How to bind to a property within a ObservableMap in JavaFX?

Question

I am trying to automatically update a JavaFX ListView when a change occurs on a Property located within an ObservableMap.

Below is my model, where I have a Project, containing a list of Seats, and each Seat in turn contains a Map of type <Layer, ObjectProperty<Category>>.

What I am trying to achieve is to bind an ui element to that ObjectProperty<Category> within the Map.

Here is the Model:

public class Seat {

    private final DoubleProperty positionX;
    private final DoubleProperty positionY;
    private final MapProperty<Layer, ObjectProperty<Category>> categoryMap;


    public Seat() {
        this.positionX = new SimpleDoubleProperty();
        this.positionY = new SimpleDoubleProperty();
        this.categoryMap = new SimpleMapProperty(FXCollections.observableHashMap());
    }
}

public class Project {

    private ObservableList<Seat> seatList;
    public Project() {

        seatList = FXCollections.observableArrayList(

                new Callback<Seat, Observable[]>() {
                    @Override
                    public Observable[] call(Seat seat) {
                        return new Observable[]{
                                seat.categoryMapProperty()
                        };
                    }
                }
        );
    }

The UI element I want to bind is a ListView with a custom cell as follows:

public class CategoryCell extends ListCell<Category>{

    private ToggleButton viewButton; 
    private Rectangle colorRect;
    private Label name;
    private Label count;
    private GridPane pane;

    public CategoryCell(ObservableList<Seat> seatList) {
        super();
        buildGui();

        itemProperty().addListener((list, oldValue, newValue) -> {
            if (newValue != null) {
                //Bind color
                colorRect.fillProperty().bind(newValue.colorProperty());

                //Bind category name
                name.textProperty().bind(newValue.nameProperty());

                //Bind number of seats assigned to this category

                LongBinding categorySeatNumProperty = Bindings.createLongBinding(() ->
                    seatList.stream().filter(seat -> seat.getCategory(newValue.getLayer()).equals(newValue)).count(), seatList);
                count.textProperty().bind(categorySeatNumProperty.asString());
            }
            if (oldValue != null) {
                name.textProperty().unbind();
                count.textProperty().unbind();
                colorRect.fillProperty().unbind();
            }
        });

    }

      private void buildGui() {
          FontIcon hidden = new FontIcon("mdi-eye-off");
          viewButton = new ToggleButton("");
          viewButton.setGraphic(hidden);
          viewButton.selectedProperty().addListener((observable,oldValue, newValue) -> {
              Category category = itemProperty().get();
              if (newValue == true) {
                  category.shownColorProperty().unbind();
                  category.setShownColor(Color.TRANSPARENT);
              }else {
                  category.shownColorProperty().bind(category.colorProperty());
              }
          });

            colorRect = new Rectangle(30,30);
            name = new Label();
            name.setMaxWidth(120);
            pane = new GridPane();
            count = new Label();
            count.setPadding(new Insets(0,0,0,10));

            ColumnConstraints nameCol = new ColumnConstraints();
            nameCol.setHgrow( Priority.ALWAYS );
            pane.getColumnConstraints().addAll(
                    new ColumnConstraints(40), 
                    new ColumnConstraints(40), 
                    nameCol,
                    new ColumnConstraints(40));

            pane.addColumn(0, viewButton);
            pane.addColumn(1, colorRect);
            pane.addColumn(2, name );
            pane.addColumn(3, count);

            this.setText(null);
            name.setOnMouseClicked(new EventHandler<MouseEvent>() {  
                  @Override  
                  public void handle(MouseEvent event) {  
                    if (event.getClickCount()==2) {  
                        //launch the category editor TODO
                    }  
                  }  
            }); 

    }

The problem is that the code below is not triggered when I change the Category value of a CategoryProperty within the MapProperty of a Seat.

//Bind number of seats assigned to this category

                LongBinding categorySeatNumProperty = Bindings.createLongBinding(() ->
                    seatList.stream().filter(seat -> seat.getCategory(newValue.getLayer()).equals(newValue)).count(), seatList);
                count.textProperty().bind(categorySeatNumProperty.asString());
            }

Any advice on how to achieve this?

===== Clarifications following James_D comment ====

1) About the model: I have actually thought and hesitated quite a bit about this. I want to allocate categories to seats in concert halls, and do this on multiple "layers/levels". Say for example a price "layer" where I could have four price tag categories, and "selling company" layer where I would have 3 companies, etc... In order to model this in my Seat class I have a Map<Layer, Category> which looks like a good choice as a seat should only be assigned to one unique category per layer. Then my Project class keeps track of Layers and their respective Categories, which is not really needed but handy to keep their user-specified display order.

2) Thank you for spotting that bug in the CategoryCell! The order of if (oldValue != null) and if (newValue != null) should indeed be reversed.

3) Now what I need to answer my initial question is a way to trigger a notification when the categoryProperty in the Map of the Seat class is modified. Actually, just refreshing the listview whenever I make a change to my Map solves the issue, but it kinds of defeat the purpose of having a Observable property...

Answer 1

Answering myself now that I understand a little more.

1) How to bind to a property within a ObservableMap?

By using the valueAt() method of a MapProperty. Instead of using ObservableMap<Layer, ObjectProperty<Category>>, use MapProperty<Layer, Category>.

2) How to trigger a notification when the objectProperty in the ObservableMap is modified?

Since we are now using a MapProperty where the value is the object and not the property wrapping it, we can just use the addListener() method of the MapProperty.