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
method) and the MIDlet is in the foreground, the
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.
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 ...