are some challenges to test your knowledge of mixins. It’s best if you try to solve them yourself, but solutions are available with the supplementary materials for this book if you get stuck.

Challenge 1: Calculator

1. Create a class called Calculator with a method called sum that prints the sum of any two integers you give it.
2. Extract the logic in sum to a mixin called Adder.
3. Use the mixin in Calculator.

Challenge 2: Heavy Monotremes

Dart has a class named Comparable, which is used by the sort method of List to sort its elements.

1. Add a weight field to the Platypus class you made earlier.
2. Then make Platypus implement Comparable so that when you have a list of Platypus objects, calling sort on the list will sort them by weight.

Key Points

• Mixins allow you to share code between classes.
• You can use any class as a mixin as long as it doesn’t extend anything besides Object.
• Using the mixin keyword means that a class can only be used as a mixin.