What are mixins in Dart and how are they different from inheritance?
TL;DR: Mixins are reusable code units that can be applied to multiple classes using 'with'. They provide implementation without being a full parent class. Use 'on' to restrict which classes can use a mixin.
Full Answer
Dart is single-inheritance. Mixins solve the need to share behavior across unrelated class hierarchies without the diamond inheritance problem.
mixin vs abstract class
A mixin cannot be instantiated. It's applied to an existing class hierarchy. The 'on' constraint requires the mixin to be applied only to classes that extend/implement a specific type — giving the mixin access to that type's API.
Flutter's own TickerProviderStateMixin and AutomaticKeepAliveClientMixin are examples. TickerProviderStateMixin uses 'on State' to access the widget's lifecycle.
Code Examples
{id: q-001, type: Question}Common Mistakes
- ✗Using extends when you need mixin behavior — extends ties you to a parent class hierarchy
- ✗Forgetting that mixin application order matters — rightmost mixin wins in case of method conflict
Interview Tip
Flutter uses mixins heavily (TickerProviderStateMixin, KeepAliveClientMixin). Show you can explain 'on' constraint — it's what enables mixins to access the base class's API safely.