如何从Firebase存储桶中获取图像，然后在传递给方法之前将其分配给变量？

Question

我正在尝试从Firebase存储桶中获取图像（正在运行），但似乎试图在通过异步接收到图像之前加载该图像。似乎在设置图片网址之前已分配findScreen。

我完全无法描述这个问题，所以我希望代码示例能更好地说明问题。

// This function is getting the image and returning it.
// It is outside of the class structure
// In file called find_screen.dart and imported in main.dart

Future<String> _getMainBg() async {
  final FirebaseApp app = await FirebaseApp.configure(
    name: 'test',
    options: FirebaseOptions(
      googleAppID:'appID',
      apiKey: 'APIKEY',
      projectID: 'sample_project',
    ),
  );
  try {
    final FirebaseStorage storage = FirebaseStorage(
        app: app, storageBucket: 'gs://storagebucket.appspot.com');
    var ref = storage.ref().child('main_bg.jpg');
    String url = (await ref.getDownloadURL()).toString();
    return url;
  } on PlatformException {
    return "some error";
  }
}


// This is a variable holding the Screen object.
// This is also outside of the class scope. 
// In file called find_screen.dart and imported in main.dart

final findScreen = new Screen(
  title: 'Some Title',
  background: new DecorationImage(
    image: new NetworkImage(_getMainBg.toString()), 
    fit: BoxFit.cover,
  ),
  contentBuilder: (BuildContext context) {.......});


// This is called from the main.dart file which sets activeScreen as findScreen


case "find":
  setState(() => activeScreen = findScreen);
break;

我希望函数_getMainBg返回存储桶中图像的URL并分配给图像：new NetworkImage（），然后将其传递给activeScreen。