The question came into my mind when I've declared a final callback in a class, and the constructor can be declared as const, and trying to make a constant value of a function like so:

class MyClass {
  final void Function() callback;

  const MyClass(this.callback);
}


void example1() {
  const foo = MyClass(() {});
}

This gives the error:

Why can I delcare a constant constructor in the first place? What would make an object of MyClass compile-time constant if no function value can be constant?

A simpler example:

typedef MyVoidCallback = void Function();

void example2() {
  const MyVoidCallback bar = () {};
}

This gives the error:

Thank you in advance

CodePudding user response：

It is because () {} is not a constant value, it is rather creating a new instance every time. All functions in dart inherit Function class which doesn't have const constructor.

However, since functions are top-level members in dart, you can pass them by name (like a variable). So if you define your function outside of any class such that it is a global function, you can pass it as a parameter value in a const constructor.

UPDATE from @randal-schwartz comment: Static functions inside a class can also be passed into these const constructors as parameters.

Below code should work:

class MyClass {
  final void Function() callback;

  const MyClass(this.callback);
}


void example1() {
  const foo = MyClass(doWork);
}

void doWork() {
  // TODO: do work
}