javax.microedition.lcdui
Class Canvas

java.lang.Object
  |
  +--javax.microedition.lcdui.Displayable
        |
        +--javax.microedition.lcdui.Canvas

public abstract class Canvas

extends Displayable

The Canvas class is a base class for writing applications that need to handle low-level events and to issue graphics calls for drawing to the display. Game applications will likely make heavy use of the Canvas class. From an application development perspective, the Canvas class is interchangeable with standard Screen classes, so an application may mix and match Canvas with high-level screens as needed. For example, a List screen may be used to select the track for a racing game, and a Canvas subclass would implement the actual game.

The Canvas provides the developer with methods to handle game actions, key events, and pointer events (if supported by the device). Methods are also provided to identify the device's capabilities and keyboard mapping. The key events are reported with respect to key codes, which are directly bound to concrete keys on the device, use of which may hinder portability. Portable applications should use game actions instead of key codes.

Like other subclasses of Displayable, the Canvas class allows the application to register a listener for commands. Unlike other Displayables, however, the Canvas class requires applications to subclass it in order to use it. The paint() method is declared abstract, and so the application must provide an implementation in its subclass. Other event-reporting methods are not declared abstract, and their default implementations are empty (that is, they do nothing). This allows the application to override only the methods that report events in which the application has interest.

This is in contrast to the Screen classes, which allow the application to define listeners and to register them with instances of the Screen classes. This style is not used for the Canvas class, because several new listener interfaces would need to be created, one for each kind of event that might be delivered. An alternative would be to have fewer listener interfaces, but this would require listeners to filter out events in which they had no interest.

Key Events

Applications receive keystroke events in which the individual keys are named within a space of key codes. Every key for which events are reported to MIDP applications is assigned a key code. The key code values are unique for each hardware key unless two keys are obvious synonyms for each other. MIDP defines the following key codes: KEY_NUM0, KEY_NUM1, KEY_NUM2, KEY_NUM3, KEY_NUM4, KEY_NUM5, KEY_NUM6, KEY_NUM7, KEY_NUM8, KEY_NUM9, KEY_STAR, and KEY_POUND. (These key codes correspond to keys on a ITU-T standard telephone keypad.) Other keys may be present on the keyboard, and they will generally have key codes distinct from those list above. In order to guarantee portability, applications should use only the standard key codes.

The standard key codes' values are equal to the Unicode encoding for the character that represents the key. If the device includes any other keys that have an obvious correspondence to a Unicode character, their key code values should equal the Unicode encoding for that character. For keys that have no corresponding Unicode character, the implementation must use negative values. Zero is defined to be an invalid key code. It is thus possible for an application to convert a keyCode into a Unicode character using the following code:

    if (keyCode > 0) {
        char ch = (char)keyCode;
        // ...
    }
 

This technique is useful only in certain limited cases. In particular, it is not sufficient for full textual input, because it does not handle upper and lower case, keyboard shift states, and characters that require more than one keystroke to enter. For textual input, applications should always use TextBox or TextField objects.

It is sometimes useful to find the name of a key in order to display a message about this key. In this case the application may use the getKeyName() method to find a key's name.

Game Actions

Portable applications that need arrow key events and gaming-related events should use game actions in preference to key codes and key names. MIDP defines the following game actions: UP, DOWN, LEFT, RIGHT, FIRE, GAME_A, GAME_B, GAME_C, and GAME_D.

Each key code may be mapped to at most one game action. However, a game action may be associated with more than one key code. The application can translate a key code into a game action using the getGameAction(int keyCode) method, and it can translate a key code into a game action using the getKeyCode(int gameAction) method. The implementation is not allowed to change the mapping of game actions and key codes during execution of the application.

Portable applications that are interested in using game actions should translate every key event into a game action by calling the getGameAction() method and then testing the result. For example, on some devices the game actions UP, DOWN, LEFT and RIGHT may be mapped to 4-way navigation arrow keys. In this case, getKeyCode(UP) would return a device-dependent code for the up-arrow key. On other devices, a possible mapping would be on the number keys 2, 4, 6 and 8. In this case, getKeyCode(UP) would return KEY_NUM2. In both cases, the getGameAction() method would return the LEFT game action when the user presses the key that is a "natural left" on her device.

