Programming a Custom User Interface - Beginning J2ME: From Novice to Professional - page 229

There is also a full-screen mode. Some Canvas implementations won't occupy all the avail-
able screen space, reserving areas of the screen for information about the state of the device or
other purposes. If the device supports an alternate full screen mode for Canvas , you can use it
by calling setFullScreenMode(true) . Setting full screen mode on or off may result in calls to the
sizeChanged() method Canvas inherits from Displayable .
Canvas also features event handler methods that will be called by the MIDP implementa-
tion as your Canvas is displayed and hidden. Each time your Canvas is shown, the showNotify()
method will be called. If another Displayable is shown, or the application manager decides to
run a different application, hideNotify() is called.
Painting and Repainting
The MIDP implementation calls a Canvas 's paint() method when the contents of the Canvas
need to be shown. This paint() method should look familiar to anyone who has ever imple-
mented a custom Swing or AWT component.
The MIDP implementation passes a Graphics object to your paint() method. Graphics
has methods for drawing shapes, text, and images on a Canvas . A typical Canvas implementa-
tion, then, looks something like this:
import javax.microedition.lcdui.*;
public class JonathanCanvas
extends Canvas {
public void paint(Graphics g) {
// Draw stuff using g.
}
}
What if you want to tell the Canvas to draw itself? You can't call paint() directly, because
you don't have a suitable Graphics to pass to paint() . Instead, you need to tell the MIDP imple-
mentation that it's time to paint the Canvas . The way you do this is by calling repaint() . The
first version of this method simply tells Canvas to paint everything.
public void repaint()
public void repaint(int x, int y, int width, int height)
The second version is a way of saying, “I only want you to paint this rectangular portion of
the screen.” If the drawing you're doing is very complicated, you can save some time by only
painting the portion of the Canvas that has changed. This is implemented using a technique
called clipping. A later section discusses clipping in more detail.
How exactly does repaint() work? When you call repaint() , paint() won't be called right
away. The call to repaint() just signals to the MIDP implementation that you want the screen
to be painted. Some time later, the implementation services the repaint request, which results
in an actual call to the paint() method of the Canvas . The MIDP implementation may even
combine several repaint requests, particularly if their repaint regions overlap.