在下拉菜单中添加订阅主题

Question

当用户选择他们选择的位置时，我希望添加一个订阅主题，任何可以帮助我将其放入我的项目的人都可以添加到我的项目中，但是当我转到firebase时，它不会订阅确定我做错了

 SizedBox(height: 8.0),
                StreamBuilder<QuerySnapshot>(
                  stream: locationValue(),
                  builder: (context, snapshot) {
                    if (!snapshot.hasData) {
                      return Text('Loading...');
                    } else {
                      List<DropdownMenuItem> clubLocations = [];
                      for (int i = 0; i < snapshot.data.documents.length; i++) {
                        DocumentSnapshot ds = snapshot.data.documents[i];
                        clubLocations.add(DropdownMenuItem(
                          child: Text(
                            ds.documentID,
                            style: TextStyle(color: Colors.black),
                          ),
                          value: '${ds.documentID}',
                        ));
                      }
                      return Row(
                        mainAxisAlignment: MainAxisAlignment.start,
                        children: <Widget>[
                          DropdownButton(
                            items: clubLocations,
                            onChanged: (locationValue) {
                              final snackBar = SnackBar(
                                content: Text(
                                  'Selected Location is $locationValue',
                                  style: TextStyle(color: Colors.black),
                                ),
                              );
                              Scaffold.of(context).showSnackBar(snackBar);
                              this.setState(() {
                                print(locationValue);
                                _selectedLocation = locationValue;
                              });
                            },
                            value: _selectedLocation,
                            isExpanded: false,
                            hint: new Text(
                              'Choose Location',
                              style: TextStyle(color: Colors.black),
                            ),
                          ),
                        ],
                      );
                    }
                  },
                ),
                SizedBox(height: 8.0),