按下按钮时如何显示带有 FutureBuilder 的对话框？

Question

我使用 FutureBuilder 上传图像 FirebaseStorage，当我按下上传按钮时，我想显示一个等待对话框。在 FireStorage 上成功上传图像，但没有显示任何内容。我的代码有什么问题？我认为 FutureBuilder 返回 Widget 并按下按钮无效。也许这就是问题所在。或者我以错误的方式调用 FutureBuilder。您对我的错误代码有什么提示或建议吗？

这是我的代码；

    Widget buildBody(BuildContext context) {
        return new SingleChildScrollView(
          child: new Column(
            children: [
              new SizedBox(
                width: double.infinity,
                child: new RaisedButton(
                  child: new Text('Upload'),
                  onPressed:  () {
                    futureBuilder();
                  },
                ),
              ),
            ],
          ),
        );
      }

     Future mediaUpload() async {
        final fileName = DateTime.now().millisecondsSinceEpoch.toString() + '.jpg';
        final StorageReference storage = FirebaseStorage.instance.ref().child(fileName);
        final StorageUploadTask task = storage.putFile(_image);
        return task.future;
      }

      Widget futureBuilder() {
        return new FutureBuilder(
          future: mediaUpload(),
          builder: (BuildContext context, AsyncSnapshot snapshot) {
            switch (snapshot.connectionState) {
              case ConnectionState.none:
                print('ConnectionState.none');
                return new Text('Press button to start.');
              case ConnectionState.active:
                print('ConnectionState.active');
                return new Text('');
              case ConnectionState.waiting:
                waitingDialog(context);
                return waitingDialog(context);
              case ConnectionState.done:
                print('ConnectionState.done');
                if (snapshot.hasError) return new Text('Error: ${snapshot.error}');
                print(snapshot.data.downloadUrl);
                Navigator.of(context).pushReplacementNamed('home');
                return new Text('Result: ${snapshot.data.downloadUrl}');
            }
          },
        );
      }

      waitingDialog(BuildContext context) {
        return showDialog(
          context: context,
          barrierDismissible: false,
          builder: (BuildContext context) {
            return new Center(
              child: new SizedBox(
                width: 40.0,
                height: 40.0,
                child: const CircularProgressIndicator(
                  value: null,
                  strokeWidth: 2.0,
                ),
              ),
            );
          },
        );
      }

Answer 1

我不是 100% 确定为什么 FutureBuilder 不起作用。但我有一个猜测。在上面的示例中，FutureBuilder 未附加到 widgetTree(screen)。未附加意味着来自FutureBuilder 的返回值不会显示在屏幕上。在这种情况下，Text 基于未附加到屏幕的snapshot.connectionState 返回。

解决方法： （我测试过，它有效，请您验证一下）

futureBuilder(BuildContext context) {
  mediaUpload().then((task) {          // fire the upload
    Navigator.of(context).maybePop();  // remove the dialog on success upload
  });                                  // we can use task(which returned from
                                       // mediaUpload()) to get the values like downloadUrl.
  waitingDialog(context);             // show the spinner dialog
}