Update UI after change variable in AppBar

Hey everybody
Inside the typical build() Widget I have a AppBar with an IconButton showing a double variable. When I click on the IconButton an Alert appears where I can change the value. With a TextButton “Save” I save the value. Everything works fine.
But the change should also refresh the UI in the Stateful Widget where the build() Widget is in place. But it doesn’t. The UI refreshes only if I go to another tab on the TabBar and back.
My question is: How can I force the refresh of the UI clicking the “Save” button on the alert?

Thank you in advance.

Below the code:
Alert:

    return Alert(
        onWillPopActive: true,
        closeIcon: null,
        context: context,
        title: 'Umgebungsradius ändern',
        content: SliderDistWidget(),
        buttons: [
          DialogButton(
            color: cButton,
            splashColor: Colors.transparent,
            child: Text(
              'Speichern',
              style: const TextStyle(color: Colors.white),
            ),
            onPressed: () {
              subFarmer.resume();
              Navigator.pop(context);
              setState(() {
                saveDistance();
              });
            },
          ),
        ]).show();
  } 

build() Widget:
 @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          leading: IconButton(
              iconSize: 20,
              splashRadius: 1,
              icon: Icon(FontAwesomeIcons.plus),
              color: cButton,
              onPressed: () {
                Navigator.push(context,
                    MaterialPageRoute(builder: (context) => FarmerSugScreen()));
              }),
          **actions: [**
**            Padding(**
**                padding: EdgeInsets.only(right: 20.0, top: 5.0),**
**                child: FutureBuilder<double>(**
**                    future: readDistance(),**
**                    builder:**
**                        (BuildContext context, AsyncSnapshot<double> snapshot) {**
**                      if (snapshot.hasData) {**
**                        int radius = snapshot.data!.toInt();**
**                        return IconButton(**
**                            splashRadius: 1,**
**                            iconSize: 50,**
**                            icon: Text('$radius' + ' Km',**
**                                textAlign: TextAlign.center,**
**                                style: TextStyle(**
**                                    fontSize: 15,**
**                                    color: cText,**
**                                    fontWeight: FontWeight.bold)),**
**                            onPressed: () {**
**                              subFarmer.pause();**
**                              actnBtn();**
**                            });**
**                      } else {**
**                        return Container();**
**                      }**
**                    }))**
**          ],**
          automaticallyImplyLeading: false,
          centerTitle: true,
          elevation: 10,
          backgroundColor: cHeader,
          toolbarHeight: toolbarHeight,
          title: SizedBox(
            child: setTitleImg(),
            height: toolbarHeight - 10,
          ),
        ),
        backgroundColor: Colors.white,
        floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
        floatingActionButton: getButtons(),
        body: FutureBuilder(
            future: getInitLoc(),
            builder: (BuildContext context, AsyncSnapshot snapshot) {
              if (snapshot.hasData) {
                return GoogleMap(
                  markers: Set<Marker>.of(markers.values),
                  onMapCreated: onMapCreated,
                  scrollGesturesEnabled: true,
                  tiltGesturesEnabled: true,
                  rotateGesturesEnabled: true,
                  myLocationEnabled: true,
                  myLocationButtonEnabled: false,
                  trafficEnabled: false,
                  zoomGesturesEnabled: true,
                  zoomControlsEnabled: false,
                  initialCameraPosition: CameraPosition(
                    target:
                        LatLng(snapshot.data.latitude, snapshot.data.longitude),
                    zoom: 12.0,
                    bearing: 0.0,
                    tilt: 0.0,
                  ),
                );
              } else {
                return loadingMapScreen();
              }
            }));
  }
}