各种widget的()的flutter含义

Question

Ofter times in flutter I come across () used in the code, and I've just been rote memorizing when they need to be there. However, I don't exactly understand what they mean, or even what they are called.

Some examples:

setState((){count++})

FlatButton(child: Text("More Beer"), 
           onPressed: ()=> sendBeer())   // <-- I'm also interested if the two ()'s mean different things.

for sendBeer(), I would call the () and indication that sendBeer is a function, and that it takes no arguments. Is that different from () by itself?

In short, can you explain what "()" in those examples are, what they are called, and what they mean? [Also, sometimes I see it as (_) or even (_,_,_) ]

Answer 1

This is unrelated to widgets/flutter. It is the syntax for defining closures, which are functions as variables.

There are two syntaxes available:

() => value;

and

() {
  return value;
}

Where () represents the arguments of your function. It doesn't have to be strictly () and could be something more complex:

final example = (int variable, {String namedParameter}) => print($variable $namedParameter');

Which is equivalent to:

void example(int variable, {String namedParameter}) {
  print($variable $namedParameter');
}