Commands

It is also possible for the user to issue commands when a canvas is current. Commands are mapped to keys and menus in a device-specific fashion. For some devices the keys used for commands may overlap with the keys that will deliver key code events to the canvas. If this is the case, the device will provide a means transparent to the application that enables the user to select a mode that determines whether these keys will deliver commands or key code events to the application. The set of key code events available to a canvas will not change depending upon the number of commands that are present on the canvas. Game developers should be aware that access to commands will vary greatly across devices, and that requiring the user to issue commands during game play may have a great impact on the ease with which the game can be played.

Event Delivery

The Canvas object defines several methods that are called by the implementation. These methods are primarily for the purpose of delivering events to the application, and so they are referred to as event delivery methods. The set of methods is:

• showNotify()
• hideNotify()
• keyPressed()
• keyRepeated()
• keyReleased()
• pointerPressed()
• pointerDragged()
• pointerReleased()
• paint()
• the CommandListener's commandAction() method

These methods are all called serially. That is, the implementation will never call an event delivery method before a prior call to any of the event delivery methods has returned. (But see the note below.) This property enables applications to be assured that processing of a previous user event will have completed before the next event is delivered.

Calls to the run() method of Runnable objects passed to Display.callSerially() will also be serialized along with calls to the event delivery methods. See callSerially() for further information.

Note: The serviceRepaints() method is an exception to this rule, as it blocks until paint() is called and returns. This will occur even if the application is in the midst of one of the event delivery methods and it calls serviceRepaints().

The key-related, pointer-related, paint(), and commandAction() methods will only be called while the Canvas is actually visible on the output device. These methods will therefore only be called on this Canvas object only after a call to showNotify() and before a call to hideNotify(). After hideNotify() has been called, none of the key, pointer, paint, and commandAction() methods will be called until after a subsequent call to showNotify() has returned. A call to a sun() method resulting from callSerially() may occur irrespective of calls to showNotify() and hideNotify().

The showNotify() method is called prior to the Canvas actually being made visible on the display, and the hideNotify() method is called after the Canvas has been removed from the display. The visibility state of a Canvas (or any other Displayable object) may be queried through the use of the Displayable.isShown() method. The change in visibility state of a Canvas may be caused by the application management software moving MIDlets between foreground and background states, or by the system obscuring the Canvas with system screens. Thus, the calls to showNotify() and hideNotify() are not under the control of the MIDlet and may occur fairly frequently. Application developers are encouraged to perform expensive setup and teardown tasks outside the showNotify() and hideNotify() methods in order to make them as lightweight as possible.

Field Summary

static int	DOWN Constant for the DOWN game action.
static int	FIRE Constant for the FIRE game action.
static int	GAME_A Constant for the general purpose "A" game action.
static int	GAME_B Constant for the general purpose "B" game action.
static int	GAME_C Constant for the general purpose "C" game action.
static int	GAME_D Constant for the general purpose "D" game action.
static int	KEY_NUM0 keyCode for ITU-T key 0.
static int	KEY_NUM1 keyCode for ITU-T key 1.
static int	KEY_NUM2 keyCode for ITU-T key 2.
static int	KEY_NUM3 keyCode for ITU-T key 3.
static int	KEY_NUM4 keyCode for ITU-T key 4.
static int	KEY_NUM5 keyCode for ITU-T key 5.
static int	KEY_NUM6 keyCode for ITU-T key 6.
static int	KEY_NUM7 keyCode for ITU-T key 7.
static int	KEY_NUM8 keyCode for ITU-T key 8.
static int	KEY_NUM9 keyCode for ITU-T key 9.
static int	KEY_POUND keyCode for ITU-T key "pound" (#).
static int	KEY_STAR keyCode for ITU-T key "star" (*).
static int	LEFT Constant for the LEFT game action.
static int	RIGHT Constant for the RIGHT game action.
static int	UP Constant for the UP game action.

Constructor Summary

protected

Canvas() Constructs a new Canvas object.

