MIDP2.0(Mobile Internet Device Profile)技术进行游戏开发中用到的最重要的包是:javax.microedition.lcdui.game，本文通过对样例游戏Tumbleweed的代码分析，将展示MIDP2.0技术的几个主要部分。游戏的主要情景是一个牛仔跳着闪避风滚草，这是一个简单的游戏，但你可以从中学到将来写复杂游戏必须具备的大部分基础知识。
从MIDlet类开始
象通常一样，应用从MIDlet类开始。本例中，我的MIDlet子类是Jump。Jump类几乎是完全继承的MIDIet子类，唯一的不同是用到了另一个独立类GameThread，用来动态设置当前窗口允许的有效按钮，比如当游戏处于非暂停状态时，玩家才可以使用暂停这个有效按钮，而激活按钮有效则是在游戏处于暂停状态时候，与此类似游戏停止后，开始按钮才是有效按钮。
Listing 1是游戏的MIDlet子类——Jump.java源码。
Listing 1. Jump.java
package net.frog_parrot.jump;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
/**
* This is the main class of the Tumbleweed game.
*
* @author Carol Hamer
*/
public class Jump extends MIDlet implements CommandListener {
//---------------------------------------------------------
// Commands
/**
* The command to end the game.
*/
private Command myExitCommand = new Command("Exit", Command.EXIT, 99);
/**
* The command to start moving when the game is paused.
*/
private Command myGoCommand = new Command("Go", Command.SCREEN, 1);
/**
* The command to pause the game.
*/
private Command myPauseCommand = new Command("Pause", Command.SCREEN, 1);
/**
* The command to start a new game.
*/
private Command myNewCommand = new Command("Play Again", Command.SCREEN, 1);
//---------------------------------------------------------
// Game object fields
/**
* The canvas that all of the game will be drawn on.
*/
private JumpCanvas myCanvas;
/**
* The thread that advances the cowboy.
*/
private GameThread myGameThread;
//-----------------------------------------------------
// Initialization and game state changes
/**
* Initialize the canvas and the commands.
*/
public Jump() {
try {
myCanvas = new JumpCanvas(this);
myCanvas.addCommand(myExitCommand);
myCanvas.addCommand(myPauseCommand);
myCanvas.setCommandListener(this);
} catch(Exception e) {
errorMsg(e);
}
}
/**
* Switch the command to the play again command.
*/
void setNewCommand () {
myCanvas.removeCommand(myPauseCommand);
myCanvas.removeCommand(myGoCommand);
myCanvas.addCommand(myNewCommand);
}
/**
* Switch the command to the go command.
*/
private void setGoCommand() {
myCanvas.removeCommand(myPauseCommand);
myCanvas.removeCommand(myNewCommand);
myCanvas.addCommand(myGoCommand);
}
/**
* Switch the command to the pause command.
*/
private void setPauseCommand () {
myCanvas.removeCommand(myNewCommand);
myCanvas.removeCommand(myGoCommand);
myCanvas.addCommand(myPauseCommand);
}
//----------------------------------------------------------------
// Implementation of MIDlet.
// These methods may be called by the application management
// software at any time, so you always check fields for null
// before calling methods on them.
/**
* Start the application.
*/
public void startApp() throws MIDletStateChangeException {
if(myCanvas != null) {
if(myGameThread == null) {
myGameThread = new GameThread(myCanvas);
myCanvas.start();
myGameThread.start();
} else {
myCanvas.removeCommand(myGoCommand);
myCanvas.addCommand(myPauseCommand);
myCanvas.flushKeys();
myGameThread.resumeGame();
}
}
}
/**
* Stop and throw out the garbage.
*/
public void destroyApp(boolean unconditional)
throws MIDletStateChangeException {
if(myGameThread != null) {
myGameThread.requestStop();
}
myGameThread = null;
myCanvas = null;
System.gc();
}
/**
* Request the thread to pause.
*/
public void pauseApp() {
if(myCanvas != null) {
setGoCommand();
}
If(myGameThread != null) {
myGameThread.pauseGame();
}
}
//----------------------------------------------------------------
// Implementation of CommandListener
/*
* Respond to a command issued on the Canvas.
* (either reset or exit).
*/
public void commandAction(Command c, Displayable s) {
if(c == myGoCommand) {
myCanvas.removeCommand(myGoCommand);
myCanvas.addCommand(myPauseCommand);
myCanvas.flushKeys();
myGameThread.resumeGame();
} else if(c == myPauseCommand) {
myCanvas.removeCommand(myPauseCommand);
myCanvas.addCommand(myGoCommand);
myGameThread.pauseGame();
} else if(c == myNewCommand) {
myCanvas.removeCommand(myNewCommand);
myCanvas.addCommand(myPauseCommand);
myCanvas.reset();
myGameThread.resumeGame();
} else if((c == myExitCommand) || (c == Alert.DISMISS_COMMAND)) {
try {
destroyApp(false);
notifyDestroyed();
} catch (MIDletStateChangeException ex) {
}
}
}
//-------------------------------------------------------
// Error methods
/**
* Converts an exception to a message and displays
* the message.
*/
void errorMsg(Exception e) {
if(e.getMessage() == null) {
errorMsg(e.getClass().getName());
} else {
errorMsg(e.getClass().getName() + ":" + e.getMessage());
}
}
/**
* Displays an error message alert if something goes wrong.
*/
void errorMsg(String msg) {
Alert errorAlert = new Alert("error", msg, null, AlertType.ERROR);
errorAlert.setCommandListener(this);
errorAlert.setTimeout(Alert.FOREVER);
Display.getDisplay(this).setCurrent(errorAlert);
}
} |
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