如何在Flutter中观察Button的onPressed函数？

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final StreamController _controller = StreamController();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Button Observer'),
        ),
        body: Center(
          child: StreamBuilder(
            stream: _controller.stream,
            builder: (BuildContext context, AsyncSnapshot snapshot) {
              return RaisedButton(
                onPressed: () {
                  _controller.add('Button Pressed');
                },
                child: Text('Button'),
              );
            },
          ),
        ),
      ),
    );
  }
}