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

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.