JavaFx automatically add listeners to dynamically added items

Question

In jquery, there is the on feature which will automatically bind events to items that are added at a later time (such as an ajax call), it looks something like this:

$(document).on("click", ".my-class", function(){
    // Do my event task here
});

Now with that every time an item with the class my-class is added to the dom that event is automatically attached.

Is there anything like that in Java or JavaFx? I would like to keep my events in my controller, and have other methods in what I like to call Helper Classes with the code that I am working on I am not sure how I can achieve that.

So I have this in my controller:

Tree tree = new Tree(currentMysqlConn);
tree.addDatabases(treeItem);

That then creates TreeItem's then that creates child items, and those child items have events when clicked. Here is an example one:

TreeItem tableNode = new TreeItem<>("Tables", tableIcon);
tableNode.expandedProperty().addListener(new ChangeListener(){

    @Override
    public void changed(ObservableValue observable, Object oldValue, Object newValue){
        if((boolean) newValue){
            TableActions ta = new TableActions(tableNode);
            ta.getTables(conn);
        }
    }
});

The issue is, that I have that in my Helper Class, so is there anything I can do to move that into my controller? Because that is inside a loop.

James_D · Accepted Answer

You can register a listener with any observable list (e.g. the list you get from TreeItem.getChildren(), or Parent.getChildren()), and recursively have that listener register the same listener with any children added to the list.

The following example updates a label displaying the total number of nodes in a tree view. To do this, we need to know when child nodes are added to any node, so we need listeners associated with the dynamically added nodes. Your use case might be a bit different, but if you understand how this works you should be able to implement what you need. Note that while I don't have any other classes here, if the contents of the UI are changed from anywhere, the controller is notified appropriately via the listeners.

CountingTreeViewController.java:

import java.util.concurrent.atomic.AtomicInteger;

import javafx.beans.binding.Bindings;
import javafx.beans.binding.IntegerBinding;
import javafx.collections.ListChangeListener;
import javafx.fxml.FXML;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;


public class CountingTreeViewController {
    @FXML
    private TextField itemTextField ;
    @FXML
    private TreeView tree ;
    @FXML
    private Label countLabel ;

    // binding that contains the total count of all nodes
    // the computeValue() method recursively counts the number of children
    // of each node
    // This binding needs to be invalidated if the list of child nodes of any
    // tree node changes...
    private final IntegerBinding count = new IntegerBinding() {

        @Override
        protected int computeValue() {
            // Note the AtomicInteger here is just used as a mutable wrapper.
            // We don't care about the atomicity since it's only updated from 
            // one thread
            return countNodes(tree.getRoot(), new AtomicInteger());
        }

        private int countNodes(TreeItem node, AtomicInteger count) {
            count.incrementAndGet();
            for (TreeItem child : node.getChildren()) {
                countNodes(child, count);
            }
            return count.intValue();
        }
    };

    // A ListChangeListener that will be associated with any tree node. It does two things:
    // First, it invalidates the count binding above if the number of child nodes changes.
    // Second, it adds the same listener to any new child nodes, and removes it from any nodes
    // that are removed from the tree.
    private final ListChangeListener> childrenChanged 
        = new ListChangeListener>() {

            @Override
            public void onChanged(
                    javafx.collections.ListChangeListener.Change> change) {
                while (change.next()) {
                    if (change.wasAdded() || change.wasRemoved()) {
                        count.invalidate();
                    }
                    if (change.wasAdded()) {
                        for (TreeItem item : change.getAddedSubList()) {
                            item.getChildren().addListener(childrenChanged);
                        }
                    } else if (change.wasRemoved()) {
                        for (TreeItem item : change.getRemoved()) {
                            item.getChildren().removeListener(childrenChanged);
                        }
                    }
                }
            }

    };

    public void initialize(){ 
        tree.getRoot().getChildren().addListener(childrenChanged);
        countLabel.textProperty().bind(Bindings.format("Count: %s", count));
    }


    // add a new child node to the selected node (or the root if nothing's selected)
    @FXML
    private void addItem() {
        TreeItem item = tree.getSelectionModel().getSelectedItem();
        if (item == null) {
            item = tree.getRoot();
        }
        TreeItem newItem = new TreeItem<>(itemTextField.getText());
        newItem.setExpanded(true);
        item.getChildren().add(newItem);
        itemTextField.setText("");
        itemTextField.requestFocus();
    }
}

CountingTree.fxml:

CountingTree.java:

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class CountingTree extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {
        FXMLLoader loader = new FXMLLoader(getClass().getResource("CountingTree.fxml"));
        primaryStage.setScene(new Scene(loader.load(), 800, 600));
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

JavaFx automatically add listeners to dynamically added items

Answers (2)

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