on Constraints and Mixin Order
Restrict and stack mixins correctly.
on Constraints and Mixin Order is a free Dart Academy lesson on CoddyKit — lesson 4 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.
Mixins With Requirements
Sometimes a mixin only makes sense on certain classes. Dart lets you set that rule with an on constraint, so misuse is caught early. 🔒
The on Keyword
Adding on SomeType restricts where a mixin can be applied. Only classes that extend or implement that type may use it.
mixin Walker on Animal {
void walk() => print('Walking');
}Why Constrain a Mixin
A constraint guarantees the mixin can safely call the base type's members. The on type becomes available inside the mixin's methods.
mixin Walker on Animal {
void move() => breathe();
}Enforced at Compile Time
Try to mix Walker into a class that is not an Animal and Dart refuses to compile. The constraint protects you before runtime.
Calling super in Mixins
A constrained mixin can call super on the base type's methods. This lets it wrap or extend behavior it knows must exist.
mixin Logged on Animal {
@override
void speak() { print('log'); super.speak(); }
}Order Matters
When you list several mixins, Dart applies them left to right. Each one layers on top of the previous, building a linear chain.
class C extends Base with A, B {}
// order: Base, then A, then BLinearization
Dart flattens the class and its mixins into one ordered line called linearization. That order decides which method wins when names clash.
Last Mixin Wins
If two mixins define the same method, the one listed last takes priority. Reorder the with list and the winning version changes.
class C with A, B {}
// B's version overrides A'ssuper Walks the Chain
Inside a mixin, super points to the next member down the linearized chain, not the original class. That is how stacked mixins cooperate.
Stacking Behavior
By chaining super calls, several mixins can each add a step around one method. You build layered behavior, like middleware around a core action.
Order as a Design Tool
Choose mixin order deliberately, since it controls overrides and super flow. Treat the with list as a meaningful sequence, not random.
Quick Check
Two mixins define the same method. Which one runs?
Recap
You restricted mixins with on, learned that order is left to right, and saw how linearization and super decide which method wins. 🎯
Frequently asked questions
Is the “on Constraints and Mixin Order” lesson free?
Yes — the full text of “on Constraints and Mixin Order” 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 “on Constraints and Mixin Order”?
Restrict and stack mixins correctly. 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 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “on Constraints and Mixin Order” 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
- extends and Method Overriding
- Calling super and Constructor Chaining
- Mixins With the with Keyword
- on Constraints and Mixin Order