What is the mixin keyword in Dart?

A mixin is a way to reuse code across multiple class hierarchies without inheritance. Declare a mixin with mixin MyMixin { ... } and apply it with the with keyword: class MyClass extends BaseClass with MyMixin. Mixins cannot have constructors. A class can apply multiple mixins. They are widely used in Flutter for capabilities like TickerProviderStateMixin (provides vsync for animations) and AutomaticKeepAliveClientMixin (preserves scroll position).