flutter - What is the difference between calling the function without parentheses and with parentheses

Question

What is the difference between calling the function without parentheses and with parentheses on onPressed or Ontap?

I just know that void function can't be called with parentheses on onPressed.

floatingActionButton: FloatingActionButton(
    onPressed: _incrementCounter,
    tooltip: 'Increment',
    child: Icon(Icons.add),
  ),

_incrementCounter has void return type

void _incrementCounter() {
    setState(() {
        _counter++;
    });  
}

But I didn't find any proper documentation.

Answer 1

_incrementCounter inside onPressed is a function reference, which basically means it is not executed immediately, it is executed after the user clicks on the specific widget.(callback)

_incrementCounter() is a function call and it is executed immediately.

Therefore, inside onPressed you can either pass a function reference or an anonymous function that will act as a callback.

floatingActionButton: FloatingActionButton(
    onPressed: _incrementCounter,
    tooltip: 'Increment',
    child: Icon(Icons.add),
  ),

or

floatingActionButton: FloatingActionButton(
    onPressed: () {
        // Add your onPressed code here!
      },
    tooltip: 'Increment',
    child: Icon(Icons.add),
  ),

The is not something specific to dart, it is also done in javascript and many other languages:

What is the difference between a function call and function reference?

Javascript function call with/without parentheses