Optional parameters

Optional parameters can be either named or positional, but not both.

Named parameters

When calling a function, you can specify named parameters usingparamName: value. For example:

enableFlags(bold: true, hidden: false);

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

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold, bool hidden}) {...}

Although named parameters are a kind of optional parameter,you can annotate them with @required to indicatethat the parameter is mandatory —that users must provide a value for the parameter.For example:

const Scrollbar({Key key, @required Widget child})

If someone tries to create a Scrollbarwithout specifying the child argument,then the analyzer reports an issue.

To use the @required annotation,depend on the meta package and import package:meta/meta.dart.

Positional parameters

Wrapping a set of function parameters in [] marks them as optionalpositional 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 optionalparameter:

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

And here’s an example of calling this function with the third parameter:

assert(say('Bob', 'Howdy', 'smoke signal') ==
    'Bob says Howdy with a smoke signal');

Default parameter values

Your function can use = to define default values for both named and positionalparameters. The default values must be compile-time constants.If no default value is provided, the default value is null.

Here’s an example of setting default values for named parameters:

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold = false, bool hidden = false}) {...}
// bold will be true; hidden will be false.
enableFlags(bold: true);

Deprecation note: Old code might use a colon (:) instead of = to set default values of named parameters. The reason is that originally, only : was supported for named parameters. That support might be deprecated, so we recommend that you use = to specify default values.

The next example shows how to set default values for positional parameters:

String say(String from, String msg,
    [String device = 'carrier pigeon', String mood]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  if (mood != null) {
    result = '$result (in a $mood mood)';
  }
  return result;
}
assert(say('Bob', 'Howdy') ==
    'Bob says Howdy with a carrier pigeon');

You can also pass lists or maps as default values.The following example defines a function, doStuff(),that specifies a default list for the listparameter and a default map for the gifts parameter.

void doStuff(
    {List<int> list = const [1, 2, 3],
    Map<String, String> gifts = const {
      'first': 'paper',
      'second': 'cotton',
      'third': 'leather'
    }}) {
  print('list:  $list');
  print('gifts: $gifts');
}