Make Function parameter optional in custom widget flutter

Question

I try to create some custom widget with some parameter in the constructor. This widget has some optional and required parameter. how can make Function type paramete

Answer 1

You can use a default value that does nothing:

class TextInputWithIcon extends StatefulWidget {
  final String iconPath;
  final String placeHolder;
  final Function(bool) onFocusChange;
  const TextInputWithIcon(
      {Key key,
      @required this.iconPath,
      this.placeHolder = "",
      this.onFocusChange = _dummyOnFocusChange})
      : assert(onFocusChange != null), super(key: key);

  @override
  _TextInputWithIconState createState() => _TextInputWithIconState();

  static dynamic _dummyOnFocusChange(bool val) {}
}

I created a static named function instead of just a closure as default value, because closures are not const and currently default values need to be const.

I added the assert(...) to ensure that an error is shown when null is passed explicitly.

Answer 2

Another option if you don't like named parameters (like me :/) is:

function_name (argument1, [argument2]) {
   // statements
}

arguments in brackets are optional.

source

Answer 3

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

Named parameters are optional by default so you don't have to assign the default value.

const TextInputWithIcon(
      {Key key,
      @required this.iconPath,
      this.placeHolder = "",
      this.onFocusChange
})
      : super(key: key);

and when calling the onFocusChange perform a null check:

if(this.onFocusChange != null) {
    this.onFocusChange(boolValue)
}

Have a look at Optional Parameters to understand better.

Edit: Thank you Jonah Williams to clarification.