1.2MIDP低級UI
與高級UI相比，低級UI就自由很多，任何時候我們可以調用repaint()產生重繪事件，調用完了repaint()會立刻返回，調用paint()回調函數則是由另一個專門的線程來完成。
相對於高級UI提供的豐富的組件，低級UI能夠對
我們從javax.microedition.lcdui.Canvas開始瞭解我們的低級UI，我們要用到低級UI必須要繼承Canvas這個抽象類，在 Canvas的核心是paint()這個方法，這個方法做是負責繪製屏幕上的畫面，每當屏幕需要重新繪製時，就會產生重繪事件時，系統就會自動調用paint(),並傳入一個Graphics對象。
任何時候我們都可以通過調用reapaint()方法來產生重繪事件，它有兩個方法，一個需要四個參數，分別用來指示起始座標（X,Y）,長寬，另一個則不需要任何參數，代表整個畫面重新繪製。
我們可以通過getWidth()和getHeight() 方法獲得Canvas的當前範圍大小。每當Canvas 範圍大小發生變化時，就會自動調用Canvas類的 sizeChanged（）方法。
下面是通用的用低級CANVAS API繪製九宮格程序的源碼:
public class MenuCanvas extends Canvas implements CommandListener {
private String[] menuName = new String[] { "login", "picture", "wap",
"phone", "call", "goto", "fuck", "music", "video" };
private int canvaWidth = -1, canvaHeight = -1;
private int maxHeight = -1, imgWidth = -1, imgHeight = -1;
private int focus = 0;
private int fontHeight = -1;
private Image menuImg[] = new Image[9];
private Command exit;
public MenuCanvas() {
exit = new Command("Exit", Command.EXIT, 1);
addCommand(exit);
setCommandListener(this);
canvaWidth = getWidth();
canvaHeight = getHeight();
try {
for (int i = 0; i < menuImg.length; i++) {
menuImg[i] = Image.createImage(28, 28);
Graphics g = menuImg[i].getGraphics();
g.translate(-28 * i, 0);
}
} catch (Exception e) {
System.out.println(e.toString());
}
imgWidth = menuImg[0].getWidth();
imgHeight = menuImg[0].getHeight();
fontHeight = Font.getDefaultFont().getHeight();
maxHeight = imgHeight + fontHeight;
}
public void paint(Graphics g) {
int color = g.getColor();
g.setColor(0xFFFFFF);
g.fillRect(0, 0, canvaWidth, canvaHeight);
int cellWidth = canvaWidth / 3;
int cellHeight = canvaHeight / 3;
g.setColor(color);
for (int i = 0; i < menuImg.length; i++) {
g.drawImage(menuImg[i], cellWidth * (i % 3)
+ (cellWidth - imgWidth) / 2, cellHeight * (i / 3)
+ (cellHeight - maxHeight) / 2, Graphics.TOP
| Graphics.LEFT);
int fontWidth = Font.getDefaultFont().stringWidth(menuName[i]);
g.drawString(menuName[i], cellWidth * (i % 3)
+ (cellWidth - fontWidth) / 2, cellHeight * (i / 3)
+ (cellHeight - maxHeight) / 2 + imgHeight + 1,
Graphics.TOP | Graphics.LEFT);
if (i == focus) {
g.drawRect(cellWidth * (i % 3) + (cellWidth - imgWidth) / 2,
cellHeight * (i / 3) + (cellHeight - maxHeight) / 2,
imgWidth + 1, imgHeight + 1);
}
}
}
public void keyPressed(int key) {
int action = getKeyAction(key);
switch (action) {
case FIRE:
Alert alert = new Alert("You select:", menuName[focus],
menuImg[focus], AlertType.INFO);
alert.setTimeout(Alert.FOREVER);
break;
case UP:
focus = focus - 3;
if (focus < 0) {
focus = focus + 9;
}
break;
……
repaint();
serviceRepaints();
}
public void commandAction(Command c, Displayable dpa) {
if (c == exit) {
}
}
}
最後得到的效果圖如下所示
