MIDP 2.0开发J2ME游戏起步之二

package net.frog_parrot.jump;
import javax.microedition.lcdui.*;

import javax.microedition.lcdui.game.*;
/**

* This class draws the background grass.

*

* @author Carol Hamer

*/

public class Grass extends TiledLayer {
//---------------------------------------------------------

// Dimension fields

// (constant after initialization)
/**

* The width of the square tiles that make up this layer.

*/

static final int TILE_WIDTH = 20;
/**

* This is the order that the frames should be displayed

* for the animation.

*/

static final int[] FRAME_SEQUENCE = { 2, 3, 2, 4 };
/**

* This gives the number of squares of grass to put along

* the bottom of the screen.

*/

static int COLUMNS;
/**

* After how many tiles does the background repeat.

*/

static final int CYCLE = 5;
/**

* The fixed Y coordinate of the strip of grass.

*/

static int TOP_Y;
//---------------------------------------------------------

// Instance fields
/**

* Which tile you are currently on in the frame sequence.

*/

private int mySequenceIndex = 0;
/**

* The index to use in the static tiles array to get the

* animated tile.

*/

private int myAnimatedTileIndex;
//---------------------------------------------------------

// Gets / sets
/**

* Takes the width of the screen and sets my columns

* to the correct corresponding number.

*/

static int setColumns(int screenWidth) {

COLUMNS = ((screenWidth / 20) + 1)*3;

return(COLUMNS);

}
//---------------------------------------------------------

// Initialization
/**

* Constructor initializes the image and animation.

*/

　public Grass() throws Exception {

super(setColumns(JumpCanvas.DISP_WIDTH), 1,

　Image.createImage("/images/grass.png"),

　TILE_WIDTH, TILE_WIDTH);

TOP_Y = JumpManager.DISP_HEIGHT - TILE_WIDTH;

setPosition(0, TOP_Y);

myAnimatedTileIndex = createAnimatedTile(2);

for(int i = 0; i < COLUMNS; i++) {

　if((i % CYCLE == 0) || (i % CYCLE == 2)) {

setCell(i, 0, myAnimatedTileIndex);

　} else {

setCell(i, 0, 1);

}

　}

}
//---------------------------------------------------------

// Graphics
/**

* Sets the grass back to its initial position.

*/

void reset() {

　setPosition(-(TILE_WIDTH*CYCLE), TOP_Y);

　mySequenceIndex = 0;

　setAnimatedTile(myAnimatedTileIndex, FRAME_SEQUENCE[mySequenceIndex]);

}
/**

* Alter the background image appropriately for this frame.

* @param left Whether the player is moving left.

*/

void advance(int tickCount) {

　if(tickCount % 2 == 0) { // Slow the animation down a little.

mySequenceIndex++;

mySequenceIndex %= 4;

setAnimatedTile(myAnimatedTileIndex, FRAME_SEQUENCE[mySequenceIndex]);

　}

}
}