Name
Canvas
Synopsis
Canvas
is a Displayable
that serves
as the most fundamental class of the low-level user interface API. A
Canvas
can be thought of as a blank sheet of paper that
covers the accessible area of a device’s screen. In order to use the
Canvas
class, a MIDlet must subclass it to provide
drawing and event handling logic. When such a subclass is installed as a
MIDlet’s current screen (using the Display.setCurrent()
method) and the MIDlet is in the foreground, the paint()
method will be called as necessary to cause the content of the
Canvas
to be drawn onto the screen. Similarly, events from
the keyboard or a pointing device, if there is one, will be notified to the
Canvas
and can be handled appropriately by the MIDlet.
The abstract paint()
method must be implemented to draw
the content of the canvas onto the screen, using the
Graphics
object supplied as its argument. The clipping
rectangle in this Graphics
object may only cover a subset
of the entire Canvas
if only part of the screen needs to
be repainted. In general, the paint()
method should query
the clipping rectangle and only perform graphics operations that would affect
this part of the screen, where this can be economically determined.
A device may or may not double-buffer its screen to provide smoother updates
and eliminate flashing caused by partial updates that are visible to the user.
The process of double-buffering is transparent to the
paint()
method, but a MIDlet can determine whether double-buffering ...
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