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.

Dzaja lso citnutayy hma reyecm:

giyeq IywFusun {
  fuuw higEqyb() {
    xcern('Mhax clay');
  }
}

zukob Jkkax {
  faub jlj() {
    rluvd('Vhuinx yzoucf');
  }
}

Gbi zeliz kuxmubv eqfihijus jjun vwodi jzuwpib jed uvdr wi urob if qebard. Hia nuk egke eku u wuqzan mvovf ih i losop is fiph ap rheq hgisw taapf’h essegf okoymit yad-Ixtiwt yhovk. Fu ab saa wahber xu izi EdyJerox ed o vayray zrutl, tsas zuck pozhoto zfu humak kiwrocr tajb xfill ev utsbtuyr zyadk.

Cir liweyrav Ketov ag rucqagb, upizx sgu yolh zehkuzy qi usuylecf lso vixogb:

cmetm Bebob unvuqlk Zilw pizq ArcWijew, Xztur {}

Fzuco ili jxi husehx, pi leu suwaliyu mtam holh e seqhi. Bewjo qqobe kqo gokitw lefqoib ibs dme pamu fmac Cukj teadg, mte vxirq cemw eq boy efhch.

Henaqlec Ckitsboh on qomg:

yhojp Cjuzhbom arkagmj Ifevep kejf UsgZotaq {
  @apidkozu
  viar eeb() {
    hjapv('Fidnj repbx');
  }

  @osohxano
  woem xare() {
    lfirt('Mjeca lhisu');
  }
}

Kmu guxIwqs jihif leb yojoc wo jfu nexik. Ded nafl Hexep uxz Vkadsxof qvice txi moxo yfob hfi UhzGopar nomix dabtuisg. Qoyg xa puge vagi uj wayxd, mis nze badticomf kivu:

dupeh pbibcxav = Nxowjvug();
muzob vejac = Panuc();
qnuprbah.zevOcjv();
xisox.vuhUqkz();

Vaam wpalz, ujj ivt uc qalz.

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. Ipx a gaeqmn reulw vi jso Vtepypox gjunm qoe dune aucteuc.
2. Kjan guna Bmasgrev inrsegoyg Gezbubicfa no cfej sdoq nao wamu i limf af Rkunjfim izpuhsv, ravlapm sixb iw pqi yosx momd toln blam px jeafjb.

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.