How to get specific instance of controller from parent controller at JavaFX2?

Question

I have 2 fxml layouts, one of which includes another. I try to update contents of inner element from controller of parent, ie when button at parent controller is pressed change image at imageview.

I use approach, proposed here . The problem is when I call method that suppose to change image inside the imageView ( questionController.setPdfPageImage(++currentPage); ), nothing happens. I tried some debugging and I believe that is because there are two completely different controller instances: one that is called from runtime and another that is called from fxml by default. Please point me to the right solution.

part of main.fxml

<BorderPane 
    fx:id="mainContainer" 
    xmlns="http://javafx.com/javafx/8" 
    xmlns:fx="http://javafx.com/fxml/1"
    fx:controller="com.package.name.MainController">
    <center>
        <Pane fx:id="center">
            <fx:include source="question.fxml"/>
        </Pane>
    </center>
    <bottom>
        <VBox fx:id="bottom"
              BorderPane.alignment="CENTER">
            <children>
                <Button fx:id="next" text="Next"/>
            </children>
        </VBox>
    </bottom>
</BorderPane>

part of question.fxml

<SplitPane fx:id="content" 
    dividerPositions="0.5" 
    xmlns="http://javafx.com/javafx/8" 
    xmlns:fx="http://javafx.com/fxml/1"
    fx:controller="com.package.name.QuestionController">
    <items>
        <AnchorPane fx:id="questionContainer">
            <children>
                <ImageView fx:id="questionView" pickOnBounds="true" preserveRatio="true"/>
            </children>
        </AnchorPane>
    </items>
</SplitPane>

part of QuestionController.java

public void setPdfPageImage(int pageNum) {
//      InputStream is = QuestionController.class.getResourceAsStream(currentPdf);
        InputStream is = this.getClass().getResourceAsStream(currentPdf);
        Image convertedImage;
        try {
            PDDocument document = PDDocument.load(is);
            List<PDPage> list = document.getDocumentCatalog().getAllPages();
            PDPage page = list.get(pageNum);
            BufferedImage image = page.convertToImage(BufferedImage.TYPE_INT_RGB, 128);
            convertedImage = SwingFXUtils.toFXImage(image, null);
            document.close();
            questionView.setImage(convertedImage);    
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

part of MainController.java

    public class MainController implements Initializable {
    QuestionController questionController;
    @FXML //  fx:id="next"
    private Button next; // Value injected by FXMLLoader
    @Override
    public void initialize(URL location, ResourceBundle resources) {
        FXMLLoader loader = new FXMLLoader(getClass().getResource("/question.fxml"));
        try {
            loader.load();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
        questionController = (QuestionController) loader.getController();
        next.setOnAction(event -> {
            //TODO remove hardcoded value 49
            if (currentPage < 49)
                questionController.setPdfPageImage(++currentPage);
            else {
                Alert alert = new Alert(Alert.AlertType.INFORMATION);
                alert.setTitle("Quiz finished");
                alert.setHeaderText(null);
                alert.setContentText("This was the last question. Thank you!");
                alert.showAndWait();
            }
        });
    }
}

Answer 1

To inject the controller of the included fxml, add the fx:id attribute to the fx:include tag:

<fx:include fx:id="question" source="question.fxml"/>

This will inject the controller to the field with the name <fx:id>Controller , ie questionController in this case (if such a field is visible to the loader).

(Also remove the loader part from the initialize method).