JavaFX - Event Filters

Event filters enable you to handle an event during the event capturing phase of event processing. Event Capturing phase in event handling is a phase where an event travels through all the nodes in a dispatch chain. A node in this dispatch chain can either have one or more filters, or no filters at all, to handle an event.

Event Filters process the events, like mouse events, scroll events, keyboard events, etc. during this Event Capturing phase. However, these Event Filters need to be registered with the node in order to provide the event handling logic to the event generated on the node.

If a node does not contain an event filter, the event is directly passed to the target node. Otherwise, a single filter can be used for more than one node and event types. To simply summarize, Event filters are used to enable the parent node to provide common processing for its child nodes; and also to intercept an event and prevent child nodes from acting on the event.

Adding and Removing Event Filter

To add an event filter to a node, you need to register this filter using the method addEventFilter() of the Node class.

//Creating the mouse event handler 
EventHandler<MouseEvent> eventHandler = new EventHandler<MouseEvent>() { 
   @Override 
   public void handle(MouseEvent e) { 
      System.out.println("Hello World"); 
      circle.setFill(Color.DARKSLATEBLUE);  
   } 
};   
//Adding event Filter 
Circle.addEventFilter(MouseEvent.MOUSE_CLICKED, eventHandler);

In the same way, you can remove a filter using the method removeEventFilter() as shown below −

circle.removeEventFilter(MouseEvent.MOUSE_CLICKED, eventHandler);

Example

Following is an example demonstrating the event handling in JavaFX using the event filters. Save this code in a file with name EventFiltersExample.java.

import javafx.application.Application; 
import static javafx.application.Application.launch; 
import javafx.event.EventHandler;
 
import javafx.scene.Group; 
import javafx.scene.Scene; 
import javafx.scene.input.MouseEvent; 
import javafx.scene.paint.Color; 
import javafx.scene.shape.Circle; 

import javafx.scene.text.Font; 
import javafx.scene.text.FontWeight;
import javafx.scene.text.Text; 
import javafx.stage.Stage; 
         
public class EventFiltersExample extends Application { 
   @Override 
   public void start(Stage stage) {      
      //Drawing a Circle 
      Circle circle = new Circle(); 
      
      //Setting the position of the circle 
      circle.setCenterX(300.0f); 
      circle.setCenterY(135.0f); 
      
      //Setting the radius of the circle 
      circle.setRadius(25.0f); 
      
      //Setting the color of the circle 
      circle.setFill(Color.BROWN); 
      
      //Setting the stroke width of the circle 
      circle.setStrokeWidth(20);      
       
      //Setting the text 
      Text text = new Text("Click on the circle to change its color"); 
      
      //Setting the font of the text 
      text.setFont(Font.font(null, FontWeight.BOLD, 15));     
      
      //Setting the color of the text 
      text.setFill(Color.CRIMSON); 
  
      //setting the position of the text 
      text.setX(150); 
      text.setY(50); 
       
      //Creating the mouse event handler 
      EventHandler<MouseEvent> eventHandler = new EventHandler<MouseEvent>() { 
         @Override 
         public void handle(MouseEvent e) { 
            System.out.println("Hello World"); 
            circle.setFill(Color.DARKSLATEBLUE);
         } 
      };  
      //Registering the event filter 
      circle.addEventFilter(MouseEvent.MOUSE_CLICKED, eventHandler);   
       
      //Creating a Group object  
      Group root = new Group(circle, text); 
         
      //Creating a scene object 
      Scene scene = new Scene(root, 600, 300); 
       
      //Setting the fill color to the scene 
      scene.setFill(Color.LAVENDER);  
      
      //Setting title to the Stage 
      stage.setTitle("Event Filters Example");       
         
      //Adding scene to the stage 
      stage.setScene(scene); 
         
      //Displaying the contents of the stage 
      stage.show(); 
   } 
   public static void main(String args[]){ 
      launch(args); 
   } 
}

Compile and execute the saved java file from the command prompt using the following commands.

javac --module-path %PATH_TO_FX% --add-modules javafx.controls EventFiltersExample.java 
java --module-path %PATH_TO_FX% --add-modules javafx.controls EventFiltersExample

Output

On executing, the above program generates a JavaFX window as shown below.

Example

We have seen how event filters processing a mouse event. Now, let us try to register event filters on other events like Keyboard events. Save this code in a file with name EventFilterKeyboard.java.

import javafx.application.Application;  
import javafx.event.EventHandler;  
import javafx.scene.Group;  
import javafx.scene.Scene;  
import javafx.scene.control.Label;  
import javafx.scene.control.TextField;  
import javafx.scene.input.KeyEvent;  
import javafx.scene.paint.Color;  
import javafx.stage.Stage;

public class EventFilterKeyboard extends Application{ 
   @Override  
   public void start(Stage primaryStage) throws Exception {  
      //Adding Labels and TextFileds to the scene   
      Label label1 = new Label("Insert Key");  
      Label label2 = new Label("Event");  
        
      label1.setTranslateX(100);  
      label1.setTranslateY(100);  
        
      label2.setTranslateX(100);  
      label2.setTranslateY(150);  
        
      TextField text1 = new TextField();  
      TextField text2 = new TextField();  
        
      text1.setTranslateX(250);  
      text1.setTranslateY(100);  
      text2.setTranslateX(250);  
      text2.setTranslateY(150);  
        
      //Creating EventHandler Object   
      EventHandler<KeyEvent> filter = new EventHandler<KeyEvent>() {  
         @Override  
         public void handle(KeyEvent event) {   
            text2.setText("Event : "+event.getEventType());  
            text1.setText(event.getText());  
            event.consume();  
         }  
      };  
        
      //Registering Event Filter for the event generated on text field   
      text1.addEventFilter(KeyEvent.ANY, filter);  
        
      //Setting Group and Scene   
      Group root = new Group();  
      root.getChildren().addAll(label1,label2,text1,text2);  
      Scene scene = new Scene(root, 500, 300);  
      primaryStage.setScene(scene);  
      primaryStage.setTitle("Adding Event Filter");  
      primaryStage.show();  
   }  
   public static void main(String[] args) {  
      launch(args);  
   }
}

Compile and execute the saved java file from the command prompt using the following commands.

javac --module-path %PATH_TO_FX% --add-modules javafx.controls EventFilterKeyboard.java 
java --module-path %PATH_TO_FX% --add-modules javafx.controls EventFilterKeyboard

Output

On executing, the above program generates a JavaFX window as shown below.