Flutter - Multiselection ListView

In my app, I do have a list, on which I have implemented the long press selection of this post of Raouf Rahiche. When the selection is enabled I do have a different appbar, that has an IconButton on it, that should disable the selection. But I do not know how to do that.

Till now it is not working the way it should. The behaviour is displayed in the video below.

The longpress-selection is a StatefulWidget:

class _SelectableItems extends State<SelectableItems> {
  bool isSelected = false;
  GoogleMaterialColors googleMaterialColors = new GoogleMaterialColors();

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return new GestureDetector(
        onLongPress: () {
          setState(() {
            isSelected = !isSelected;
          });
          widget.callback();
        },
        onTap: () {
          setState(() {
            isSelected = !isSelected;
          });
          if (widget.longPressEnabled) {
            widget.callback();
          } else {
            Navigator.push(
              context, 
              MaterialPageRoute(builder: (context)=>RecipeDetails(widget.name))
            );
          }
        },
        child: ListTile(
          leading: CircleAvatar(
            child: (isSelected
              ? Icon(
                Icons.check,
                color: Colors.white,
              )
              : (widget.image != "no image"
                ? Container(
                  width: 40.0,
                  height: 40.0,
                  decoration: new BoxDecoration(
                    image: new DecorationImage(
                      colorFilter: ColorFilter.mode(Colors.black.withOpacity(0.2), BlendMode.darken),
                      image: AssetImage(widget.image),
                      fit: BoxFit.cover,
                    ),
                    borderRadius: new BorderRadius.all(new Radius.circular(50.0)),
                  ),
                )
                : Text(
                  widget.name[0].toUpperCase(),
                  style: TextStyle(
                    color: Colors.white,
                    fontSize: 21.0,
                    fontWeight: FontWeight.w400
                  ),
                )
              )
            ),
            backgroundColor: (isSelected
              ? googleMaterialColors.primaryColor()
              : widget.color.withOpacity(1.00)
            )
          ),
          title: Padding(
            padding: EdgeInsets.only(top: 25.0),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: <Widget>[
                widget.title
              ],
            ),
          ),
        ),
      );
  }
}

I am calling this widget inside a SideHeaderListView like this:

bool longPressFlag = false;
List<String> indexList = new List();
//other code

                return SideHeaderListView(                  
                    hasSameHeader: (int a, int b){
                      return snapshot.data[a].name[0] == snapshot.data[b].name[0];                  
                    },
                    itemCount: snapshot.data.length,
                    headerBuilder: (BuildContext context, int index){
                      return new Padding(
                        padding: EdgeInsets.only(top: 30.0, left: 20.0, right: 25.0),
                        child: Container(
                          width: 10.0,
                          child: Text(
                            snapshot.data[index].name[0].toUpperCase(),
                            style: TextStyle(
                              color: googleMaterialColors.primaryColor().withGreen(120),                        
                              fontFamily: "Google-Sans",
                              fontSize: 15.0,
                              fontWeight: FontWeight.w600
                            ),
                          ),
                        ),
                      );
                    },
                    itemExtend: 70.0,
                    itemBuilder: (BuildContext context, int index){

                      Color usedColor = convertColor.convertToColor(snapshot.data[index].backgroundColor);                    
                      String image = snapshot.data[index].image;


                      return SelectableItems(
                        color: usedColor,
                        name: snapshot.data[index].name,
                        title: (searchController.text.isEmpty
                          ? Text(snapshot.data[index].name)
                          : recipeName(searchCondition, snapshot.data[index].name)
                        ),
                        index: index,
                        image: image,
                        longPressEnabled: longPressFlag,
                        //isSelected: selectedFlag,
                        callback: () {
                          if (indexList.contains(snapshot.data[index].name)) {
                            indexList.remove(snapshot.data[index].name);
                          } else {
                            indexList.add(snapshot.data[index].name);
                          }
                          longPress();
                        },
                      );
                    },
                  );

void longPress() {
    setState(() {
      if (indexList.length == 0) {
        longPressFlag = false;
      } else {
        longPressFlag = true;
      }
    });
  }

I hope somebody would be able to solve my problem. Thanks in advance.

from Flutter - Multiselection ListView