How to store function as class member variable

Question

Is it possible in Dart to store a callback function with return and argument type information? It appears I can do the following:

class MyClass {
  void addCallback( callback( int ) )
  {
    _callback = callback;
  }

  var _callback;
}

But I thought it would be nice if _callback wasn't declared as var, and instead had information about its return and argument types. I couldn't find info on this in the docs, anyone know?

Answer 1

Dart 2 supports a function type syntax:

class MyClass {
  void addCallback( callback( int ) )
  {
    _callback = callback;
  }

  void Function(int) _callback;
}

The Effective Dart Design Guide states that this form is preferred over typedefs.

Answer 2

You can typedef a Function signature like this:

typedef bool Filter(num x);

List<num> filterNumbers(List<num> numbers, Filter filter) {
  return numbers.where(filter).toList();
}

For more great information like this, check out this article: https://www.dartlang.org/articles/idiomatic-dart/