/**     *  画布重画事件,替代Canvas中的paint()事件, 根据不同的游戏状态（gameState）画出游戏画面     *  本方法由thread每次重新启动, 最后执行flushGraphics()重画缓冲区     */    public synchronized void paintCanvas(Graphics g){               if(gameState == GAME_INIT){            //游戏第一次启动            //设置为全屏模式并清屏            this.setFullScreenMode(true);                       g.setColor(255, 255, 255);            g.fillRect(0, 0, mainWidth, mainHeight);                       if(!COMMAND_ADD_FLAG){                //添加响应命令及监听器                this.addCommand(pauseCommand);                this.addCommand(exitCommand);                this.setCommandListener(this);                COMMAND_ADD_FLAG = true;            }                       if(fishCollectionVector != null){                fishCollectionVector = null;                fishCollectionVector = new Vector();            }                       if(this.layerManager != null){                this.layerManager = null;                this.layerManager = new LayerManager();                  if(userViewWindow){                    this.layerManager.setViewWindow(xViewWindow, yViewWindow, wViewWindow, hViewWindow);                }            }                       if(mySub != null){                mySub = null;                mySub = new Sub(this, SubMIDlet.createImage("/res/sub.png"), getWidth() / 3, getHeight() / 3, layerManager);                           }                       //创建背景图层            this.createSandBackground();            this.createSunkenBoat();                       this.createFishCollection(0, FishCollection.NUM_FISH_TYPES);            this.createMysub();            this.createSeaBackground();                         mySub.setPosition(getWidth() / 3, getHeight() / 3);            gameState = GAME_RUN;                   }else if(gameState == GAME_RUN){            //游戏处于运行状态            //提供游戏运行标识Flag,保证对用户操作的响应和"敌人"的运行动作只有在运行的时候生效            //在性能过耗（可用内存不到当前内存总量的4/5时），进行垃圾回收GC                       if(rt.freeMemory() < (rt.totalMemory() * 4 / 5)){                rt.gc();            }                              }else if(gameState == GAME_SUSPEND){            //下一轮游戏                       //更新数据            mySub.setHpLife(15);            mySub.setPosition(mainWidth / 3, mainHeight / 3);            if(PLAYER_LEVEL >= 4){                gameState = GAME_OVER;            }else{                PLAYER_LEVEL ++;                controller.EventHandler(Controller.EVENT_NEXTROUND);                               //目前游戏只设计了四级关卡                ENEMY_MAX       = PLAYER_LEVEL * 10;                 ENEMY_CURRENT   = 0;                TRIGGER_COUNT   = 0;                TICK_COUNT      = 0;                ENEMY_CURRENT_LIMIT = PLAYER_LEVEL * 2;                               Layer layer = null;                //暂时删除layerManager中所有文件，清空鱼雷与敌人潜艇数据                //为更新图层做准备                this.tinfishCollectionVector.removeAllElements();                this.enemyCollectionVector.removeAllElements();                this.fishCollectionVector.removeAllElements();                               //             for(int i = 0; i < layerManager.getSize(); i++){ //                 layer = layerManager.getLayerAt(i); //                 layerManager.remove(layer); //             }                                              layer = null;                               //更新海底图层数据                this.SEABACK_DENSITY = (SEABACK_DENSITY + 1) % 4;                gameState = GAME_INIT;                this.unActive();                           }          }else if(gameState == GAME_OVER){            //游戏结束            threadAlive = false;            controller.EventHandler(Controller.EVENT_MENU_GAMEOVER);            gameState = this.GAME_INIT;                   }               //在缓冲区重画        this.layerManager.paint(g, 0, (getHeight() - WORLD_HEIGHT) / 2);        this.flushGraphics();    }         public void commandAction(Command command, Displayable display) {        if(command == startCommand){                       if(this.gameState == GAME_OVER){                gameState = GAME_INIT;            }else{                gameState = GAME_RUN;            }                       this.removeCommand(this.startCommand);            this.addCommand(pauseCommand);                   }else if(command == pauseCommand){            gameState = GAME_PAUSE;                       this.removeCommand(this.pauseCommand);            this.addCommand(startCommand);                   }else if(command == exitCommand){            gameState = GAME_OVER;                   }    }       /**     * 创建海底沙地背景图层     */    protected void createSandBackground(){        Image bottomTitles = SubMIDlet.createImage("/res/bottom.png");               //将图片bottomTitles切成指定大小(TILE_WIDTH, TILE_HEIGHT)        //创建一个指定维数(1, WIDTH_IN_TILES)的背景数组        TiledLayer layer   = new TiledLayer(WIDTH_IN_TILES, 1,                bottomTitles, TILE_WIDTH, TILE_HEIGHT);               for(int column = 0; column < WIDTH_IN_TILES; column++){            //将海底图层数组中的每个小格用原始图片的第i块来填充            int i = SubMIDlet.createRandom(NUM_DENSITY_LAYER_TILES) + 1;            layer.setCell(column, 0, i);        }        layer.setPosition(0, WORLD_HEIGHT - bottomTitles.getHeight());        layerManager.append(layer);               bottomTitles = null;    }          /**     * 创建海底水层背景图片     */    protected void createSeaBackground(){               if(this.SEABACK_DENSITY >= 4){            SEABACK_DENSITY = 0;        }               Image image = SubMIDlet.createImage("/res/densityLayer" + this.SEABACK_DENSITY + ".png");               layerSeaback = new TiledLayer(WIDTH_IN_TILES, HEIGHT_IN_TILES,                image, TILE_WIDTH, TILE_HEIGHT);               for(int row = 0; row < HEIGHT_IN_TILES; row++){            for(int column = 0; column < WIDTH_IN_TILES; column++){                layerSeaback.setCell(column, row, SubMIDlet.createRandom(NUM_DENSITY_LAYER_TILES) + 1);            }        }        layerSeaback.setPosition(0, 0);        layerManager.append(layerSeaback);               image = null;    }      /**     * 创建沉船图层     */    protected void createSunkenBoat(){        Image imageSunkenBoat = SubMIDlet.createImage("/res/sunkenBoat.png");        //出现的沉船数量        int numSunkenBoats = 1 + (WORLD_WIDTH / (3 * imageSunkenBoat.getWidth()));               Sprite sunkenBoat;        int bx = 0;        for(int i = 0; i < numSunkenBoats; i++){            sunkenBoat = new Sprite(imageSunkenBoat, imageSunkenBoat.getWidth(), imageSunkenBoat.getHeight());            sunkenBoat.setTransform(this.rotations[SubMIDlet.createRandom(this.rotations.length)]);                       //随机定义沉船位置            bx = (WORLD_WIDTH - imageSunkenBoat.getWidth()) / numSunkenBoats;                       bx = (i * bx) + SubMIDlet.createRandom(bx);                       sunkenBoat.setPosition(bx, WORLD_HEIGHT - imageSunkenBoat.getHeight());                       //添加图层            this.layerManager.append(sunkenBoat);            //mySub.addCollideable(sunkenBoat);        }        imageSunkenBoat = null;    }      /**     * 创建玩家潜艇     */    protected void createMysub(){        this.layerManager.append(mySub);    }          /**创建鱼群背景     * @param startId     * @param endId     */    protected void createFishCollection(int startId, int endId){        for(int id = startId; id < endId; id++){            int w  = WORLD_WIDTH / 4;            int h  = WORLD_HEIGHT / 4;            int x  = SubMIDlet.createRandom(WORLD_WIDTH - w);            int y  = SubMIDlet.createRandom(WORLD_HEIGHT - h);            int vx = FishCollection.randomVelocity(TILE_WIDTH / 4);            int vy = FishCollection.randomVelocity(TILE_HEIGHT / 4);                       //初始化鱼类图层, 同时把图层添加到图层管理器上            FishCollection fishCollection = new FishCollection(layerManager, id, x, y, w, h,                    vx, vy, 0, 0, WORLD_WIDTH, WORLD_HEIGHT);                       this.fishCollectionVector.addElement(fishCollection);        }    }       /**重新设置图层显示区域     * @param x          玩家潜艇位置x     * @param y          玩家潜艇位置y     * @param width      玩家潜艇宽     * @param height     玩家潜艇高     */    public void adjustViewWindow(int xSub, int ySub, int width, int height) {        if (this.userViewWindow)         {             xViewWindow = xSub + (width / 2) - (wViewWindow / 2);             if (xViewWindow < 0)             {                 xViewWindow = 0;             }             if (xViewWindow > (WORLD_WIDTH - wViewWindow))             {                 xViewWindow = WORLD_WIDTH - wViewWindow;             }               yViewWindow = ySub + (height / 2) - (hViewWindow / 2);             if (yViewWindow < 0)             {                 yViewWindow = 0;             }             if (yViewWindow > (WORLD_HEIGHT - hViewWindow))             {                 yViewWindow = WORLD_HEIGHT - hViewWindow;             }               layerManager.setViewWindow(xViewWindow,                                        yViewWindow,                                        wViewWindow,                                        hViewWindow);         }    }        /** 获取玩家潜艇      * @return 返回 mySub。      */     public Sub getMySub() {         return mySub;     }     /**      * @return 返回 threadAlive。      */     public boolean isThreadAlive() {         return threadAlive;     }     /**      * @param threadAlive 要设置的 threadAlive。      */     public void setThreadAlive(boolean threadAlive) {         this.threadAlive = threadAlive;     }         public void active(){         this.showNotify();     }         public void unActive(){         this.hideNotify();     } }   6.6    小结 MIDP 2.0添加的游戏API大大简化了MIDP游戏的开发，使得开发者能够更关注游戏的逻辑和底层架构，同时还提高了游戏性能。遗憾的是，目前支持MIDP 2.0的手机还不多，使用MIDP 2.0 Game API编写的游戏无法在MIDP 1.0平台上运行。   本章节关注了基本的GAME API的使用，作为一名游戏开发人员熟练掌握这些系统提供的API是一个基本功，但仅仅有这些是不够的。游戏开发中还有很多话题，大大超出了基本API的使用。读者们应该大量的阅读和实践才能一窥游戏开发的全貌。如果需要更多的资料请参阅www.j2medev.com文章区之游戏开发专题的文章，会给你更大的收获。

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