Method Summary

int	getGameAction(int keyCode) Gets the game action associated with the given key code of the device.
int	getHeight() Gets height of the displayable area in pixels.
int	getKeyCode(int gameAction) Gets a key code that corresponds to the specified game action on the device.
String	getKeyName(int keyCode) Gets an informative key string for a key.
int	getWidth() Gets width of the displayable area in pixels.
boolean	hasPointerEvents() Checks if the platform supports pointer press and release events.
boolean	hasPointerMotionEvents() Checks if the platform supports pointer motion events (pointer dragged).
boolean	hasRepeatEvents() Checks if the platform can generate repeat events when key is kept down.
protected void	hideNotify() The implementation calls hideNotify() shortly after the Canvas has been removed from the display.
boolean	isDoubleBuffered() Checks if the Graphics is double buffered by the implementation.
protected void	keyPressed(int keyCode) Called when a key is pressed.
protected void	keyReleased(int keyCode) Called when a key is released.
protected void	keyRepeated(int keyCode) Called when a key is repeated (held down).
protected abstract void	paint(Graphics g) Renders the Canvas.
protected void	pointerDragged(int x, int y) Called when the pointer is dragged.
protected void	pointerPressed(int x, int y) Called when the pointer is pressed.
protected void	pointerReleased(int x, int y) Called when the pointer is released.
void	repaint() Requests a repaint for the entire Canvas.
void	repaint(int x, int y, int width, int height) Requests a repaint for the specified region of the Screen.
void	serviceRepaints() Forces any pending repaint requests to be serviced immediately.
protected void	showNotify() The implementation calls showNotify() immediately prior to this Canvas being made visible on the display.

Methods inherited from class javax.microedition.lcdui.Displayable

addCommand, isShown, removeCommand, setCommandListener

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

UP

public static final int UP

Constant for the UP game action.

Constant value 1 is set to UP.

DOWN

public static final int DOWN

Constant for the DOWN game action.

Constant value 6 is set to DOWN.

LEFT

public static final int LEFT

Constant for the LEFT game action.

Constant value 2 is set to LEFT.

RIGHT

public static final int RIGHT

Constant for the RIGHT game action.

Constant value 5 is set to RIGHT.

FIRE

public static final int FIRE

Constant for the FIRE game action.

Constant value 8 is set to FIRE.

GAME_A

public static final int GAME_A

Constant for the general purpose "A" game action.

Constant value 9 is set to GAME_A.

GAME_B

public static final int GAME_B

Constant for the general purpose "B" game action.

Constant value 10 is set to GAME_B.

GAME_C

public static final int GAME_C

Constant for the general purpose "C" game action.

Constant value 11 is set to GAME_C.

GAME_D

public static final int GAME_D

Constant for the general purpose "D" game action.

Constant value 12 is set to GAME_D.

KEY_NUM0

public static final int KEY_NUM0

keyCode for ITU-T key 0.

Constant value 48 is set to KEY_NUM0.

KEY_NUM1

public static final int KEY_NUM1

keyCode for ITU-T key 1.

Constant value 49 is set to KEY_NUM1.

KEY_NUM2

public static final int KEY_NUM2

keyCode for ITU-T key 2.

Constant value 50 is set to KEY_NUM2.

KEY_NUM3

public static final int KEY_NUM3

keyCode for ITU-T key 3.

Constant value 51 is set to KEY_NUM3.

KEY_NUM4

public static final int KEY_NUM4

keyCode for ITU-T key 4.

Constant value 52 is set to KEY_NUM4.

KEY_NUM5

public static final int KEY_NUM5

keyCode for ITU-T key 5.

Constant value 53 is set to KEY_NUM5.

KEY_NUM6

public static final int KEY_NUM6

keyCode for ITU-T key 6.

Constant value 54 is set to KEY_NUM6.

KEY_NUM7

public static final int KEY_NUM7

keyCode for ITU-T key 7.

Constant value 55 is set to KEY_NUM7.

KEY_NUM8

public static final int KEY_NUM8

keyCode for ITU-T key 8.

Constant value 56 is set to KEY_NUM8.

KEY_NUM9

public static final int KEY_NUM9

keyCode for ITU-T key 9.

Constant value 57 is set to KEY_NUM09.

KEY_STAR

public static final int KEY_STAR

keyCode for ITU-T key "star" (*).

Constant value 42 is set to KEY_STAR.

KEY_POUND

public static final int KEY_POUND

keyCode for ITU-T key "pound" (#).

Constant value 35 is set to KEY_POUND.

Constructor Detail

Canvas

protected Canvas()

Constructs a new Canvas object.

Method Detail

getWidth

public int getWidth()

Gets width of the displayable area in pixels. The value is unchanged during the execution of the application and all Canvases will have the same value.

Returns:

width of the displayable area

getHeight

public int getHeight()

Gets height of the displayable area in pixels. The value is unchanged during the execution of the application and all Canvases will have the same value.

Returns:

height of the displayable area

isDoubleBuffered

public boolean isDoubleBuffered()

Checks if the Graphics is double buffered by the implementation.

Returns:

true if double buffered, false otherwise.

hasPointerEvents

public boolean hasPointerEvents()

Checks if the platform supports pointer press and release events.

