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java.lang.Object
|
+--javax.microedition.lcdui.Displayable
|
+--javax.microedition.lcdui.Canvas
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.
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.
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.
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.
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:
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 |
public static final int UP
Constant for the UP game action.
Constant value 1 is set to UP.
public static final int DOWN
Constant value 6 is set to DOWN.
public static final int LEFT
Constant value 2 is set to LEFT.
public static final int RIGHT
Constant value 5 is set to RIGHT.
public static final int FIRE
Constant value 8 is set to FIRE.
public static final int GAME_A
Constant value 9 is set to GAME_A.
public static final int GAME_B
Constant value 10 is set to GAME_B.
public static final int GAME_C
Constant value 11 is set to GAME_C.
public static final int GAME_D
Constant value 12 is set to GAME_D.
public static final int KEY_NUM0
Constant value 48 is set to KEY_NUM0.
public static final int KEY_NUM1
Constant value 49 is set to KEY_NUM1.
public static final int KEY_NUM2
Constant value 50 is set to KEY_NUM2.
public static final int KEY_NUM3
Constant value 51 is set to KEY_NUM3.
public static final int KEY_NUM4
Constant value 52 is set to KEY_NUM4.
public static final int KEY_NUM5
Constant value 53 is set to KEY_NUM5.
public static final int KEY_NUM6
Constant value 54 is set to KEY_NUM6.
public static final int KEY_NUM7
Constant value 55 is set to KEY_NUM7.
public static final int KEY_NUM8
Constant value 56 is set to KEY_NUM8.
public static final int KEY_NUM9
Constant value 57 is set to KEY_NUM09.
public static final int KEY_STAR
Constant value 42 is set to KEY_STAR.
public static final int KEY_POUND
Constant value 35 is set to KEY_POUND.
| Constructor Detail |
protected Canvas()
| Method Detail |
public int getWidth()
public int getHeight()
public boolean isDoubleBuffered()
public boolean hasPointerEvents()
public boolean hasPointerMotionEvents()
public boolean hasRepeatEvents()
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.
gameAction - the game actionpublic 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))
keyCode - the key code being requestedpublic 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.
keyCode - the key codeprotected 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.
keyCode - The key code of the key that was pressed.protected void keyRepeated(int keyCode)
keyCode - The key code of the key that was repeatedhasRepeatEvents()protected void keyReleased(int keyCode)
keyCode - The key code of the key that was released
protected void pointerPressed(int x,
int y)
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.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)
protected void pointerReleased(int x,
int y)
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.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)
protected void pointerDragged(int x,
int y)
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.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)
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.
x - the x coordinate of the rectangle to be repaintedy - the y coordinate of the rectangle to be repaintedwidth - the width of the rectangle to be repaintedheight - the height of the rectangle to be repaintedDisplay.callSerially(Runnable),
serviceRepaints()public final void repaint()
repaint(0, 0, getWidth(), getHeight());
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.
Display.callSerially(Runnable)protected void showNotify()
protected void hideNotify()
protected abstract void paint(Graphics g)
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:
Font.getDefaultFont();SOLID;true.g - the Graphics object to be used for rendering the Canvas
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