Flutter：来自后退导航的回调产生“错误的 state：调用关闭后无法添加新事件”

Question

I have a list page (stateful widget) that is a list of subsidiaries of a firm and a separate editing page (stateful widget) for editing and removing specific subsidiary. Every row of the list is a widget which has a button that takes user to the editing page where he can also remove the corresponding subsidiary. Below is the code of this button

          FlatButton(
                  onPressed: () => Navigator.of(context).push(MaterialPageRoute(
                        builder: (context) =>
                            BlocProvider<SubsidiaryEditBloc>(
                                 builder: (BuildContext context) => SubsidiaryEditBloc(),
                                 child: SubsidiaryEditPage(subsidiary: subsidiary)
                             ) 
                            )).then((value) {
                              subsidiaryListBloc.dispatch(Reset()); 
                            }),
                  child: Text(
                    "Edit",
                  ),
                )

When editing/removal is done the user navigates back, and I want the list to reflect the update/removal. To achieve this I use the "then" callback on the "push" action where I dispatch "Reset" event on the bloc attached to the list page which makes bloc fetch subsidiaries from repository as if they were requested for the first time. However it does not quite work out. When a subsidiary is edited, saved, and the user navigates back, nothing bad happens, the list is successfully updated. But when a subsidiary is removed, navigating back yields error mentioned in the subject. I am new to flutter and don't yet know all the pitfalls nor clearly understand the good and the bad practices. Is this callback a clear source of this error? I assume that it is bound to specific instance of subsidiary widget so that when list elements change, the widget and bloc are disposed - is that correct? Or am I not allowed to dispatch any bloc events on navigation for some other reason?

Answer 1

Using the "then" keyword usually doesn't work. I use await before the navigator so it will await the page to be popped and then run you dispatch code!

Try this:

 FlatButton(
  onPressed: () async{
   await Navigator.of(context).push(MaterialPageRoute(
       builder: (context) =>
          BlocProvider<SubsidiaryEditBloc>(
                builder: (BuildContext context) => SubsidiaryEditBloc(),
                child: SubsidiaryEditPage(subsidiary: subsidiary)
            ) 
          ));
    subsidiaryListBloc.dispatch(Reset()); 
     },

              child: Text(
                "Edit",
              ),
            )