Mixins

For Dart see here. Mixins classes are not allowed to have a constructor.

Kotlin does not have mixins, but it has instead something similar that is called delegation Also Kotlin interfaces with method implementations can be used, in some cases, instead of mixins.

Kotlin

interface Person {
    val name:String
    val surname:String
}

// interface with method implentations and properties
// (but no actual data)
interface Musical : Person{
  var canPlayPiano:Boolean
  var canCompose:Boolean
  var canConduct:Boolean

  fun entertainMe() {
    println("${name} ${surname} is")
    if (canPlayPiano) {
      println("Playing piano")
    }
    if (canConduct) {
      println("Waving hands")
    }
    if (canCompose) {
      println("Humming to self")
    }
    println("")
  }
}

// MusicalImpl is the class  we will "delegate to" 
// for implementing the "Musical" interface.
// it is equivalent to the "Musical" mixin in Dart code
class MusicalImpl(
  override val name:String,override val surname:String):Musical {
  override var canPlayPiano:Boolean=false
  override var canCompose:Boolean=false
  override var canConduct:Boolean=false
}

// a class that has "Person" as superclass and
// delegate implementation of the "Musical" interface 
// to the MusicalImpl class
class Pianist(name:String, surname:String)
 : Musical by MusicalImpl(name,surname)
{
  init {
      canPlayPiano=true //from  "Musical" 
    }
}

// a class that has "Person" as superclass and
// delegate implementation of the "Musical" interface 
// to the MusicalImpl class
class Maestro(name:String, surname:String)
 : Musical by  MusicalImpl(name,surname)
{
  init {
    canConduct=true //from  "Musical"
  }
}

// a class that has "Person" as superclass and
// delegate implementation of the "Musical" interface 
// to the MusicalImpl class
class Composer(name:String, surname:String)
 : Musical by  MusicalImpl(name,surname)
{
  init {
    canCompose=true //from  "Musical" 
  }
}

var p=Pianist("Glenn","Gould")
var m=Maestro("Zubin","Mehta")
var c=Composer("Frédéric","Chopin")
c.canPlayPiano=true //chopin was also a virtuoso Pianist

p.entertainMe() //Glenn Gould is Playing piano
m.entertainMe() // Zubin Mehta is Waving hands
c.entertainMe() //Frédéric Chopin is Playing piano Humming to self

Dart

class Person {
  final String name;
  final String surname;
  Person(this.name,this.surname);
}

// a mixin in Dart
// defines a class that CANNOT BE instantiated, 
// but can be included in another class as a mixin
// with the "with" keyword
// the "on <class name>" class restricts the classes 
// that can use the mixin to only the classes that derives from
// some specific superclass, in this case "Person" 
// because the mixin implementation depends on
// the availability of fields and methods
// from that superclass
// All the mixin methods and fields will be available to the class that
// include it with the "with" keyword
mixin Musical on Person {
  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    print("${name} ${surname} is");
    if (canPlayPiano) {
      print('Playing piano');
    }
    if (canConduct) {
      print('Waving hands');
    }
    if (canCompose) {
      print('Humming to self');
    }
    print("");
  }
}



// a class that has "Person" as superclass and
// use the "Musical" mixin
class Pianist extends Person with Musical {
  Pianist(super.name,super.surname) {
      canPlayPiano=true; //from the "Musical" mixin
    }
}

// a class that has "Person" as superclass and
// use the "Musical" mixin
class Maestro extends Person  with Musical {
  Maestro(super.name,super.surname) {
    canConduct=true; //from the "Musical" mixin
  }
}

// a class that has "Person" as superclass and
// use the "Musical" mixin
class Composer extends Person  with Musical {
  Composer(super.name,super.surname) {
    canCompose=true; //from the "Musical" mixin
  }
}


var p=Pianist("Glenn","Gould");
var m=Maestro("Zubin","Mehta");
var c=Composer("Frédéric","Chopin");
c.canPlayPiano=true; //chopin was also a virtuoso Pianist

p.entertainMe(); //Glenn Gould is Playing piano
m.entertainMe(); // Zubin Mehta is Waving hands
c.entertainMe(); //Frédéric Chopin is Playing piano Humming to self