Multiple mixins
- Dart course
- Intermediate
- Lesson 27/116
- 5 min
Learn to apply multiple mixins.
Multiple mixins
A class can apply many mixins, separated by commas. This shows Dart's power to combine behaviors; the single inheritance limit is overcome.
Example: class Duck with Flyer, Swimmer { ... }. Here the Duck class applies both the Flyer and the Swimmer mixin. Methods of both mixins are reachable on an instance: Duck().fly() and Duck().swim(). The mixin list is read left to right; the leftmost is applied first. When two mixins share a method name, the rightmost (last applied) wins.
Application order
The mixin list is applied left to right; order matters. When two mixins share a method name, the rightmost mixin wins and overrides the previous one.
Example: class A with M1, M2. Here M1 is applied first, then M2. If both define hello(), the class calls M2.hello(); M1.hello() is hidden. This behavior is how priority is set in behavior composition. Changing the order changes the results; this is why mixin list design needs care.
Practice Apply two mixins to Duck
Define two mixins named Flyer and Swimmer. In Flyer add a void fly() { print('Flying'); } method, in Swimmer add a void swim() { print('Swimming'); } method. Write a class named Duck: class Duck with Flyer, Swimmer { Duck(); }. In main, create an instance with var d = Duck(); and call d.fly() and d.swim().
Starter code
// Define the Flyer and Swimmer mixins
// Apply both mixins to the Duck class
void main() {
// Create a Duck instance and call fly() and swim()
}Sample solution
// Define the Flyer and Swimmer mixins
mixin Flyer {
void fly() {
print('Flying');
}
}
mixin Swimmer {
void swim() {
print('Swimming');
}
}
class Duck with Flyer, Swimmer {
Duck();
}
void main() {
var d = Duck();
d.fly();
d.swim();
}Expected output
Flying Swimming
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is class Duck with Flyer, Swimmer { } written?
- class Duck with Flyer; Swimmer { }
- class Duck extends Flyer implements Swimmer { }
- class Duck with Flyer, Swimmer { } Correct answer
- class Duck with Flyer with Swimmer { }
Explanation: Multiple mixins are separated by commas.
2 Match the multiple mixin parts
| Term | Meaning |
|---|---|
, |
List separator |
Flyer, Swimmer |
Mixin list |
Duck |
Class name |
3 Add the second mixin
class Duck with Flyer____ Swimmer {
Duck();
}class Duck with Flyer, Swimmer {
Duck();
}Explanation: A comma separates the mixin list.