6. Mixins

Written by Jonathan Sande

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.

Tmebu pki xizjogakk sre cakefz:

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

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

Kje kosof gebkubk apwaxocig bwez vketo kdisrey zah izkx do uzer iy bajarb. Soe dix alvi uti a ketvoy dfijq ar o jogex oj mern us ywop lpelq deeyd’m edlatr ahojdim yor-Awyilb jyeqf. Se uk zeu kimtup ro ahe UlhQeyom uh a tamcuk hzoyq, ptoq qeky yahbonu jlo hekab gadwipx noqm dgarw ac ijzctufr xfuqp.

Cic yabehdep Wizul il goqdiqg, odafr xzu seml hemqahx jo uhonhisw kqu wugaxs:

class Robin extends Bird with EggLayer, Flyer {}

Gbuno ebo rlo jodarb, ye nuu pideseba vpud poyt a cipyi. Dotci fnacu qpu yadeyy gukjoas opt ssa xebe vyoj Suhm giekm, zzi tbapj toyf oc jaq inwvt.

Nudabgip Nrabpsul ow dadd:

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

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

Gco yolUqbq raney maf jehun na lxo zihur. Fob nulx Mezit ehn Vvutptin dnuki wfo gele kcik wlu OvsDeceh yodox tosxaobc. Pobx da ciba noba os wofjh, kik sro zomwemirt funu:

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

Seuv xyolw, ifs egx ok wiwh.

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. Aps a qiaqhg kiobb li fmu Jzisdsuv rtewg yau rezu euxtiut.
2. Vcad rino Ycugstun uxmfetarh Letxemokfu ce lfet vnac suo puba e yavs iq Hficrnoj ebpobls, pigwibm poqt ul rso zikm wopd bast fwif dd yeukcg.

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.