Flutter VoidCallback field default value in Constructor

Question

This should be an easy answer but I do not find the solution yet on the internet.

Let say I have 1 VoidCallback field name onStart in a constructor, and I want this field to have a default value when there is no parameter pass it to the constructor. I already try some code but the IDE gives me a warning. How to provide this default value?

class DefaultCallbackExample {
  final VoidCallback onStart;
  final VoidCallback onFinish;
  final bool isEnable;

  DefaultCallbackExample({
    this.onStart = (() => {}), // IDE Warning
    required this.onFinish,
    this.isEnable = true,
  });
}

class DefaultCallbackExample {
  final VoidCallback onStart;
  final VoidCallback onFinish;
  final bool isEnable;

  DefaultCallbackExample({
    this.onStart = const (() => {}), // IDE Warning
    required this.onFinish,
    this.isEnable = true,
  });
}

There is a solution from jamesdin, but I hope there is a simpler solution not to have to write constant sentinel value. Maybe in the future dart version, there is a possible solution

Answer 1

Either the callback should be nullable or it must be initialized with a default value. Default arguments must be constants, so as usual, if you can't provide a constant directly, use a constant sentinel value:

class DefaultCallbackExample {
  final VoidCallback onStart;
  final VoidCallback onFinish;
  final bool isEnable;

  DefaultCallbackExample({
    VoidCallback? onStart,
    required this.onFinish,
    this.isEnable = true,
  }) : onStart = onStart ?? (() {});
}

Also note that () => {} creates an anonymous function that returns an empty Set. An anonymous function with an empty body should be just () {}. Function bodies either should use => (if possible) or curly braces, but not both.