不断更新多个小部件

Question

我正在尝试为我的应用实现简单的下载管理器。我有文件：

downloader.dart ：

List<downloader> downloads = new List<downloader>(); // Used as global variable, to store different downloads.

class downloader{
  double progress;

  void download() async{
    await dio.download(url, path, onProgress: (received, total) { //dio is library to download files
      progress = (received / total).toDouble();
    });
  }

  Stream<double> getStream() async* {
    yield progress;
  }
}

downloadmanager.dart ：

import 'package:appname/downloader.dart' as d; 
//DEFAULT STATEFUL WIDGET STUFF
class DownloadManager extends StatefulWidget {
  @override
  _DownloadManagerState createState() => _DownloadManagerState();
}

class _DownloadManagerState extends State<DownloadManager> {

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: new Text("Download Manager")),
      body:
        new StreamBuilder(
            stream: d.downloads[0].getStream(), //Hardcoded first element of the array, for testing purproses.
            initialData: 0,
            builder: (BuildContext context, AsyncSnapshot<double> snapshot) {
          return new Text(snapshot.data.toString());
        })
    );
  }
}

当我加载DownloadManager StatefulWidget时，它会显示加载时的下载进度，但不会更新。如何使DownloadManager显示当前进度和更新？

注意：加载DownloadManager时，downloads列表中已经有元素，并且download()函数正在执行/完成。

谢谢。