Motorola J2ME Specifications download pdf (Page 41)

Gaming API and Sound

• public static final int VOLUME_MIN - The minimum volume for

playing sound effects. This constant has a value of 0.

GameScreen Methods

The GameScreen class defines the following methods:

•

protected Graphics getGraphics() - Obtains the Graphics object for

rendering GameScreens. The Graphics object renders to an off-screen buffer whose

size is equal to that of the GameScreen (use getWidth() and getHeight() to determine

the size of the GameScreen). The buffer is initially filled with white pixels. Rendering

operations do not appear on the display until flushGraphics() is called; flushing the

buffer does not change its contents (that is, the pixels are not cleared as a result of

the flushing operation). Only one image buffer is supported because without a

vertical sync blanking period or its equivalent, there is little or no benefit from having

multiple image buffers. Only one Graphics object exists for each GameScreen

instance.

•

public int getKeyStates() - Gets the states of the physical keys. Each bit

in the returned integer represents a specific key on the device. A key's bit will be set if

the key is currently pressed or was pressed at least once since the last time this

method was called. The bit will be 0 if the key is not currently pressed and was not

pressed at all since the last time this method was called. This latching behavior

ensures that a rapid key press and release will always be caught by the game loop,

regardless of how slowly the loop runs. This method may be called twice to check if a

key is currently pressed; that is, calling this method twice effectively disables the

latching behavior. The lower bits are defined by UP_KEY, DOWN_KEY, LEFT_KEY,

etc.; the remaining bits may be mapped to device-specific keys.

The following is a code sample to show implementation of public int

getKeyStates()

Public int getKeyStates ()

// Get the key state and store it

int keyState = gameScreenObject.getKeyStates();

if ((keyState & LEFT_KEY) != 0) {

positionX--;

} else if ((keyState & RIGHT_KEY) != 0) {

positionX++;

}

•

public void enableKeyEvents(boolean enabled) - Enables or

disables key event calls to this GameScreen. If disabled, the Canvas key event

methods (keyPressed, keyRepeated, keyReleased) are not called when keys are

pressed or released; however, the developer can still call getKeyStates to query the

state of the keys. For games that poll key state and do not need event-driven key

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Motorola J2ME Specifications Page 41

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