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

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.

See Question&Answers more detail:os

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_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


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