Returns:

true if the device supports pointer events

hasPointerMotionEvents

public boolean hasPointerMotionEvents()

Checks if the platform supports pointer motion events (pointer dragged). Applications may use this method to determine if the platform is capable of supporting motion events.

Returns:

true if the device supports pointer motion events

hasRepeatEvents

public boolean hasRepeatEvents()

Checks if the platform can generate repeat events when key is kept down.

Returns:

true if the device supports repeat events

getKeyCode

public int getKeyCode(int gameAction)

Gets a key code that corresponds to the specified game action on the device. The implementation is required to provide a mapping for every game action, so this method will always return a valid key code for every game action. See above for further discussion of game actions.

Note that a key code is associated with at most one game action, whereas a game action may be associated with several key codes. Then, supposing that g is a valid game action and k is a valid key code for a key associated with a game action, consider the following expressions:

    g == getGameAction(getKeyCode(g))     // (1)
    k == getKeyCode(getGameAction(k))     // (2)
 

Expression (1) is always true. However, expression (2) might be true but is not necessarily true.

The mapping between key codes and game actions will not change during the execution of the application.

Parameters:

gameAction - the game action

Returns:

a key code corresponding to this game action

Throws:

IllegalArgumentException - if gameAction is not a valid game action

getKeyName

public String getKeyName(int keyCode)

Gets an informative key string for a key. The string returned will resemble the text physically printed on the key. This string is suitable for displaying to the user. For example, on a device with function keys F1 through F4, calling this method on the keyCode for the F1 key will return the string "F1". A typical use for this string will be to compose help text such as "Press F1 to proceed."

This method will return a non-empty string for every valid key code.

There is no direct mapping from game actions to key names. To get the string name for a game action GAME_A, the application must call

    getKeyName(getKeyCode(GAME_A))
 

Parameters:

keyCode - the key code being requested

Returns:

a string name for the key

Throws:

IllegalArgumentException - if keyCode is not a valid key code

getGameAction

public int getGameAction(int keyCode)

Gets the game action associated with the given key code of the device. Returns zero if no game action is associated with this key code. See above for further discussion of game actions.

The mapping between key codes and game actions will not change during the execution of the application.

Parameters:

keyCode - the key code

Returns:

the game action corresponding to this key, or 0 if none

Throws:

IllegalArgumentException - if keyCode is not a valid key code

keyPressed

protected void keyPressed(int keyCode)

Called when a key is pressed.

The getGameAction() method can be called to determine what game action, if any, is mapped to the key. Class Canvas has an empty implementation of this method, and the subclass has to redefine it if it wants to listen this method.

Parameters:

keyCode - The key code of the key that was pressed.

keyRepeated

protected void keyRepeated(int keyCode)

Called when a key is repeated (held down). The getGameAction() method can be called to determine what game action, if any, is mapped to the key. Class Canvas has an empty implementation of this method, and the subclass has to redefine it if it wants to listen this method.

Parameters:

keyCode - The key code of the key that was repeated

See Also:

hasRepeatEvents()

keyReleased

protected void keyReleased(int keyCode)

Called when a key is released. The getGameAction() method can be called to determine what game action, if any, is mapped to the key. Class Canvas has an empty implementation of this method, and the subclass has to redefine it if it wants to listen this method.

Parameters:

keyCode - The key code of the key that was released

pointerPressed

protected void pointerPressed(int x,
                              int y)

Called when the pointer is pressed. The hasPointerEvents() method may be called to determine if the device supports pointer events. Class Canvas has an empty implementation of this method, and the subclass has to redefine it if it wants to listen this method.

Parameters:

x - The horizontal location where the pointer was pressed (relative to the Canvas)

y - The vertical location where the pointer was pressed (relative to the Canvas)

pointerReleased

protected void pointerReleased(int x,
                               int y)

Called when the pointer is released. The hasPointerEvents() method may be called to determine if the device supports pointer events. Class Canvas has an empty implementation of this method, and the subclass has to redefine it if it wants to listen this method.

Parameters:

x - The horizontal location where the pointer was released (relative to the Canvas)

y - The vertical location where the pointer was released (relative to the Canvas)

pointerDragged

protected void pointerDragged(int x,
                              int y)

Called when the pointer is dragged. The hasPointerMotionEvents() method may be called to determine if the device supports pointer events. Class Canvas has an empty implementation of this method, and the subclass has to redefine it if it wants to listen this method.

