深入分析J2ME平台MIDP低级事件处理机制

      在前面的文章中我们讲述了如何实现CommandListener和ItemStateListener接口来完成高级事件处理。但是来自于键盘输入的低级事件是不能由Command来完成的。你必须继承Canvas类并覆盖其中的方法来完成低级事件的处理。

      通常低级事件的处理是为了满足游戏开发的需要，因为游戏开发相关的问题非常复杂。因此不在这里进行阐述，有兴趣的朋友可以和我索要相关的游戏开发资料。MIDP定义了如下键值在Canvas类内

• KEY_NUM0
• KEY_NUM1
• KEY_NUM2
• KEY_NUM3
• KEY_NUM4
• KEY_NUM5
• KEY_NUM6
• KEY_NUM7
• KEY_NUM8
• KEY_NUM9
• KEY_STAR
• KEY_POUND

• 这些都是按键基本值，当然有些手机是触摸屏的。事件的传递是通过如下方法来实现的。

• protected void keyPressed(int keyCode)
• protected void keyReleased(int keyCode)
• protected void keyRepeated(int keyCode)
• protected void pointerPressed(int x, int y)
• protected void pointerDragged(int x, int y)
• protected void pointerReleased(int x, int y)
• protected void showNotify()
• protected void hideNotify()
• protected abstract void paint(Graphics g)
• commandAction() method of the CommandListener interface

• 开发者应该继承Canvas类的同时覆盖相应的方法来实现低级事件的处理，低级事件的处理是连续的、串行的，在前面的事件返回之前后面的事件不会执行，当然也有例外，你可以参考API doc得到答案。下面是一个例子，它的功能就是在屏幕上显示出你按下键的键值，界面非常简单因此没有抓图。代码如下所示：
  //LowLevelMIDlet.java
  
  import javax.microedition.lcdui.Display;
  import javax.microedition.midlet.MIDlet;
  import javax.microedition.midlet.MIDletStateChangeException;

  /*
   * Created on 2004-6-24
   *
   * TODO To change the template for this generated file go to
   * Window - Preferences - Java - Code Style - Code Templates
   */

  /**
   * @author P2800
   *
   * TODO To change the template for this generated type comment go to Window -
   * Preferences - Java - Code Style - Code Templates
   */
  public class LowLevelMIDlet extends MIDlet
  {

      private Display display;
      private KeyCanvas keyCanvas;

      /*
       * (non-Javadoc)
       *
       * @see javax.microedition.midlet.MIDlet#startApp()
       */
      protected void startApp() throws MIDletStateChangeException
      {
          // TODO Auto-generated method stub
          display = Display.getDisplay(this);
          keyCanvas = new KeyCanvas();
          display.setCurrent(keyCanvas);

      }

      /*
       * (non-Javadoc)
       *
       * @see javax.microedition.midlet.MIDlet#pauseApp()
       */
      protected void pauseApp()
      {
          // TODO Auto-generated method stub

      }

      /*
       * (non-Javadoc)
       *
       * @see javax.microedition.midlet.MIDlet#destroyApp(boolean)
       */
      protected void destroyApp(boolean arg0) throws MIDletStateChangeException
      {
          // TODO Auto-generated method stub

      }

  }

  //KeyCanvas.java
  
  import javax.microedition.lcdui.Canvas;
  import javax.microedition.lcdui.Command;
  import javax.microedition.lcdui.CommandListener;
  import javax.microedition.lcdui.Displayable;
  import javax.microedition.lcdui.Graphics;

  /*
   * Created on 2004-6-25
   *
   * TODO To change the template for this generated file go to
   * Window - Preferences - Java - Code Style - Code Templates
   */

  /**
   * @author P2800
   *
   * TODO To change the template for this generated type comment go to Window -
   * Preferences - Java - Code Style - Code Templates
   */
  public class KeyCanvas extends Canvas implements CommandListener
  {
      private String key = "Press any key";

      /*
       * (non-Javadoc)
       *
       * @see javax.microedition.lcdui.Canvas#paint(javax.microedition.lcdui.Graphics)
       */
      protected void paint(Graphics g)
      {
          // TODO Auto-generated method stub
          g.setColor(255, 255, 255);
          g.fillRect(0, 0, getWidth(), getHeight());
          g.setColor(0, 0, 0);
          g.drawString(key, getWidth() / 2, getHeight() / 2, Graphics.TOP
                  | Graphics.HCENTER);

      }

      public void keyPressed(int keyCode)
      {
          key = keyCode + " is pressed";
          repaint();
      }

      public void keyReleased(int keyCode)
      {
          key = keyCode + " is released";
          repaint();
      }

      /*
       * (non-Javadoc)
       *
       * @see javax.microedition.lcdui.CommandListener#commandAction(javax.microedition.lcdui.Command,
       *      javax.microedition.lcdui.Displayable)
       */
      public void commandAction(Command arg0, Displayable arg1)
      {
          // TODO Auto-generated method stub

      }

  }