Java Programming. Events and Listeners

Transcription

1 Java Programming Events and Listeners Alice E. Fischer April 19, 2015 Java Programming - Events and Listenersldots 1/12

2 Events and Listeners An event is generated when The user clicks a GUI button or CheckBox or RadioButton The human user types a key. The human user uses the mouse, An internal timer dings. A window is opened or closed An observable object changes. These events are caught by the Java System, which will notify YOUR code of the event if a listener is registered for it. Java Programming - Events and Listenersldots 2/12

3 Catching Events A program can react to an event by registering a listener and an event handler. Any object in your widget tree can be used as a listener. An event handler is a method inside the listener class. For button and timer events, you need a method handle( ActionEvent ev) For mouse events, you need a method handle( MouseEvent ev) For key events, you need a method handle( KeyEvent ev) Java Programming - Events and Listenersldots 3/12

4 Event Handlers An event handler is a method that defines what the program should do to respond to the event. The parameter to the handle() method will be the event that happened. The parameter name can be used inside the method to get information about the event, such as the key that was typed or the position of a mouse click. Using that information, your program should make some appropriate response. Commonly, something is added to a scene, removed from a scene, or some feature of the scene is changed (color, position, text). Java Programming - Events and Listenersldots 4/12

5 Listeners A program can react to an event by creating a listener and a handler for it. There are at least four ways to create a handler for an event. Define an inner class that contains a method for handle. Define an anonymous inner class that contains a method for handle. Define a lambda-handler. Register the Application as a listener and define a handle function. Java Programming - Events and Listenersldots 5/12

6 Choosing a Listener A Pane, or anything you can put into a Pane can be used to listen for events. Sometimes you want the pane to listen. Sometimes you want a visible object inside the pane to listen. Java Programming - Events and Listenersldots 6/12

7 For each event you want to use, a handler must be created and a listener must be registered. This is often done in the same line of code, and there are several ways to do it. The event handler can be an instance of an inner class. The Listener can be an instance of an anonymous inner class. You can use a Lambda-handler, which is a simplified syntax for the anonymous inner class. If you have only one kind of event, you can register this to handle it. The registration methods use different syntax but all work the same way. Java Programming - Events and Listenersldots 7/12

8 Use an Inner Class: Egg Suppose you have an object named egg in your scene and want to respond when the mouse is clicked in it. At the end of your controller class, define an inner class with a handle() method. class DoClick implements EventHandler<MouseEvent> { public void handle( MouseEvent e ){ egg.setfill(color.gold); xclick = e.getx(); yclick = e.gety(); } } The registration syntax is: egg.setonmouseclicked ( new DoClick() ); Java Programming - Events and Listenersldots 8/12

9 Often you need more than one inner class. Now suppose you also want to respond when a key is pressed. You need another inner class named DoKey. class DoKey implements EventHandler<KeyEvent> { public void handle( KeyEvent e ){ // Put the key-event actions here. } } The registration syntax would be: egg.setonkeypressed(new DoKey()); Java Programming - Events and Listenersldots 9/12

10 Use an Anonymous Inner Class With this technique, the code that registers the listener is combined with the code that handles the event. Here is an example from the JackOLantern program: stopbut.setonaction(new EventHandler<ActionEvent>(){ public void handle(actionevent ev) { System.out.println("Time to quit."); Platform.exit(); } }); Java Programming - Events and Listenersldots 10/12

11 Use a Lambda Handler A lambda handler is a shortened syntax for the anonymous inner class. The code inside the braces is the body of handle. This example is from the Egg program: egg.setonmousepressed( ev -> { System.out.println("Mouse pressed"); egg.setfill( lightgreen ); }); You can use a lambda handler to call any method defined in the class. this example is from the EggCarton program: buybutton.setonaction ( ev -> { buy( ev ); }); Java Programming - Events and Listenersldots 11/12

12 Register this as a listener. The Application class can be the listener for events. This example is from the TicTacToe program. Declare that the class implements the right kind of handler: public class Game extends Application implements EventHandler<ActionEvent> Then the application needs to be registered as a listener for some action element: newbutton.setonaction( this ); Finally, the class must define a method for handle. public void handle( ActionEvent e ) { System.out.println( Starting a new game. ); outputfield.settext( Choose a square ); resetboard(); } Java Programming - Events and Listenersldots 12/12