Make Function parameter optional in custom widget flutter

Question

I try to create some custom widgets with some parameters in the constructor. This widget has some optional and required parameters.

how can make Function type parameter optional in my Widget.

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})
      : super(key: key);

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

class _TextInputWithIconState extends State<TextInputWithIcon> {
@override
  Widget build(BuildContext context) {
    return MY_WIDGET;
   }
}

@GünterZöchbauer. if function not set `The method 'call' was called on null.` — Saman, Jan 30 '19 at 15:57
That's not about the parameter being optional, that's about when no function is passed to that parameter, then of course you can't call it. Can you please explain what the expected behavior is? — Günter Zöchbauer, Jan 30 '19 at 15:58
I try to get focus change in 'TextField' on parent widget. but just in some screen focus change does not matter — Saman, Jan 30 '19 at 16:02
@GünterZöchbauer, thank you for help. `this.onFocusChange != null` solve my problem — Saman, Jan 30 '19 at 16:08

Umair M · Accepted Answer · 2023-04-11T15:27:01.113

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.

If a parameter is optional but can’t be null, provide a default value.

With null safety

class TextInputWithIcon extends StatefulWidget {
  final String iconPath;
  final String placeHolder;
  final Function(bool)? onFocusChange; // nullable and optional
  
  const TextInputWithIcon(
      {Key? key,
      required this.iconPath, // non-nullable and required
      this.placeHolder = "", // non-nullable but optional with a default value
      this.onFocusChange, // nullable and optional
      })
      : super(key: key);

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

}

Without null safety

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

Usage:

void _focusChanged(bool value) {

    // using null-aware operator (for both with and without null safety)
    onFocusChange?.call(value);
    
    // or without null-aware operator 
    
    // with null safety
    if(onFocusChange != null) {
      onFocusChange!(value);
    }

    // without null safety
    if(onFocusChange != null) {
      onFocusChange(value);
    }

  }

Dart 2.17 update:

Although it often makes sense to place positional arguments first, named arguments can be placed anywhere in the argument list when it suits your API:

repeat(times: 2, () {
  ...
});

Have a look at Optional Parameters to understand better.

Edit: Thank you Jonah Williams to clarification.

Is value assignment not required to make named parameters optional? — Umair M, Jan 30 '19 at 18:19
@UmairM need the same solution for Null Safety. Please share any suggestion. Thanks. — Kamlesh, May 16 '21 at 17:27
@Kamlesh I updated the answer, please me know if I missed anything :) — Umair M, May 25 '21 at 10:01
"this.placeHolder = "", // non-nullable but optional with a default value" helped me. Thanks Umair. — Kamlesh, May 25 '21 at 12:57

score 17 · Answer 2 · answered Feb 02 '21 at 10:33

17

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

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

arguments in brackets are optional.

source

answered Feb 02 '21 at 10:33

Giorgos Kavalieratos

415
4
13

What if i have argument3 and argument4 and i also want the argument3 and argument4 to be optional together with argument2 – Emmanuel Njorodongo Oct 18 '22 at 14:01
If you need more arguments, you can add them like: [arg2, arg3, arg4, ...] Then you have to pass the values in the same order since they are not named. – Giorgos Kavalieratos Oct 20 '22 at 05:31
This means you have to pass all of the arguments that are optional you cant parse arg2 and for example arg4 only, right ? – Emmanuel Njorodongo Oct 21 '22 at 12:14

score 15 · Answer 3 · edited Feb 26 '21 at 02:39

15

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 a 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.

edited Feb 26 '21 at 02:39

answered Jan 30 '19 at 16:08

Günter Zöchbauer

623,577
216
2,003
1,567

if my function don't take argument also not return anything it'll be >> (static _dummyOnFocusChange() {} ) is this way correct because i tried and give error at compile time so i tried ( static dynamic _dummyOnFocusChange() {} ) and worked with me. Can you tell me why worked ?? – Magician Apr 07 '20 at 21:19
If you need to pass a parameter depends on `final Function(bool) onFocusChange`. If you don't intend to return anything, use `void`. `dynamic` is kind of a wildcard. – Günter Zöchbauer Apr 08 '20 at 03:18
for my case I need to pass a custom model. I can't just declare the whole model if null, I need extra info that it's not there – Dani Feb 13 '21 at 11:52
@GünterZöchbauer how can we pass optional params and use them in a custom widget? Kindly suggest. Thanks. – Kamlesh May 16 '21 at 17:29

Make Function parameter optional in custom widget flutter

3 Answers3

With null safety

Without null safety

Usage:

Dart 2.17 update:

Linked