Is it possible to extend private Dart classes?

Question

I am trying to expand upon the functionality of the Dismissible widget, which is a StatefulWidget, with its state being a private class.

Since I need to change the functionality inside of _DismissibleState, is it possible to somehow extend from it?

If not, is there an alternative or recommended way how to extend upon Flutter standard classes, apart from copying the whole source?

Solution

You can only extend a class that you can refer to. If _DismissibleState is declared in a different library, then you cannot refer to it, and so you can't extend it.

There is no workaround. That's what it means to be private.

You also cannot extend Dismissible to return a different state because its interface contains _DismissibleState createState(). There is no way you can return a state which satisfies that interface restriction, and you also cannot override it with a different return type unless that type also implements _DismissibleState, which was the original unsolvable problem.