Flutter: Passing a function as parameter to a Stateful Widget Class

Question

I have a stateful widget that returns a scaffold as follows:

class TimerPage extends StatefulWidget {
  const TimerPage({Key? key, required this.title}) : super(key: key);

  final String title;

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

class TimerPageState extends State<TimerPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          getMaterialTextButton('1', 'Roboto', 24, keypadPressed('1')),   /// ERROR
          getMaterialTextButton('2', 'Roboto', 24, keypadPressed2('2')),  /// ERROR
        ]
      ),
    );
  }
}

So what I'm trying to do is pass a generic function inside getMaterialTextButton() that will return a material button as follows:

// this function is within the TimerPageState class
Widget getMaterialTextButton(String text, String fontname, double fontsize, Function onPressAction) {
  return (
    MaterialButton(
      onPressed: () {
        onPressAction(text);
      },
      color: Colors.blue,
      textColor: Colors.white,
      child: Text(
          text,
          style: TextStyle(fontFamily: fontname, fontSize: fontsize)
      ),
      padding: const EdgeInsets.all(24),
      shape: const CircleBorder(),
    )
  );
}

// this function is to be called when the button 1 is pressed
// also resides inside TimerPageState class
void keyPressed (String text) {
   print('Pressed: $text');
}

// this function is also to be called when button 2 is pressed
// also resides inside TimerPageState class
void keyPressed2 (String text) {
   print('Pressed: $text');
}

But this doesn't seem to work as dart is giving me an exception: Exception has occurred. _TypeError (type 'Null' is not a subtype of type 'Function') . How can I do this operation properly?

Answer 1

you are passing the return value of the function, not the function itself

change this

getMaterialTextButton('1', 'Roboto', 24, keypadPressed('1')),

to this

getMaterialTextButton('1', 'Roboto', 24, keypadPressed),