CSIS0329_Tutorial2.ppt

1、2009.01,CSIS0329 Computer Game Design and Programming,Tutorial 2 Simple J2ME game programming,Outline,Basic game loop functions Try/catch statements General screen configurations LayerManager Adding a background color Loading an image Making an image move Creating random objects starting at random p

2、ositions Making an image visible and invisible Collision detection Getting current time Drawing texts Sample game,Basic game loop functions,protected void startApp () Contains statements that will be executed when the application is started E.g. initialize some variables public void pauseApp () What

3、 should be done when the application is paused? E.g. display some messages public void destroyApp (boolean unconditional) What should be done when the application is stopped? destroyApp(true) destroy all threads created,Basic game loop functions,public void commandAction(Command c, Displayable s) Ha

4、ndles key commands E.g. public void commandAction(Command c, Displayable s) if (c.getCommandType( ) = Command.EXIT) destroyApp(true); ,Outline,Basic game loop functions Try/catch statements General screen configurations LayerManager Adding a background colour Loading an image Making an image move Cr

5、eating random objects starting at random positions Making an image visible and invisible Collision detection Getting current time Drawing texts Sample game,Try/catch statements,The try/catch statement encloses some code and is used to handle errors and exceptions that might occur in that code. Gener

6、al syntax of the try/catch statement: try body-code catch (exception-classname variable-name) handler-code Four parts: The body-code contains code that might throw the exception that we want to handle. The exception-classname is the class name of the exception we want to handle. The variable-name sp

7、ecifies a name for a variable that will hold the exception object if the exception occurs. The handler-code contains the code to execute if the exception occurs.,Try/catch statements,Example: try hdCanvas = new HardDriveCanvas(this, /car.png, /obstacle.png); catch (IOException ioe) System.err.printl

8、n(Problem loading image +ioe); ,Outline,Basic game loop functions Try/catch statements General screen configurations LayerManager Adding a background colour Loading an image Making an image move Creating random objects starting at random positions Making an image visible and invisible Collision dete

9、ction Getting current time Drawing texts Sample game,General screen configurations LayerManager,You need to have a LayerManager to contain everything. private LayerManager layerManager; layerManager = new LayerManager(); You need to define the size of the visible window. int width = getWidth(); / wi

10、dth of the phone screen int height = getHeight(); / height of the phone screen layerManager.setViewWindow(1,1,width-2, height-2) / left upper corner at (1,1) and right lower corner at (width-2, height-2). i.e. having 1 unit margin on each side,Adding a background color,You need to first create a Gra

11、phics object: Graphics g = getGaphics(); Then set its colour and size: g.setColor(255, 255, 255); / a white color g.fillRect(0, 0, getWidth()-3, getHeight()-3); Then draw it into the LayerManager: layerManager.paint(g, 0, 0);,Loading an image,You need to create an Image object and load your image fi

12、le to it. String carImageName = “/car.png”; / remember / means in the “res” folder Image carImage = Image.createImage(carImageName); You need to use a Sprite object to contain the Image object. Sprite carSprite; carSprite = new Sprite(carImage);,Loading an image,You can set the position of the sprit

13、e using the “setPosition()” method. carSprite.setPosition(width/2, height-30); Finally, you need to “append” your image to the LayerManager. layerManager.append(carSprite);,Making an image move,Getting the key states using getKeyStates() method: int keyStates = getKeyStates(); Getting the current po

14、sition of a sprite using getY() and getX() methods: int CurrentY = carSprite.getY(); int CurrentX = carSprite.getX();,Making an image move,Checking which key is pressed and setting the position of a sprite: if (keyStates / Math.max(a, b) returns the greater number among a and b / Math.min(a, b) retu

15、rns the smaller number among a and b,Making an image move,Math.PI Math.E Math.LN10 Math.LN2 Math.LOG10E Math.LOG2E Math.SQRT1_2 square root of 1/2 Math.SQRT2 square root of 2,Math.abs() Math.round() Math.ceil() Math.floor() Math.max() Math.min() Math.power() Math.random(),Please refer to .hk/jstutor

16、/ch15/ch15.htm for details.,Making an image move,Finally, you need to refresh the screen so that your motion can take effect. flushGraphics();,Creating random objects starting at random positions,You can have an array of sprite objects which all use the same image: String obsImageName = “/obstacle.p

17、ng”; Image obsImage = Image.createImage(obsImageName); obstacles = new Sprite10; for (int i=0; i 10; i+) obstaclesi = new Sprite(obsImage); layerManager.append(obstaclesi); In Java, you can also use the length property to get the number of elements in an array. E.g. obstacles.length is equivalent to

18、 10.,Creating random objects starting at random positions,Then set the position of each sprite randomly: java.util.Random random = new java.util.Random(); for (int i=0; I 1) % 170; int xPos = (random.nextInt()1) % 170; obstaclesi = setPosition(xPos, -yPos); ,Bitwise operators in Java,Making an image

19、 visible and invisible,You can make your sprites visible and invisible at different time to make your game more interesting obstaclesi.setVisible(true); / visible obstaclesi.setVisible(false); / invisible,Collision detection,You can make use of the “collidesWith()” method to detect whether two sprit

20、es collide with each other. for (int i = 0; i obstacles.length; i+) if (carSprite.collidesWith(obstaclesi.true) / some operations ,Getting current time,You can make use of the “System.currentTimeMillis()” method to get the current time. Its type is “long” and so you have to use a “long” variable to

21、store it. long CurrTime = System.currentTimeMillis();,Drawing texts,Besides text boxes, you can draw some simple texts on a screen using methods in the Graphics class. int width = getWidth(); int height = getHeight(); g.setColor(255,100,0); g.fillRect(0, 0, width - 3, height - 3); g.setFont(Font.get

22、Font (Font.FACE_PROPORTIONAL, Font.STYLE_BOLD, Font.SIZE_LARGE); int centerX = width / 2; int centerY = height / 2;,Drawing texts,int fontHeight = g.getFont().getHeight(); int textHeight = 3 * fontHeight; int topY = centerY - textHeight / 2; g.drawString(GAME OVER, centerX, topY, Graphics.HCENTER |

23、Graphics.TOP); g.drawString(Time: + time + s, centerX, topY + fontHeight, Graphics.HCENTER | Graphics.TOP); g.drawString(Score: + score), centerX, topY + 2 * fontHeight, Graphics.HCENTER | Graphics.TOP);,Sample game,A left/right moving car There are some obstacles Can avoid hitting an obstacle, 1 mark will be given Hit an obstacle, game over Print out time spent and score when game is over,Sample screens,Sample game,4 classes HardDriveMIDlet.java Main game loop HardDriveCanvas.java Control of the moving car ObstacleManager.java Control of the obstacle