Events based Programming

1. Events based Programming Chris North cs3724: HCI

2. Typical command line program • Non-interactive • Linear execution program: main() { code; code; code; code; code; code; code; code; code; code; code; code; }

3. Interactive command line program program: main() { decl data storage; initialization code; loop { get command; switch(command) { command1: code; command2: code; … } } } • User input commands • Non-linear execution • Unpredictable order • Much idle time

4. Typical GUI program GUI program: main() { decl data storage; initialization code; create GUI; register callbacks; main event loop; } Callback1() //button1 press { code; } Callback2() { code; } … • User input commands • Non-linear execution • Unpredictable order • Much idle time • Event callback procs

5. GUI Events App1 App2 mouseclick OK OK Cancel Cancel App2 code: OKbtn_click() { do stuff; } OKbtn_mouseover() { do more stuff; } CancelBtn_click() { do different stuff; } App1 event loop WindowSystem event loop App2 event loop whichapp? inputdevice whichcallback?

6. GUI program GUI program: Class{ main() { decl data storage; initialization code; create GUI objects; register listeners; } listener1() { do stuff; } listener2() { do stuff; } … • Event loop automaticin separate thread

7. Example Example: draw program MyDrawClass{ main() { DataStruct drawn_shapes; drawn_shapes.clear(); create Frame, Panel, buttons, … register listeners; } DrawPanel_listener_click() { drawn_shapes.add(new shape); } UndoButton_listener_click() { drawn_shapes.deleteLast(); } …

8. Listeners • Register with a component to receive events • Give component a ref to your Listener object • JButton1.addMouseListener(new myMouseListener) • Receive events from component • Component will call callback procs on your Listener object • myMouseListener.mouseClicked(event) click JButton1 1. addMouseListener( ) 2. mouseClicked( ) myMouse-Listener

9. Listener API • Listeners must inherit from Listener base classes • ActionListener, KeyListener, MouseListener, MouseMotionListener, WindowListener, … • Abstract base classes: xxxxListener • Stubbed base classes: xxxxAdapter • MouseListener: • mouseClicked(), mouseEntered(), mouseExited(), mousePressed(), mouseReleased()

10. Code button1 = new JButton(“press me”); myListener = new myListenClass; button1.addMouseListener(myListener); // extending a class (“subclassing”): class myListenClass extendsMouseAdapter { public void mouseClicked(MouseEvent e){ // button clicked, do stuff here } } // OR “implementing an interface”: class myListenClass implements MouseListener { public void mouseClicked(MouseEvent e){ // button clicked, do stuff here } … } An abstract base class (methods, no code)

11. Event objects • mouseClicked(MouseEvent e) • MouseEvent: • getX( ), getY( ), getClickCount( ), getSource( ), … • For each listener type: • Component.addxxxxListener( ) • xxxxListener abstract base class • xxxxAdapter stubbed base class • xxxxEvent