When calling a function, you can specify optional named parameters using paramName: value. For example:

enableFlags(bold: true, hidden: false);

When defining a function, use {param1, param2, …} to specify named parameters:

void enableFlags({bool bold, bool hidden}) {...}

Wrapping a set of function parameters in [] marks them as optional positional parameters:

String say(String from, String msg, [String device]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  return result;
}

Here’s an example of calling this function without the optional parameter:

assert(say('Bob', 'Howdy') == 'Bob says Howdy');

if the parameter is required, add @required annotation in front of the parameter, otherwise everything will be optional

theme in MaterialApp object

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(primarySwatch: Colors.deepOrange),
      home: Scaffold(
        appBar: AppBar(
          title: Text("Easy App Flutter"),
        ),
        body: ProductManager("Food Tester"),
      ),
    );
  }
}


RaisedButton(
    color: Theme.of(context).primaryColor,
    onPressed: () {
        setState(() {
        _products.add('Advance Food Test');
        });
    },
child: Text("Add Location")

deepOrange theme with button based on context theme:

Here parameters in Text(),add() and ProductManager() are positional parameters. primarySwatch is a named parameters.

Const and Final

final is a reference which cannot reassign value to the variable, but the object is still mutable. const is not only a immutable reference but also immutable for the value.

final List<String> products = [];
products.add("new");        //this can work.

final List<String> prices = const [];
prices.add("100");          //cannot work.