mixin

Learn reusable code with mixin.

What is a mixin?

A mixin is a reusable block of behavior. It is applied to classes with the with keyword; this way the behavior is shared across many classes. Dart uses single inheritance; mixins are the way to stretch that limit.

Example: mixin Flyer { void fly() { print('Flying'); } }. We apply this mixin to a class with class Bird with Flyer { ... }. The Bird class can call fly() directly. Mixins cannot hold constructors and cannot be instantiated alone; they make sense only when applied to a class. We will dig deeper into the with keyword in the next lesson.

Defining a mixin

A mixin is declared with the mixin keyword; followed by a name and a body. The body is similar to a class body; but it cannot hold a constructor.

Example: mixin Greeter { String hello() { return 'Hello'; } }. Here the Greeter mixin carries a hello() method. A mixin body can hold methods, fields and even behaviors that call other methods. A mixin cannot be called on its own: Greeter() is a compile error. When applied to a class with with it becomes part of that class and is called via an instance.

Exercise Define and apply the Flyer mixin

Define a mixin named Flyer; add a void fly() { print('Flying'); } method. Write a class named Bird that applies the Flyer mixin: class Bird with Flyer { Bird(); }. In main, create an instance with var b = Bird(); and call b.fly().

Show example solution

// Define the Flyer mixin
mixin Flyer {
  void fly() {
    print('Flying');
  }
}

class Bird with Flyer {
  Bird();
}

void main() {
  var b = Bird();
  b.fly();
}

Test yourself

Why use a mixin?
- To speed up runtime
- To shrink the class size
- To hide constructors
- To reuse behavior across many classes
Show answer

To reuse behavior across many classes — A mixin is the clean way to share behavior across many classes.

Complete the mixin declaration

class Flyer____ {
  void fly() {
    print('Flying');
  }
}

Show answer

class Flyermixin {
  void fly() {
    print('Flying');
  }
}

Match the mixin parts

mixin

Keyword
Flyer

Mixin name
with

Application keyword

Next lesson: Using mixins

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