Passing values between classes in Vaadin

Question

This might be more of a Java question, but how would you access values (say from a textfield) of a given view/class from a different class? For example if there was a TextField t1 that is in the MainView, and I wanted to get its current value for a computation in a different class. And is there a more Vaadin-specific approach here?

Answer 1

That can depend on the use case specifically. Since you mentioned a TextField value I assume the value is not yet stored in the DB, it's just on the UI yet -> I rule out singleton spring services.

A few ideas:

1. If the MainView and the different class are nested components and it's viable and not really complicated across a lot of classes... then probably passing it down the way when creating the sub-component. This is a naive solution - it can get pretty messy.

   MainView() {
       var t1 = new TextField();
       var d = new Different(t1);
   }
   

2. Fire and listen to Vaadin Component events. If you want really loose coupling, the most universal would be to use the UI instance as the event bus.

   // listen in different class
   ComponentUtil.addListener(attachEvent.getUI(), CloseMenuEvent.class, e -> closeMenu());
   // fire change in MainView
   ComponentUtil.fireEvent(ui, new CloseMenuEvent(ui))
   

3. A more specific version of number 2. is to pass the ValueChangeListener of the MainView's t1 to the different class.

   MainView() {
       var t1 = new TextField();
       var d = new DifferentClass();
       t1.addValueChangeListener(d::t1Changed)
       add(t1, d);
   }
   

4. Extract the common field to a third party, to a third class. Use a @UIScoped spring bean (@SpringComponent, @Service, ...) that will hold that field, and inject it to both MainView and the different class.

   @Route
   public class MainView extends VerticalLayout {
       public MainView(Model m, Different d) {
           add(m.t1, d);
       }
   }
   
   @Scope(SCOPE_PROTOTYPE)
   public class Different extends Component {
       public Different(Model m) {
           // something with m.t1
       }
   }
   
   @UIScoped
   public class Model {
       public final TextField t1 = new TextField(); // TODO use getter
   }
   

   You could change the 4th approach by keeping String in Model and having a value change listener that updates it.