Dart / Flutter - class doesn't have to implement abstract class's abstract methods?

Question

in Flutter we have class

abstract class PreferredSizeWidget implements Widget {
  Size get preferredSize;
}

and

@immutable
abstract class Widget extends DiagnosticableTree {
  const Widget({ this.key });
  final Key? key;
  @protected
  @factory
  Element createElement();
  @override
  String toStringShort() {
    final String type = objectRuntimeType(this, 'Widget');
    return key == null ? type : '$type-$key';
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.defaultDiagnosticsTreeStyle = DiagnosticsTreeStyle.dense;
  }

  @override
  @nonVirtual
  bool operator ==(Object other) => super == other;

  @override
  @nonVirtual
  int get hashCode => super.hashCode;
  static bool canUpdate(Widget oldWidget, Widget newWidget) {
    return oldWidget.runtimeType == newWidget.runtimeType
        && oldWidget.key == newWidget.key;
  }
  static int _debugConcreteSubtype(Widget widget) {
    return widget is StatefulWidget ? 1 :
           widget is StatelessWidget ? 2 :
           0;
    }
}

My question is: Why if we mix in PreferredSizeWidget class we don't have to have implementation for all of the abstract class Widget methods and abstract methods?

I tried to mock up some code

class A implements B {}

abstract class B extends C {}

abstract class C {
  void test() {
    print("test");
  }
}

and got error

Error: The non-abstract class 'A' is missing implementations for these members:
 - C.test
class A implements B {
      ^
lib/main.dart:16:8:

Answer 1

Haven’t run it myself, but in your case A isn’t abstract anymore so it has to implement everything, in the case of PreferredSizeWidget - you usually extend/implement/ mixin it on a widget (which already implements everything from widget) thus leaving you with only preferredSize

@Norbert515's answer.

The main point is that even though if we mixin/extend PreferredSizeWidget we actually implement all of the Widget methods but as we usually mixin it on a Widget we don't have to make an implicit override of the methods.