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Dart Academy · Lesson

Mixins With the with Keyword

Add capabilities without single inheritance.

Mixins With the with Keyword is a free Dart Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Dart Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Beyond Single Parents

A Dart class can extend only one parent, but you often want abilities from several sources. A mixin lets you blend in extra behavior cleanly. 🧩

Declaring a Mixin

You define one with the mixin keyword. It looks like a class but is meant to be mixed into others, not instantiated on its own.

mixin Swimmer {
  void swim() => print('Swimming');
}

The with Keyword

You apply a mixin using with after the class declaration. The class instantly gains every method the mixin defines.

class Fish with Swimmer {}
// Fish().swim() works

Mixing Several In

Separate multiple mixins with commas. A class can blend in many mixins at once, each adding its own slice of behavior.

class Duck with Swimmer, Flyer {}

Combining extends and with

Mixins work alongside inheritance. Write extends first for the single parent, then with for the mixins you want layered on.

class Duck extends Bird with Swimmer, Flyer {}

Mixins Carry State

A mixin can hold fields too, not just methods. Any class that mixes it in gains that state as part of itself.

mixin Counter {
  int count = 0;
  void tick() => count++;
}

Not for Instantiation

You cannot write new on a pure mixin. It only exists to be folded into real classes, so Dart blocks direct construction.

Reuse Across Trees

Unlike a superclass, the same mixin can be added to totally unrelated classes. That is how you share code without a forced family line.

Mixins Are Types

A mixed-in class also counts as that mixin's type. You can store a Fish in a Swimmer variable and call its shared methods.

Swimmer s = Fish();
s.swim();

Composition Over Inheritance

Mixins favor assembling small, focused capabilities. You compose behavior from reusable pieces rather than forcing one deep hierarchy.

Keep Mixins Focused

A good mixin does one job well, like logging or serialization. Small, single-purpose mixins combine cleanly without surprising conflicts.

Quick Check

Recall how you attach a mixin to a class.

Recap

You declared a mixin, applied it with the with keyword, combined several, and shared behavior across unrelated classes. 🧩

Frequently asked questions

Is the “Mixins With the with Keyword” lesson free?

Yes — the full text of “Mixins With the with Keyword” is free to read here on the web, and the Dart Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Dart Academy course, upgrade to CoddyKit PRO.

What will I learn in “Mixins With the with Keyword”?

Add capabilities without single inheritance. You practise Dart Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Dart Academy?

No prior experience is required. Dart Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Mixins With the with Keyword” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Dart Academy lesson?

Yes. Every Dart Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. extends and Method Overriding
  2. Calling super and Constructor Chaining
  3. Mixins With the with Keyword
  4. on Constraints and Mixin Order
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