What does the empty parentheses after the onPressed property mean in Dart?

Question

I know the syntax for calling a function after onPressed and onTap for a widget. There are two options We can use either the () => functio

Answer 1

They are not the same thing. According the the language docs, the fat arrow is syntactical sugar for a return statement.

https://www.dartlang.org/guides/language/language-tour#functions

() => function()

is comparable to this line

(){ return function(); }

not this statement

() { function(); } //returns void

I guess you got away with it because both handlers have a tendency to be void.

https://docs.flutter.io/flutter/dart-ui/VoidCallback.html

https://docs.flutter.io/flutter/gestures/GestureTapCallback.html

https://docs.flutter.io/flutter/material/ListTile/onTap.html

https://docs.flutter.io/flutter/material/IconButton/onPressed.html

void main() {
  num add(a,b) => a + b;
  num add_void(a,b) { a+b; }
  for (int i = 0; i < 5; i++) {
    print('hello ${i + 1}');
    print(add(i,i));
    print(add_void(i,i));
  }
}