Use VoidCallback type

Check the line comments on the code as well for more information:

class Button extends StatelessWidget {
  final String text;
  final VoidCallback callback; // Notice the variable type

  Button(this.text, this.callback);

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 50,
      child: SizedBox(
        width: double.infinity,
        child: RaisedButton(
          onPressed: callback, // Simply put the function name here, DON'T use ()
          child: Padding(
              padding: EdgeInsets.symmetric(vertical: 13),
              child: Text(
                text,
                style: TextStyle(fontWeight: FontWeight.bold),
              )),
          color: COLOR_BLUE,
          textColor: Colors.white,
          shape:
              RoundedRectangleBorder(borderRadius: BorderRadius.circular(25)),
        ),
      ),
    );
  }
}

• By omitting the parentheses, you are passing the function itself as a parameter. If you added the parentheses, the function would be executed and you would pass the return of the function to the widget.

Then, initialize your widget:

Button("Log in", myMethod)

or

Button("Log in", (){
    print('something');
})