Mixin in Dart

Introduction

In this section, you will learn about dart mixins to reuse the code in multiple classes. Before learning about mixins you should have a basic understanding of class, object, constructor, and inheritance.

Mixin In Dart

Mixins are a way of reusing the code in multiple classes. Mixins are declared using the keyword mixin followed by the mixin name. Three keywords are used while woring with mixins: mixin, with, and on. It is possible to use multiple mixins in a class.

Info

Note: The with keyword is used to apply the mixin to the class. It promotes DRY(Don’t Repeat Yourself) principle.

Rules For Mixin

  • Mixin can’t be instantiated. You can’t create object of mixin.
  • Use the mixin to share the code between multiple classes.
  • Mixin has no constructor and cannot be extended.
  • It is possible to use multiple mixins in a class.

Syntax

mixin Mixin1{
  // code
}

mixin Mixin2{
  // code
}

class ClassName with Mixin1, Mixin2{
  // code
}

Example 1: Mixin In Dart

In this example below, there are two mixins named ElectricVariant and PetrolVariant. The ElectricVariant mixin has a method electricVariant() and the PetrolVariant mixin has a method petrolVariant(). The Car class uses both the ElectricVariant and PetrolVariant mixins.

mixin ElectricVariant {
  void electricVariant() {
    print('This is an electric variant');
  }
}

mixin PetrolVariant {
  void petrolVariant() {
    print('This is a petrol variant');
  }
}
// with is used to apply the mixin to the class
class Car with ElectricVariant, PetrolVariant {
  // here we have access of electricVariant() and petrolVariant() methods
}

void main() {
  var car = Car();
  car.electricVariant();
  car.petrolVariant();
}

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Example 2: Mixin In Dart

In this example below, there are two mixins named CanFly and CanWalk. The CanFly mixin has a method fly() and the CanWalk mixin has a method walk(). The Bird class uses both the CanFly and CanWalk mixins. The Human class uses the CanWalk mixin.

mixin CanFly {
  void fly() {
    print('I can fly');
  }
}

mixin CanWalk {
  void walk() {
    print('I can walk');
  }
}

class Bird with CanFly, CanWalk {
 
}

class Human with CanWalk {
 
}

void main() {
  var bird = Bird();
  bird.fly();
  bird.walk();

  var human = Human();
  human.walk();
}

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On Keyword

Sometimes, you want to use a mixin only with a specific class. In this case, you can use the on keyword.

Syntax Of On Keyword

mixin Mixin1 on Class1{
  // code
}

Example 3: On Keyword In Mixin In Dart

In this example below, there is abstract class named Animal with properties name and speed. The Animal class has an abstract method run(). The CanRun mixin is only used by class that extends Animal. The Dog class extends the Animal class and uses the CanRun mixin. The Bird class cannot use the CanRun mixin because it does not extend the Animal class.

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
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 { } 

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What Is Allowed For Mixin

  • You can add properties and static variables.
  • You can add regular, abstract, and static methods.
  • You can use one or more mixins in a class.

What Is Not Allowed For Mixin

  • You can’t define a constructor.
  • You can’t extend a mixin.
  • You can’t create an object of mixin.

Video

Watch our video on mixin in Dart.