custom_action_page.dart

import 'package:flutter/material.dart';

class CustomActionPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Custom Action Page")),
      body: Center(
        child: CustomActionButton(
          doAction: openDialog,
        ),
      ),
    );
  }

  openDialog(context) {
    showDialog(
      context: context,
      builder: (context) {
        return AlertDialog(
          content: Text(
            "It's an alert",
          ),
          actions: <Widget>[
            FlatButton(
              child: const Text('OK'),
              onPressed: () {
                Navigator.pop(context);
              },
            )
          ],
        );
      },
    );
  }
}

class CustomActionButton extends StatelessWidget {
  final Function doAction;

  const CustomActionButton({Key key, this.doAction}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return RaisedButton(
      child: Text("Press me"),
      onPressed: () {
        doAction(context);
      },
    );
  }
}