Clojure and JavaFX 2.0 — How to access element's ID from clojure callback function

Question

Following the JavaFX Tutorial here: http://docs.oracle.com/javafx/2/get_started/fxml_tutorial.htm, trying to make it run in Clojure. For now I'm just doing lein run after setting up :aot :all and stuff with (:gen-class) etc. It took a few days of figuring out, but now it seems to be mostly working.

In src/jfxtwo/ClojureExampleController.clj:

(defn -handleSubmitButtonAction [^ActionEvent event]
  (let [actiontarget (Text.)]
    (println "event button pressed")
    (println "Event instance:" event)
    (println "Event class:" (class event))
    (.setText actiontarget "Sign in button pressed...")))

In resources/fxml_example.fxml:

<GridPane fx:controller= "jfxtwo.ClojureExampleController"
      xmlns:fx= "http://javafx.com/fxml" 
      alignment= "center" hgap= "10" vgap= "10"
      styleClass= "root" >
...
    <Button text= "Sign In"
    onAction= "#handleSubmitButtonAction" />

...
    <Text fx:id= "actiontarget"
        GridPane.columnIndex= "1" GridPane.rowIndex= "6" />
...

I have my clojure code able to read the fxml and css file to generate the proper GUI. When I press the button I can see the event handler being called, but I don't know how to access the Text I want to change, or the ActionEvent instance that is associated with the button press. I tried (println event) and (println (class event)) expecting to see something about an ActionEvent instance, but this only results in showing me that for whatever reason the callback function thinks the event is a ClojureExampleController, even though the type hint says it should be an ActionEvent:

event button pressed
Event instance: #<ClojureExampleController jfxtwo.ClojureExampleController@3e61061d>
Event class: jfxtwo.ClojureExampleController

The Java code to do this looks like this:

public class JFXAppSampleController  {
   @FXML private Text actiontarget;
   @FXML protected void handleSubmitButtonAction(ActionEvent event) {
       actiontarget.setText("Sign in button pressed");
   }
}

Clearle the @FXML annotation is doing the magic here. What's going on, and how do I make this work in Clojure?

Also, is there a way to bind the button press to the text change directly in the FXML so I don't really have to handle a gui->gui change in the code-behind, and instead only deal with the logic associated with the button press? I'm guessing yes, but I haven't gotten to that point in the tutorial.

thanks!

Michał Marczyk Michał Marczyk · Accepted Answer · 2013-05-19T07:17:42

You need an explicit this argument in your handler:

(defn -handleSubmitButtonAction [this ^ActionEvent event]
  ...)

Perhaps a more accurate way of putting this is that event is the this argument in your handler as exhibited in the question text and you need to add a second argument to accept the event in and move the type hint to it (and probably rename the first argument to this for readability).

Given the fact that your code gets called at all, it would seem that JavaFX is happy to call a handler without passing it the event at all if it doesn't care about it (as evidenced by not having a formal parameter corresponding to it).

The type hint's only purpose is to allow the Clojure compiler to avoid emitting reflective code when ActionEvent methods are called on event. This will not prevent passing an object of a different type to the function.

Clojure and JavaFX 2.0 — How to access element's ID from clojure callback function

1 Answers