UID – Event Handling and Listeners

Boriana Koleva(bnk@cs.nott.ac.uk)

Event handling and listeners

• What is an event? user actions and context event sources and listeners

• Why should my programs be event-driven? User interaction with the GUI

Some typical component events and listeners

Act that results in event Listener

User clicks a button, presses return while typing in a text field, or chooses a menu item

ActionListener

User closes a window WindowListener

User presses a mouse button while the cursor is over a component

MouseListener

User moves the mouse over a component MouseMotionListener

Component becomes visible ComponentListener

Component gets the keyboard focus FocusListener

Table or list selection changes ListSelectionListener

Implementing listeners (1)

• Three key bits of code 1) add interface 2) register 3) handle

• Components can have multiple listeners

• A simple JButton ActionListener…

Implementing listeners (2)

public class myClass … implements ActionListener {

…

// where setting up occurs (e.g. constructor)

JButton button = new JButton(“I am a button”);

button.addActionListener(this);

…

public void actionPerformed(ActionEvent e) {

… // respond to event

} // end response method

} // end class

Types of event listeners (1)

• Global component listeners may be used for any Swing components Types

ComponentListener (changes in size, position, visibility)

FocusListener (whether ability for keyboard input) KeyListener (key press events, only with focus) MouseListener (clicks and movement into/out of

component area) MouseMotionListener (changes in position over

component)

Types of event listeners (2)

• Component-specific listeners relevant to specific components’ actions Types

ActionListener CaretListener ChangeListener DocumentListener ItemListener ListSelectionListener WindowListener etc.

• See:http://java.sun.com/docs/books/tutorial/uiswing/events/eventsandcomponents.html

Working with event listeners

• Getting event information

• Low-level events

• Semantic events

• Adapters for event handling

• Inner classes for event handling

Getting event information

• EventObject class - use sub classes of this to determine what’s happened.

• Get the firing object with getSource();• Actual event classes sometimes have specific types

e.g. the ComponentListener uses a sub-class of EventObject : ComponentEvent that has getComponent();

• Event classes may define methods that return more information e.g. ActionEvent has a method for getting modifiers

(Shift, Alt, Ctrl)

Low-level and semantic events (1)

• Low-level events - window-system level e.g. mouse, key, component, container, focus,

window trigger component-independent

• Semantic events everything else! – e.g. action, item, list selection trigger can differ by component

e.g. button click and textfield ‘return’ action events

Low-level and semantic events (2)

• Listen for semantic events whenever possible

Gives robust and portable code eg Button - listen for action event rather than mouse

event. Means that button responds to keyboard shortcuts.

Compound components eg combo box - no real way of guaranteeing low level

listeners on all look and feel specific components used to form the compound component.

Adapters for event handling (1)

• Classes which implement listener interfaces must implement all listener methods e.g. MouseListener has 5 methods: mouseClicked, mouseReleased, mousePressed, mouseEntered, mouseExited

• This leads to cluttered code Say you only want mouseClicked to do

something then all others have to be implemented but empty

• Alternative….

Adapters for event handling (2)

• ... is to extend a MouseAdapter class inherits empty definitions of all five

mouseListener methods. Eg:

public class MyClass extends MouseAdapter {...

someObject.addMouseListener(this); ... public void mouseClicked(MouseEvent e) {

//Event handler implementation goes here...

}

}

Inner classes for event handling (1)

• Don’t want to / cant inherit from an adapter class? there’s no multiple inheritance in Java

eg can’t extend JPanel AND MouseAdapter Solution: use an inner class

• public class MyClass extends JPanel {…anObject.addMouseListener(new myAdapter());…class myAdapter extends MouseAdapter {

public void mouseClicked(MouseEvent e) {// blah

} // end mouseClicked} // end inner class

} // end MyClass

Inner classes for event handling (2)

• Anonymous classes - used to simplify code good when only 1 instance will ever be needed

• public class MyClass extends JPanel { ... someObject.addMouseListener(new MouseAdapter() {

public void mouseClicked(MouseEvent e) { //Event handler implementation goes here

} }); ...

}

Threads and Swing (1)

• Why use them? Improved perceived performance Can remove time consuming task from event

thread to keep GUI responsive Initialisation of program so GUI appears faster

• Potential problems Deadlock the application if access any realised

swing components from non event threads.

Threads and Swing (2)

• Remember the rule: Once a Swing component has been realised, all code that might

affect or depend on the state of that component should be executed in the event-dispatching thread.

• If code does not need to be in event thread then:public void actionPerformed(ActionEvent e) {

final SwingWorker worker = new SwingWorker() { public Object construct() { //---code that might take a while to execute is here...

return someValue; }

}; worker.start(); //required for SwingWorker 3

}

Threads and Swing (3)

• invokeLater() requests that event thread runs certain code can be called from any thread code goes in run method of Runable object returns immediately without waiting for

event thread to execute code.

Runnable updateAComponent = new Runnable() { public void run() {component.doSomething(); }

};SwingUtilities.invokeLater(updateAComponent);

Threads and Swing (4)

• invokeAndWait() identical to invokeLater() except doesn’t return

till event thread has finished executing the code. Should use this if possible - less chance of

deadlock.

void showHelloThereDialog() throws Exception { Runnable showModalDialog = new Runnable() { public void run() { JOptionPane.showMessageDialog(myMainFrame, "Hello There"); } }; SwingUtilities.invokeAndWait(showModalDialog);

}

Summary - but not the end...

• Implementing event listeners• Types of event listeners• Handling event listeners

getting event information low-level and semantic events adapters inner classes - named and anonymous

• Threads

What Covered So Far?

• What is Swing?• Containers

Frames Dialogs (applets)

• Components Loads to choose from

• Layout Managers ‘Educated Trial and Error’

• Events and User Interaction

A simple Swing program

• Uses components in containers• Lays components out correctly• Listens for events

• An example: SwingExample.java (revisited)… Code on Course Website…

A (Slightly) More Complex Swing program

• Uses components in containers (again)

• Lays components out correctly (again - but more complex)

• Listens for events - Multiple listeners

• Another example: SwingExample2.java