在游戏开发中,有时候我们需要一个时钟来记录游戏的时间,如果时间结束则结束游戏。本文介绍如何在J2ME中使用Timer和TimerTask来实现这样一个时钟,并给出具体代码实例。

  在java.util包中有一个TimerTask类,你可以扩展这个类并且实现他的run()方法,在run()方法中编写我们的逻辑代码。如果我们想制作一个游戏时钟,那么非常简单我们编写一个GameClock类扩展TimerTask,GameClock需要维持一个实例变量 timeLeft,这样我们就可以记录游戏剩余的时间了,在每次run()运行的时候把timeLeft减1就可以了。有时候我们需要始终暂停以及重新启动,这并不复杂,在GameClock中添加一个boolean类型的标记就可以了。下面给出GameClock的代码:

/*
 * GameClock.java
 *
 * Created on 2005年7月18日, 上午11:00
 *
 * To change this template, choose Tools | Options and locate the template under
 * the Source Creation and Management node. Right-click the template and choose
 * Open. You can then make changes to the template in the Source Editor.
 */

package com.j2medev.gameclock;
import java.util.TimerTask;
/**
 *
 * @author Administrator
 */
public class GameClock extends TimerTask{
   
    private int timeLeft = 60;//时钟的默认时间
    private boolean pause = false;
    /** Creates a new instance of GameClock */
    public GameClock() {
    }
   
    public GameClock(int value){
        timeLeft = value;
    }
   
    public void run(){
        if(!pause){
            timeLeft--;
        }
    }
   
    public void pause(){
        pause = true;
    }
   
    public void resume(){
        pause = false;
    }
   
    public int getTimeLeft(){
        return timeLeft;
    }
   
    public void setTimeLeft(int _value){
        this.timeLeft = _value;
    }
}

  当我们使用这个时钟的时候,只需要把它的一个实例作为参数传给Timer的schedule()方法即可。例如

      clock = new GameClock(30);
      timer.schedule(clock,0,1000);

  接下来我们编写一个简单的游戏界面测试一下时钟。我们在程序启动的时候开始计时,每隔一秒钟timeLeft会减少1,并且在手机屏幕上显示当前剩余的时间。如果timeLeft为0的时候代表游戏已经结束了。因此我们需要这样判断游戏的状态。

    public void verifyGameState(){
        timeLeft = clock.getTimeLeft();
        if(timeLeft == 0){
            going = false;
        }
    }

  为了测试时钟的暂停功能,我们接收用户的按键行为,如果左键被按下,那么调用clock的pause()方法,如果右键被按下则调用clock的resume()方法。

    public void userInput(){
        int keyStates = this.getKeyStates();
        if((keyStates & GameCanvas.LEFT_PRESSED) != 0){
            clock.pause();
        }else if((keyStates & GameCanvas.RIGHT_PRESSED) != 0){
            clock.resume();
        }
           
    }

  下面给出MIDlet和Canvas的代码:

/*
 * ClockCanvas.java
 *
 * Created on 2005年7月18日, 上午11:04
 *
 * To change this template, choose Tools | Options and locate the template under
 * the Source Creation and Management node. Right-click the template and choose
 * Open. You can then make changes to the template in the Source Editor.
 */

package com.j2medev.gameclock;
import java.util.Timer;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.game.*;

/**
 *
 * @author Administrator
 */
public class ClockCanvas extends GameCanvas implements Runnable {
   
    private Timer timer = new Timer();
    private GameClock clock = null;
    private boolean going = true;
    int timeLeft = 0;
    /** Creates a new instance of ClockCanvas */
    public ClockCanvas() {
        super(false);
    }
   
    public void run(){
        clock = new GameClock(30);
        timer.schedule(clock,0,1000);
        while(going){
            verifyGameState();
            userInput();
            repaint();
            try{
                Thread.sleep(100);
            }catch(Exception e){
                e.printStackTrace();
            }
           
        }
    }
   
    public void userInput(){
        int keyStates = this.getKeyStates();
        if((keyStates & GameCanvas.LEFT_PRESSED) != 0){
            clock.pause();
        }else if((keyStates & GameCanvas.RIGHT_PRESSED) != 0){
            clock.resume();
        }
           
    }
   
    public void paint(Graphics g){
        int color = g.getColor();
        g.setColor(0xffffff);
        g.fillRect(0,0, this.getWidth(), this.getHeight());
        g.setColor(color);
       
        if(timeLeft == 0){
            g.drawString("游戏结束", this.getWidth()/2, this.getHeight()/4, Graphics.HCENTER|Graphics.BOTTOM);
        }else{
            g.drawString("游戏剩余时间:"+timeLeft, this.getWidth()/2, this.getHeight()/4, Graphics.HCENTER|Graphics.BOTTOM);
           
        }
       
       
    }
   
    public void verifyGameState(){
        timeLeft = clock.getTimeLeft();
        if(timeLeft == 0){
            going = false;
        }
    }
   
    public void start(){
        Thread t = new Thread(this);
        t.start();
    }
   
    public void stop(){
        going = false;
    }
   
}

/*
 * TestMidlet.java
 *
 * Created on 2005年7月18日, 上午11:00
 */

package com.j2medev.gameclock;

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

/**
 *
 * @author  Administrator
 * @version
 */
public class TestMidlet extends MIDlet {
   
    private Display display = null;
   
    public void startApp() {
        display = Display.getDisplay(this);
        ClockCanvas canvas = new ClockCanvas();
        canvas.start();
        display.setCurrent(canvas);
    }
   
    public void pauseApp() {
    }
   
    public void destroyApp(boolean unconditional) {
    }
}