How to do it...

We will work with a new file in our project, called platform_alert.dart. Let's get started:

1. Open this new file and create a constructor that will accept a title and message body. This class is just going to be a simple dart object:

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

class PlatformAlert {
  final String title;
  final String message;

  const PlatformAlert({@required this.title, @required 
    this.message})
      : assert(title != null),
        assert(message != null);
}

2. PlatformAlert is going to need a show method that will look at the app's context to determine what type of device it's running on and then show the appropriate dialog widget. 
3. Add this method just after the constructor:

void show(BuildContext context) {
  final platform = Theme.of(context).platform;

  if (platform == TargetPlatform.iOS) {
    _buildCupertinoAlert(context);
  } else {
    _buildMaterialAlert...