Question 397206 in Flutter Question posted in
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Flutter – Can't call onTap function

It always appears error below when I run the code. Could anyone give an advice? Thanks.

Error: Not a constant expression.

page.dart

MyButton(text: 'Get Started',onTap: (){Navigator.pushNamed(context,'/menupage');},),

button.dart

class MyButton extends StatelessWidget {final String text;final void Function()? onTap;

const MyButton({super.key,required this.text,required this.onTap,});

@override
Widget build(BuildContext context){
    return GestureDetector(
    onTap: onTap,
    child: Container(
        decoration: BoxDecoration(color: Color.white,),
        padding: EdgeInsets.all(20),
        child: Row(
            mainAxisAlignment: MainAxisAlignment.center
            children: [
                Text(
                    text,
                    style: TextStyle(
                        color: Colors.white,
                        fontWeight: FontWeight.bold,),),],
Tags:

2

Answers


  1. 0

    Remove the const keyword from the constructor:

    class MyButton extends StatelessWidget {
  final String text;
  final void Function()? onTap;

  MyButton({super.key, required this.text, required this.onTap,});
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  2. 0

    Given your code for MyButton is this:

    
class MyButton extends StatelessWidget {
  final String text;
  final void Function()? onTap;
  const MyButton({
    super.key,
    required this.text,
    required this.onTap,
  });
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onTap,
      child: Container(
        decoration: BoxDecoration(
          color: Colors.white,
        ),
        padding: const EdgeInsets.all(20),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              text,
              style: const TextStyle(
                color: Colors.white,
                fontWeight: FontWeight.bold,
              ),
            ),
          ],
        ),
      ),
    );
  }
}

    Based on your comments, I’m assuming this is how you’re taking an instance of the MyButton widget:

      const MyButton(text: 'Get Started',onTap: (){Navigator.pushNamed(context,'/menupage');},);
  ^^^^^

    Note that you should NOT use const keyword in front of the MenuButton line. This is because you’re assigning a function to the onTap parameter, which is not a compile-time constant.

    If you don’t use a const right in front of the MenuButton, you might be using const keyword in parent widget’s constructor. Please remove that to fix the issue.

    For example:

      const Row(
  ^^^^^
    children: [
      MyButton(text: 'Get Started', onTap: (){Navigator.pushNamed(context,'/menupage');},),
    ],
  );

    Hope this helps 🙂

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