该类的功能主要有以下几个:

1.跟踪贪食蛇实例，同时生成"污渍"的实例，并保证在游戏世界中只存在一个"污渍"

2.提供一个方法，以时间轴为基准的方式来更新贪食蛇（每0.5秒移动一个单元格的距离）

3.跟踪游戏得分，为吃到"污渍"数量的十倍

4.在每吃掉一个"污渍"以后，增加贪食蛇的移动速度，以增加游戏挑战性

5.跟踪贪食蛇是否存活，用此判断游戏是否应该结束

6.在贪食蛇吃掉一个"污渍"后，再生成一个新的"污渍"

package com.badlogic.androidgames.mrnom;

import java.util.Random;

public class World {
    static final int WORLD_WIDTH = 10;
    static final int WORLD_HEIGHT = 13;
    static final int SCORE_INCREMENT = 10;
    static final float TICK_INITIAL = 0.5f;
    static final float TICK_DECREMENT = 0.05f;

    public Snake snake;
    public Stain stain;
    public boolean gameOver = false;;
    public int score = 0;
    //初始游戏的坐标系
    boolean fields[][] = new boolean[WORLD_WIDTH][WORLD_HEIGHT];
    Random random = new Random();
    float tickTime = 0;
    float tick = TICK_INITIAL;
    //初始化蛇对象,放置污渍
    public World() {
        snake = new Snake();
        placeStain();
    }

    //放置污渍
    private void placeStain() {
    	//先把每个单元格坐标设为false
        for (int x = 0; x < WORLD_WIDTH; x++) {
            for (int y = 0; y < WORLD_HEIGHT; y++) {
                fields[x][y] = false;
            }
        }
        //得到蛇身长度
        int len = snake.parts.size();
        //然后把蛇身占据屏幕的坐标位置设成true
        for (int i = 0; i < len; i++) {
            SnakePart part = snake.parts.get(i);
            fields[part.x][part.y] = true;
        }
        //随机产生目标物坐标
        int stainX = random.nextInt(WORLD_WIDTH);
        int stainY = random.nextInt(WORLD_HEIGHT);
        
        //如果不是蛇身占据的位置(等于false的时候)
        while (true) {
            if (fields[stainX][stainY] == false)
                break;
            stainX += 1;
            //x,y坐标如果超过屏幕边界,那么重新置为0后再递增
            if (stainX >= WORLD_WIDTH) {
                stainX = 0;
                stainY += 1;
                if (stainY >= WORLD_HEIGHT) {
                    stainY = 0;
                }
            }
        }
        //放置污渍并随机产生一个污渍类型
        stain = new Stain(stainX, stainY, random.nextInt(3));
    }

    public void update(float deltaTime) {
        if (gameOver)
            return;

        tickTime += deltaTime;

        while (tickTime > tick) {
            tickTime -= tick;
            snake.advance();
            //如果蛇咬到自身,游戏结束
            if (snake.checkBitten()) {
                gameOver = true;
                return;
            }

            SnakePart head = snake.parts.get(0);
            //如果吃到目标物,判断方式是污渍坐标与蛇头部坐标重合
            if (head.x == stain.x && head.y == stain.y) {
            	//吃到一次目标物,分数累加10分
                score += SCORE_INCREMENT;
                //eat方法使蛇身加长一节
                snake.eat();
                //如果蛇身达到最长,占满整个屏幕,游戏结束
                if (snake.parts.size() == WORLD_WIDTH * WORLD_HEIGHT) {
                    gameOver = true;
                    return;
                } else {
                    placeStain();
                }

                //每得100分,蛇的移动速度加快0.05秒,TICK_DECREMENT就是那个增量,tick为移动的间隔时间
                if (score % 100 == 0 && tick - TICK_DECREMENT > 0) {
                    tick -= TICK_DECREMENT;
                }
            }
        }
    }
}