Pass a function with parameters to a VoidCallback

Issue

Is it possible to pass a Function with parameters to a VoidCallback?

for example something like this:

class MyClass {
  void doSomething(int i){

  }

  MyOtherClass myOtherClass = new MyOtherClass(doSomething);
}


class MyOtherClass {
  final VoidCallback callback(int);

  MyOtherClass(this.callback);

  callback(5);
}

Solution

The declaration of VoidCallback is

typedef void VoidCallback();

That is the type of functions that can be called with zero arguments and which does not return a useful value. That does not seem to be what you want.
It’s not entirely clear what you do want since the program isn’t syntactically valid, but would this work for you:

class MyClass { 
  static doSomething(int i) { /* ... */ }
  MyOtherClass myOtherClass = new MyOtherClass(doSomething);
}
class MyOtherClass {
  final void Function(int) callback;
  MyOtherClass(this.callback);
  void callCallaback() { callback(5); }
}

Here we define the type of the callback field to be the type of functions that can be called with one integer argument and which returns no useful value. The doSomething method has that type, so it can be assigned to callback.

You could also use a typedef to name the function:

typedef Int2VoidFunc = void Function(int);
// or: typedef void Int2VoidFunc(int arg);
class MyOtherClass {
  final Int2VoidFunc callback;
  MyOtherClass(this.callback);
  void callCallaback() { callback(5); }
}

The effect is exactly the same, it just allows you to use a shorter name for the function type, but that only really makes sense if you use it a lot.

Answered By – lrn

Answer Checked By – Terry (FlutterFixes Volunteer)

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