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

Answer 2

Here is the difference:

onPressed: _incrementCounter is a reference to an existing function is passed.

This only works if the parameters of the callback expected by onPressed and _incrementCounter are compatible.

onPressed: _incrementCounter() _incrementCounter() is executed and the returned result is passed to onPressed. This is a common mistake when done unintentionally when actually the intention was to pass a reference to _incrementCounter instead of calling it.

Answer 3

incrementCounter is a reference to the function. You're passing the function along as a parameter to be invoked somewhere later. This is usually used as callback function, if you have a child widget

incrementCounter() will invoke the function call. For this case, your counter will automatically add 1 when the widget builds, which you don't want to happen.

And usually it is not right to call the function directly, you should write it like this:

onPressed: () {
   _incrementCounter();
},

OR

onPressed: () => _incrementCounter();