Dart Mixins

last modified February 15, 2025

In this article, we show how to use mixins in Dart language. Mixins are a way to reuse code across multiple class hierarchies without using inheritance. They allow you to add functionality to a class without creating a full parent-child relationship.

Mixins are particularly useful when you want to share behavior between classes that don’t share a common ancestor. They are declared using the mixin keyword and can be added to a class using the with keyword.

Dart Mixins Simple Example

The following example demonstrates how to create and use a simple mixin in Dart.

main.dart

mixin Logging { void log(String message) { print(‘Log: $message’); } }

class User with Logging { String name;

User(this.name);

void greet() { log(‘User $name says hello!’); } }

void main() { final user = User(‘Alice’); user.greet(); }

In this program, we define a mixin Logging that provides a log method. We then use this mixin in the User class to log a greeting message.

mixin Logging { void log(String message) { print(‘Log: $message’); } }

We define a mixin named Logging that contains a log method.

class User with Logging { String name;

User(this.name);

void greet() { log(‘User $name says hello!’); } }

We create a User class that uses the Logging mixin with the with keyword. The greet method uses the log method from the mixin.

void main() { final user = User(‘Alice’); user.greet(); }

We create an instance of the User class and call the greet method, which logs a message.

$ dart main.dart Log: User Alice says hello!

Dart Mixins with Multiple Classes

Mixins can be used with multiple classes to share common behavior.

main.dart

mixin Logging { void log(String message) { print(‘Log: $message’); } }

class User with Logging { String name;

User(this.name);

void greet() { log(‘User $name says hello!’); } }

class Admin with Logging { String name;

Admin(this.name);

void announce() { log(‘Admin $name makes an announcement!’); } }

void main() { final user = User(‘Alice’); user.greet();

final admin = Admin(‘Bob’); admin.announce(); }

In this program, we use the Logging mixin in both the User and Admin classes to share logging functionality.

class User with Logging { String name;

User(this.name);

void greet() { log(‘User $name says hello!’); } }

class Admin with Logging { String name;

Admin(this.name);

void announce() { log(‘Admin $name makes an announcement!’); } }

Both the User and Admin classes use the Logging mixin to log messages.

$ dart main.dart Log: User Alice says hello! Log: Admin Bob makes an announcement!

Dart Mixins with Overriding

Mixins can override methods from the classes they are used in.

main.dart

mixin Logging { void log(String message) { print(‘Log: $message’); } }

class User with Logging { String name;

User(this.name);

void log(String message) { print(‘User Log: $message’); }

void greet() { log(‘User $name says hello!’); } }

void main() { final user = User(‘Alice’); user.greet(); }

In this program, the User class overrides the log method from the Logging mixin.

void log(String message) { print(‘User Log: $message’); }

The User class overrides the log method to provide custom logging behavior.

$ dart main.dart User Log: User Alice says hello!

Dart Mixins with Constraints

Mixins can specify constraints using the on keyword to restrict their usage to specific classes.

main.dart

class Person { String name;

Person(this.name); }

mixin Logging on Person { void log(String message) { print(‘Log: $name says $message’); } }

class User extends Person with Logging { User(String name) : super(name);

void greet() { log(‘Hello!’); } }

void main() { final user = User(‘Alice’); user.greet(); }

In this program, the Logging mixin is constrained to be used only with classes that extend Person.

mixin Logging on Person { void log(String message) { print(‘Log: $name says $message’); } }

The Logging mixin uses the on keyword to specify that it can only be used with classes that extend Person.

class User extends Person with Logging { User(String name) : super(name);

void greet() { log(‘Hello!’); } }

The User class extends Person and uses the Logging mixin.

$ dart main.dart Log: Alice says Hello!

Source

Dart Mixins - Language Documentation

In this article, we have covered the basics of using mixins in Dart. Mixins are a powerful feature for code reuse and can be used to share behavior across multiple classes.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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