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'This expression has a type of 'void' so its value can't be used' error in Flutter

I am making an app and am now getting this error. I don’t know why. I’ve tried looking at every other StackOverflow post about this error, and I tried all the suggestions, but nothing worked. Here is the code:

class ParentWidgetState extends State<ParentWidget> {
int counter = 0;
List scrambles = ["F R U R' U' F'", 'F U R B D', 'D B2 U2 S'];

void incrementCounter() {
setState(() {
  counter++;
  });
}

bool active = false;

void _handleTapboxChanged(bool newValue) {
setState(() {
  active = newValue;
    });
}

@override
Widget build(BuildContext context) {
return SizedBox(
  child: TapboxB(
    active: active,
    onChanged: _handleTapboxChanged,
    scrambles: scrambles,
    counter: counter,
    incrementCounter: incrementCounter(), **This is where the error occurs**
        ),
    );
  }
 }

And here you can see I made this ‘incrementCounter’ method required for TapBoxB. The reason why I made this ‘incrementCounter’ required was so that I could call the method inside the onPressed(){} of the TextButton(). Any help is appreciated.

class TapboxB extends StatelessWidget {
 final bool active;
 final ValueChanged<bool> onChanged;

  final List scrambles; 
 final int counter; 
 final int incrementCounter;

 const TapboxB({
super.key,
this.active = false,
required this.incrementCounter,
required this.onChanged,
required this.scrambles, 
required this.counter, 
 });

 void _handleTap() {
onChanged(!active);
 }

  @override
  Widget build(BuildContext context) {
   return GestureDetector(
  onTap: _handleTap,
  child: Container(
    width: 200,
    height: 200,
    decoration: const BoxDecoration(color: null),
    child: Center(
      child: TextButton(
        onPressed: () {
          incrementCounter;
        },
        child: Text('${scrambles[counter % scrambles.length]}'),
      ),
      // child: Text('Ignore this)
      //     ),
    ),
  ),
);
 }
    }

If anyone can provide an answer and explain how they fixed the error, that’d be great.

Tags:

3

Answers


  1. 0

    The problems is within your code:

    SizedBox(
      child: TapboxB(
        active: active,
        onChanged: _handleTapboxChanged,
        scrambles: scrambles,
        counter: counter,
        incrementCounter: incrementCounter(), // This line is the problem since it's calling the function instead of passing the function
      ),
    );

    You are calling the function using parenthesis (),instead you should just pass a reference to it.

    TapboxB(
          active: active,
          onChanged: _handleTapboxChanged,
          scrambles: scrambles,
          counter: counter,
          incrementCounter: incrementCounter // without the () since it's just a reference
          )

    Also, in your code, you are accepting it as an int but it should really be a function.

    Instead of:

    class TapboxB extends StatelessWidget {
  final int incrementCounter;
  final bool active;
  final ValueChanged<bool> onChanged;

    Use:

    class TapboxB extends StatelessWidget {
  final void Function() incrementCounter;
  final bool active;
  final ValueChanged<bool> onChanged;
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  2. 0

    While it is just about getting tap event, I will suggest using VoidCallback.

    class TapboxB extends StatelessWidget {
  final VoidCallback incrementCounter;

  const TapboxB({
    super.key,
    required this.incrementCounter,
  });

  @override
  Widget build(BuildContext context) {
    return TextButton(
      onPressed: incrementCounter,
    );
  }
}

    And use like

    TapboxB(
  incrementCounter: incrementCounter,
),
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  3. 0

    method incrementCounter() doesn’t return a value as evidenced by the keyword void:

    void incrementCounter() {
setState(() {
  counter++;
  });
}

    in the same time TapboxB(...) expect incrementCounter that is int.
    that’s why you got an error:
    ‘This expression has a type of ‘void’ so its value can’t be used’.

    you should pas int:

    Tabbox(
  incrementCounter: <--- here
)

    or you should use void Function() or VoidCallback in the Tabbox:

    class TapboxB extends StatelessWidget {
  final VoidCallback onIncrementCounter;

  const TapboxB({
    super.key,
    required this.onIncrementCounter,
  });

  @override
  Widget build(BuildContext context) {
    return TextButton(
      onPressed: onIncrementCounter,
    );
  }
}
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