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Event Handling In this class we will cover: Keyboard Events Mouse Events Focus Events Action Interface Multicasting Keyboard Events Table 8-1 on page 298 shows all AWT listener interfaces, events, and event sources. Keyboard Events

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in this class we will cover
In this class we will cover:
  • Keyboard Events
  • Mouse Events
  • Focus Events
  • Action Interface
  • Multicasting
keyboard events
Keyboard Events
  • Table 8-1 on page 298 shows all AWT listener interfaces, events, and event sources.
  • Keyboard Events
    • When the user pushes a key, a key_Pressed KeyEvent is generated.
    • When the user releases a key, a key_Released KeyEvent is generated.
    • You trap these events in the keyPressed and the keyReleaesd methods of any class that implements the KeyListener interface.
    • A third method, keyTyped, combines the two and it reports on the characters that were generated by the user’s keystrokes.
keyboard events4
Keyboard Events
  • The difference between characters and virtual key codes:
    • Virtual key codes correspond to keys on the keyboard.
      • e.g. VK_A denotes the key marked A
      • There is no separate lowercase virtual key code - the keyboard does not have separate lowercase keys.
      • Virtual key codes are indicated with a prefix of VK_ such as VK_A or VK_SHIFT
    • Here are the five events that are generated when a user tyes an uppercase A
      • Pressed the shift key (keyPressed called for VK_SHIFT)
      • Pressed the A key (keyPressed called for VK_A)
      • Typed “A” (keyTyped called for an “A”)
      • Released the A key (keyReleased called for VK_A)
      • Released the shift key (keyReleased called for VK_SHIFT)
keyboard events5
Keyboard Events
  • So, how can you see the virtual key codes:public void keyPressed (KeyEvent event) { int keyCode = event.getKeyCode();}
  • You can use the isShiftDown, isControlDown, isAltDown, and isMetaDown instead of looking for the virtual key codespublic void keyPressed (KeyEvent event) { int keyCode = event.getKeyCode(); if (keyCode == KeyEvent.VK_RIGHT && event.isShiftDown()) {…}}
keyboard events6
Keyboard Events
  • To obtain the actual character that was typed use the keyTyped method and call the getKeyChar method.public void keyTyped (KeyEvent event) { char keyChar = event.getKeyChar();}
  • Note: not all keystrokes result in a call to keyTyped. Only those keystrokes that generate a Unicode character can be captured in the keyTyped method. You need the keyPressed method to check for cursor keys and other command keys.
mouse events
Mouse Events
  • When a user clicks a mouse button, 3 listener methods are called:
    • mousePressed, mouseReleased and mouseClicked
  • You can use the getX and getY methods of the MouseEvent argument to get the x and y- coordinates of the mouse pointer.
  • If you want to distinguish between single, double and triple clicks, use the getClickCount method.
  • You can also change the cursor moved by the mouse with the getPredefinedCursor method of the Cursor class. See pg. 307 for sample cursor shapes.
  • If the user presses a button while the mouse is in motion, mouseDragged calls are generated instead of mouseClicked calls.
mouse events8
Mouse Events
  • How do we listen to mouse events?
  • Mouse clicks are reported through the mouseClicked procedure, which is part of the MouseListener interface.
  • Mouse move and drag events are defined in a separate interface called MouseMotionListener.
    • because mouse move events occur much more frequently
  • See www2.bc.edu/~bernier/MC697/LectureNotes/Sketch.java
focus events
Focus Events
  • When you type a keystroke, you keystrokes must go to a specific screen object.
  • The window manager (e.g. Windows) directs all keystrokes to the active window.
  • When the active window is controlled by a Java program, the Java window receives the keystrokes and directs them towards a particular component.
  • This component is then said to have focus.
  • When a component has focus, it usually gives a visual cue
    • text fields have a blinking cursor
    • a button has a rectangle around it
  • When a text field has focus, you can type in it.
  • When a button has focus, you can click on it.
focus events10
Focus Events
  • Only one component at a time can have focus.
  • When a user selects a component, that component is said to gain focus.
  • Conversely, when a user selects a different component, the original component is said to lose focus.
  • Some components, such as labels and panels, do not get focus by default because it is assumed that they are just there for decorations or grouping.
    • To override this (for example, to implement a drawing program with panels that paint something in reaction to keystrokes), you need to call:panel.setFocusable(true);
actions
Actions
  • It is common to have multiple ways to activate the same command.
  • You can do this by linking all he events to the same listener.
    • For example, you can have a button that changes the background color and you can have an alternative way of changing the background color by using a set of keystrokes (e.g. alt-b).
  • The Swing package provides a useful mechanism to encapsulate commands and attach them to multiple event sources - the Action interface
action interface
Action Interface
  • The Action object encapsulates the following:
    • a description of the command (as a text string and optional icon)
    • parameters that are necessary to carry out the command (e.g. color to set).
  • You can store and retrieve arbitrary name/value pairs in the aciton object using the putValue and getValue methods
    • action.putValue(Action.NAME, “Blue”);action.putValue(Action.SMALLICON, new ImageIcon(“blue-ball.gif”));
action interface13
Action Interface
  • The Action interface and the AbstractAction adapter class are very useful when using menus. We will discuss this more when we talk about Swing User Interfaces.
  • Read more about the Action interface on page 316.
putting it all together
Putting it all together
  • See www2.bc.edu/~bernier/MC697/LectureNotes/Sketch.java
final notes on event handling
Final Notes on Event Handling
  • Multicasting
    • All AWT event sources support can send the same event to multiple listeners.
    • There is no guarantee about the order in which the events are delivered to the set of registered listeners.
  • Customizing Events
    • You can manipulate the event queue directly to add or remove events. This can be useful sometimes.
    • You can also create a new custom event type and insert it into the event queue and then have it dispatched to a listener - just like regular AWT events.
    • Read more about customizing events on page 32.
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