Binding labels textProperty to object's property held by another final ObjectProperty

Question

In app I'm bulding I used data model presented by James_D here: Applying MVC With JavaFx

I just can find a way to bind labels text to property of object held in DataModel

Data is structured like this: model class Student

//partial class
public class Student {

private final StringProperty displayName = new SimpleStringProperty();

public final StringProperty displayNameProperty(){
    return this.displayName;
}

public Student(){

}

public final String getDisplayName() {
    return this.displayNameProperty().get();
}

public final void setDisplayName(String displayName) {
    this.displayNameProperty().set(displayName);
}
}

Student instaces are held by StudentDataModel class

public class StudentDataModel {
// complete student list
private final ObservableList<Student> studentList = FXCollections.observableArrayList();
private final ObjectProperty<Student> selectedStudent = new SimpleObjectProperty<>(new Student());
public final Student getSelectedStudent() {
     return selectedStudent.get();
}

public final ObjectProperty<Student> selectedStudentProperty() {
     return selectedStudent;
}

 public final void setSelectedStudent(Student student) {
     selectedStudent.set(student);
}
}

StudentList is displayed by Table View, there is change listener that sets selectedStudent like this:

    public class TableViewController {
    public void initModel(StudentDataModel studentDM) {
    // ensure model is set once
    if (this.studentDM != null) {
        throw new IllegalStateException("StudentDataModel can only be initialized once");
    }

    this.studentDM = studentDM;
    tableView.getSelectionModel().selectedItemProperty().addListener((obs, oldSelection, newSelection) -> {
        if (newSelection != null) {
            studentDM.setSelectedStudent(newSelection);
        }
    });
   }}

There is another controller ActionsBarController that has label to display selected student (this seems redundant, but there is option for selecting multiple objects to perform bulk operations). StudentDataModel is initialized properly (I can see it in debuger) but below doesn't do anything:

chosenStudentLabel.textProperty().bind(studentDM.getSelectedStudent().displayNameProperty());

//this shows class name with instance number changing correctly
chosenStudentLabel.textProperty().bind(studentDM.selectedStudentProperty().asString());

I could inject ActionsBarController to TableViewController and change label text from change Listener there, but this seems counter productive with data model. What am I doing wrong?

Answer 1

Your code doesn't work, because you call (and evaluate) getSelectedStudent() at the time the binding is created (i.e. when you initialize the model). As a consequence, you only bind to the displayName property of the student that is selected at that time. (If nothing is selected, you'll get a NullPointerException.) The binding will only change if that initially-selected student's display name changes; it won't change if the selection changes.

You need a binding that unbinds from the old selected student's display name, and binds to the new selected student's display name, when the selected student changes. One way to do this is:

chosenStudentLabel.textProperty().bind(new StringBinding() {
    {
        studentDM.selectedStudentProperty().addListener((obs, oldStudent, newStudent) -> {
            if (oldStudent != null) {
                unbind(oldStudent.displayNameProperty());
            }
            if (newStudent != null) {
                bind(newStudent.displayNameProperty());
            }
            invalidate();
        });
    }
    @Override
    protected String computeValue() {
        if (studentDM.getSelectedStudent() == null) {
            return "" ;
        }
        return studentDM.getSelectedStudent().getDisplayName();
    }
});

Note that there is also a "built-in" way to do this, but it's a bit unsatisfactory (in my opinion) for a couple of reasons. Firstly, it relies on specifying the name of the "nested property" as a String, using reflection to access it. This is undesirable because it has no way to check the property exists at compile time, it requires opening the module for access, and it is less good performance-wise. Secondly, it gives spurious warnings if one of the properties in the "chain" is null (e.g. in this case if the selected student is null, which is will be initially), even though this is a supported case according to the documentation. However, it is significantly less code:

chosenStudentLabel.textProperty().bind(
    Bindings.selectString(studentDM.selectedStudentProperty(), "displayName")
);