Null value return from push

Question

When I click button The value is the return value of the Navigator.push() - method. "because pressing the Button which returns null, does not make the Future go null, but its content. This leads to the unintentional behaviour"

enter image description here home Screen

PopupMenuButton(
            icon: Icon(Icons.more_vert,color: Colors.black,),
            offset: Offset(0, 40),
            itemBuilder: (context) => [
              PopupMenuItem(
                child: Text('View Cart'),
                onTap: () async {
                final result = await Navigator.push(
                    context,
                    MaterialPageRoute(
                        builder: (context) => CartScreen(cart: cart)),
                  ) ;
                  if(result)
                  {
                    setState(() {
                      
                    });
                  }
                },
              ),

CartScreen

 WillPopScope(

      onWillPop: () {
        Navigator.pop(context,true);

         return new Future(() => false);;
      },
      child: Scaffold(
        appBar: AppBar(
          backgroundColor: Colors.white,
          iconTheme: IconThemeData(color: Colors.black),
          elevation: 0,
        ),

when i click the button to navigate page

Answer 1

I was trying to use the onSelected property

PopupMenuButton<int>(
                      onSelected: (result) {
                        if (result == 1) {
                          nav(context, snap.data![index]);
                        }
                      },
                      icon: Icon(Icons.more_vert_outlined),
                      itemBuilder: (context) => [
                        PopupMenuItem(
                            onTap: () async {
                             
                            },
                            value: 1,
                            child: Text("Edit")),
                        PopupMenuItem(
                            onTap: () async {
                              Map<String, dynamic>? res = await api
                                  .deletepost(snap.data![index].id!);
                              ScaffoldMessenger.of(context).showSnackBar(
                                  SnackBar(
                                      backgroundColor: Colors.amber,
                                      content:
Text(
                                  " ${snap.data![index].id} isdeleted ")));
                            },
                            value: 2,
                            child: Text("Delete")),
                      ],
                    ),
                 


nav(BuildContext context, TODO td) async {
API api = API();
List<dynamic>? data = await Navigator.push(
  context,
  MaterialPageRoute(
      builder: (context) => UpdateTodo(
            td: td,
          )));
print(data);

if (data != null) {
var res = await api.updatePost(data);
print(res);

ScaffoldMessenger.of(context).showSnackBar(
    SnackBar(backgroundColor: Colors.amber, content: Text("Edit 
succes")));
}
}

Note: Popup routes Routes don't have to obscure the entire screen. [PopupRoute]s cover the screen with a [ModalRoute.barrierColor] that can be only partially opaque to allow the current screen to show through. Popup routes are "modal" because they block input to the widgets below.

There are functions which create and show popup routes. For example: [showDialog], [showMenu], and [showModalBottomSheet]. These functions return their pushed route's Future as described above. Callers can await the returned value to take an action when the route is popped, or to discover the route's value.enter code here

There are also widgets which create popup routes, like [PopupMenuButton] and [DropdownButton]. These widgets create internal subclasses of PopupRoute and use the Navigator's push and pop methods to show and dismiss them.

Answer 2

Flutter doesn't recognize the type of result, and casts it as dynamic. In order to avoid this, I suggest you to change the following:

final result = await Navigator.push(
                    context,
                    MaterialPageRoute<bool>(
                        builder: (context) => CartScreen(cart: cart)),
                  ) ;
               if(result == null) return ...;
               if(result)
                  {
                    setState(() {
                      ...
                    });

In particular, specify explicitly the return type of MaterialPageRoute and implement something when return is null.

Answer 3

I have like this for example :

Main Page

  await Navigator.of(context).pushNamed(
        PageNames.routeTest,
      ).then(
        (result) {
            if (result != null && result as bool) {
                [...do something...]
            }
          }
        },
      );

Test Page

Could be just a

/// Could be just like :
Navigator.of(context).pop(true);

/// Or
 onWillPop: () {
    Navigator.of(context).pop(true);
    return true;
 },

Try to use .then in the future function and use Navigator.of(context).pop. Tell me if that works