Dart: Mixins

Carlos Costa

In brief, a mixin is a class that contains methods for use by other classes without having to be the parent class of those other classes.

Example:

mixin BasicOperations {
  num value = 0;

  BasicOperations add(num arg) {
    value += arg;
    return this;
  }
}

class Calc with BasicOperations {}

void main(List<String> args) {
  var calc = Calc();

  calc
    ..add(1)
    ..add(2)
    ..add(3);

  print(calc.value); // 6
}

In this example, the Calc class has access to the BasicOperations methods, without having to extend it.

A mixin can’t be instantiated or extended, it can only be used with the with keyword.

Multiple mixins

You can use multiple mixins in a class, just separate them with a comma.

mixin BasicOperations {
  num value = 0;

  BasicOperations add(num arg) {
    value += arg;
    return this;
  }
}

mixin AdvancedOperations {
  num value = 0;

  AdvancedOperations multiply(num arg) {
    value *= arg;
    return this;
  }
}

class Calc with BasicOperations, AdvancedOperations {}

void main(List<String> args) {
  var calc = Calc();

  calc
    ..add(1)
    ..add(2)
    ..add(3)
    ..multiply(2);

  print(calc.value); // 12
}

Restricted Mixins

You can restrict the use of a mixin to a specific type of class, using the on keyword.

class Calc {}

mixin BasicOperations on Calc {
  num value = 0;

  BasicOperations add(num arg) {
    value += ;
    return this;
  }
}

class Calc2 extends Calc with BasicOperations {}

void main(List<String> args) {
  var calc = Calc2();

  calc
    ..add(1)
    ..add(2)
    ..add(3);

  print(calc.value);
}

Abstract mixin class

You can create an abstract mixin class, using the abstract keyword.

abstract mixin class Calc {
  int value = 0;

  Calc add(int arg);
}

class Calc1 with Calc {
  @override
  Calc add(int arg) {
    value += arg;
    return this;
  }
}

class Calc2 extends Calc {
  @override
  Calc add(int arg) {
    value += arg;
    return this;
  }
}

void main(List<String> args) {
  var calc1 = new Calc1();
  var calc2 = new Calc2();

  print(calc1.add(10).add(2).value);
  print(calc2.add(10).add(2).value);
}

If a extends o use Calc class it will be forced to implement the add method.

Resume

A mixin can be:

Used with the with keyword
Used with multiple mixins
Restricted to a specific type of class
Abstract

A mixin can’t be:

Instantiated
Extended

References

Dart: Mixins