Parameters:

x - The horizontal location where the pointer was dragged (relative to the Canvas)

y - The vertical location where the pointer was dragged (relative to the Canvas)

repaint

public final void repaint(int x,
                          int y,
                          int width,
                          int height)

Requests a repaint for the specified region of the Screen. Calling this method may result in subsequent call to paint(), where the passed Graphics object's clip region will include at least the specified region.

If the canvas is not visible, or if width and height are zero or less, or if the rectangle does not specify a visible region of the display, this call has no effect.

The call to paint() occurs independently of the call to repaint(). That is, repaint() will not block waiting for paint() to finish. The paint() method will either be called after the caller of repaint() returns to the implementation (if the caller is a callback) or on another thread entirely.

To synchronize with its paint() routine, applications can use either Display.callSerially() or serviceRepaints(), or they can code explicit synchronization into their paint() routine.

The origin of the coordinate system is above and to the left of the pixel in the upper left corner of the displayable area of the Canvas. The X-coordinate is positive right and the Y-coordinate is positive downwards.

Parameters:

x - the x coordinate of the rectangle to be repainted

y - the y coordinate of the rectangle to be repainted

width - the width of the rectangle to be repainted

height - the height of the rectangle to be repainted

See Also:

Display.callSerially(Runnable), serviceRepaints()

repaint

public final void repaint()

Requests a repaint for the entire Canvas. The effect is identical to

repaint(0, 0, getWidth(), getHeight());

serviceRepaints

public final void serviceRepaints()

Forces any pending repaint requests to be serviced immediately. This method blocks until the pending requests have been serviced. If there are no pending repaints, or if this canvas is not visible on the display, this call does nothing and returns immediately.

WARNING: This method blocks until the call to the application's paint() method returns. The application has no control over which thread calls paint(); it may vary from implementation to implementation. If the caller of serviceRepaints() holds a lock that the paint() method acquires, this may result in deadlock. Therefore, callers of serviceRepaints() must not hold any locks that might be acquired within the paint() method. The Display.callSerially() method provides a facility where an application can be called back after painting has completed, avoiding the danger of deadlock.

See Also:

Display.callSerially(Runnable)

showNotify

protected void showNotify()

The implementation calls showNotify() immediately prior to this Canvas being made visible on the display. Canvas subclasses may override this method to perform tasks before being shown, such as setting up animations, starting timers, etc. The default implementation of this method in class Canvas is empty.

hideNotify

protected void hideNotify()

The implementation calls hideNotify() shortly after the Canvas has been removed from the display. Canvas subclasses may override this method in order to pause animations, revoke timers, etc. The default implementation of this method in class Canvas is empty.

paint

protected abstract void paint(Graphics g)

Renders the Canvas. The application must implement this method in order to paint any graphics.

The Graphics object's clip region defines the area of the screen that is considered to be invalid. A correctly-written paint() routine must paint every pixel within this region. Applications must not assume that they know the underlying source of the paint() call and use this assumption to paint only a subset of the pixels within the clip region. The reason is that this particular paint() call may have resulted from multiple repaint() requests, some of which may have been generated from outside the application. An application that paints only what it thinks is necessary to be painted may display incorrectly if the screen contents had been invalidated by, for example, an incoming telephone call.

Operations on this graphics object after the paint() call returns are undefined. Thus, the application must not cache this Graphics object for later use or use by another thread. It must only be used within the scope of this method.

The implementation may postpone visible effects of graphics operations until the end of the paint method.

The contents of the Canvas are never saved if it is hidden and then is made visible again. Thus, shortly after showNotify() is called, paint() will always be called with a Graphics object whose clip region specifies the entire displayable area of the Canvas. Applications must not rely on any contents being preserved from a previous occasion when the Canvas was current. This call to paint() will not necessarily occur before any other key, pointer, or commandAction() methods are called on the Canvas. Applications whose repaint recomputation is expensive may create an offscreen Image, paint into it, and then draw this image on the Canvas when paint() is called.

The application code must never call paint(); it is called only by the implementation.

The Graphics object passed to the paint() method has the following properties:

• the destination is the actual display, or if double buffering is in effect, a back buffer for the display;
• the clip region includes at least one pixel within this Canvas;
• the current color is black;
• the font is the same as the font returned by Font.getDefaultFont();
• the stroke style is SOLID;
• the origin of the coordinate system is located at the upper-left corner of the Canvas; and
• the Canvas is visible, that is, a call to isShown() will return true.

Parameters:

g - the Graphics object to be used for rendering the Canvas