How do I correctly Mixin on State?

Question

I can create and use simple Mixins but to access setState, mounted, context etc in the mixin's methods I need to pass them to from the State class as parameters.

When I to create a Mixin on a State with some boilerplate code to use it on any statefull widget's State, but I get two errors:

error: 
The class '_ProfilePageState' cannot implement both 'State<ProfilePage>' and 'State<StatefulWidget>'
because the type arguments are different.

error: 
Type parameters could not be inferred for the mixin 'NotificationHandlers'
because no type parameter substitution could be found matching the mixin's supertype constraints.

The mixin definition looks like this:

mixin NotificationHandlers on State {
  void foo(Map bla) {
    //use setState / context / other properties or State here
  }
}

Answer 1

You should define your mixin like this:

mixin NotificationHandlers<T extends StatefulWidget> on State<T> {
  // Now you can access all of State's members and use the mixin with State classes.
  // Example:
  @override
  void initState() {
    // ...
    super.initState();
  }
}

This ensures that the generic type of your mixin is the same as the generic type for your State class. If you omit the type for on State, it is going to default to StatefulWidget, but you need to match the exact type, which you can with the syntax I provided.

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The important syntax is mixin YourMixin<T extends StatefulWidget> on State<T>.