Nokia UI学习范例FishTank研究[3]

        int i = 0;
        int k = 0;
        try
        {
            for (i = 0; i < TYPE_NUM; i++)
            {
                for (k = 0; k < FRAME_NUM; k++)
                {
                    fishImage[i][k] = Image.createImage("/fish" + i + k
                            + ".png");
                }
            }
        } catch (java.io.IOException e)
        {
            fishImage[i][k] = null;
        }

        // This MIDlet implicitly assumes that all the Images
        // are the same width and height.

        fishWidth = fishImage[0][0].getWidth();
        fishHeight = fishImage[0][0].getHeight();
    }

为了让鱼儿或者水草具有更好的随机性，代码是通过如下的算法来控制他们的方向的，下面以方向x为例说明
        // mostly continue as we are, but sometimes randomly change
        if ((ticksSinceLastChangeX > 20) && (rand(20) == 0))
        {
            vx = rand(2) * 2 - 1; // -1 or 1
            ticksSinceLastChangeX = 0;
        }

        // if moving would take us off the left or right of the water,
        // reverse
        if (((vx < 0) && (x + vx < 0))
                || ((vx > 0) && (x + fishWidth + vx > FishTankCanvas.waterWidth)))
        {
            vx = -vx;
            ticksSinceLastChangeX = 0;
        }
当鱼接触到水的边缘的时候，它的速度方向会改变。在此前的代码是为了增加他的随机性。Fish的重新绘制也比较简单，这里用到了Nokia UI中的水平翻转的扩展功能
    public void draw(Graphics g)
    {
        DirectGraphics dg = DirectUtils.getDirectGraphics(g);

        Image img = fishImage[type][frame];
        if (img != null)
        {
            if (vx < 0)
            {
                dg.drawImage(img, x, y, (Graphics.LEFT | Graphics.TOP),
                        DirectGraphics.FLIP_HORIZONTAL);
            } else
            {
                dg.drawImage(img, x, y, (Graphics.LEFT | Graphics.TOP), 0);
            }
        }

        frame = (frame + rand(2)) % FRAME_NUM;
    }

我们可以看到每次绘画结束后都会把frame的编号随机设置一下，这样可以得到鱼儿本身运动的效果。

    MIDlet的代码比较简单，直接给出
package example.fishtank;

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

public class FishTankMIDlet extends MIDlet
{
    private final FishTankCanvas canvas;