我试图创建一个下载功能，从URL下载文件到手机存储，我做了一个小部件来显示，如果下载开始，也要使用进度计数器显示getx obs

final ApiServiceController uController = Get.put(ApiServiceController());

downloading(String fileUrl, String fileName) async {
    Dio dio = Dio();
    fToast("Downloading... $fileName", pop);

    try {
      var dir = await downloadDirectory();
      uController.isDwd.value = true;

      await dio.download(fileUrl, "${dir.path}/$fileName",
          onReceiveProgress: (rec, total) {
        print("Rec: $rec , Total: $total");

        uController.dwdP.value = ((rec / total) * 100).toStringAsFixed(0) + "%";
      });
      fToast("Downloaded", pop);
    } catch (e) {
      print(e);
    }
    uController.isDwd.value = false;

    print("Download completed");
  }

要显示的控件

Stack(children: [
///.. Container widget here,
Obx(() {
    return uController.isDwd.value
        ? loading(context, uController.dwdP.value)
        : SizedBox(height: 0);
  })
])

但每当我点击下载按钮开始下载它抛出错误

════════ Exception caught by widgets library ═══════════════════════════════════
The following assertion was thrown building Obx(has builder, dirty, state: _ObxState#291e4):
visitChildElements() called during build.

The BuildContext.visitChildElements() method can't be called during build because the child list is still being updated at that point, so the children might not be constructed yet, or might be old children that are going to be replaced.
The relevant error-causing widget was
Obx

错误将显示在屏幕上，直到下载完成。我看到用这个就能修好

WidgetsBinding.instance.addPostFrameCallback((_) {
      // executes after build
});

但我不知道在这种情况下如何使用它。请有一种方法来解决这个问题，如果你需要更多的解释或信息，请告诉我