Android实例剖析笔记（五）

这个系列的前四篇文章介绍了Android sdk中自带的NotePad程序，算是开了个头，这篇开始介绍Android sdk中自带的第二个示例程序—Snake（贪食蛇）。本文将主要介绍我对这个示例程序进行的一些修改。

游戏暂停/继续机制

由于原来的代码中在游戏运行时没有提供控制选项（比如暂停/继续），因此除非你死了，否则只能玩到底。我这里对代码进行一些修改，加入一个Option Menu来提供暂停/继续机制。

首先加入一个变量记录游戏当前状态

private int mState = SnakeView.READY;

然后重载onCreateOptionsMenu函数，创建一个控制菜单项，并对其进行处理，提供暂停/继续机制。

   /*
     * @see android.app.Activity#onOptionsItemSelected(android.view.MenuItem)
     * @Author:phinecos
     * @Date:2009-08-28
     */
    @Override
    public boolean onOptionsItemSelected(MenuItem item)
    {
        switch (item.getItemId())
        {
            case MENU_CONTROL:
            {
                if (mState == SnakeView.PAUSE)
                {//此前状态是"停止"，则转为"运行"
                    mState = SnakeView.RUNNING;
                    mSnakeView.setMode(SnakeView.RUNNING);
                    item.setIcon(android.R.drawable.ic_media_pause).setTitle(R.string.cmd_pause);
                }
                else if(mState == SnakeView.RUNNING)
                {//此前状态是"运行",则转为“暂停"
                    mState = SnakeView.PAUSE;
                    mSnakeView.setMode(SnakeView.PAUSE);
                    item.setIcon(android.R.drawable.ic_media_play).setTitle(R.string.cmd_run);
                }
                else if(mState == SnakeView.READY)
                {//此前是"初始状态"，则转为"运行"
                    mState = SnakeView.RUNNING;
                }
    return true;
            }
        }
        return super.onOptionsItemSelected(item);
    }
    /*
     * @see android.app.Activity#onOptionsItemSelected(android.view.MenuItem)
     * @Author:phinecos
     * @Date:2009-08-28
     */
    @Override
    public boolean onCreateOptionsMenu(Menu menu)
    {
         super.onCreateOptionsMenu(menu);
         menu.add(0, MENU_CONTROL, 0, R.string.cmd_pause).setIcon(android.R.drawable.ic_media_pause);
         return true;
    }

修改后运行截图如下：

当然，这段代码还是有问题的，游戏刚开始时，必须先点击菜单确认，再按上方向键才能开始。（以后再来修改。。。）

穿墙贪食蛇

第二个修改是把这个普通的贪食蛇改成可以穿墙(呵呵，这样就可以不死了。。。)。想必要修改的就是撞墙检测那段代码？没错，就是下面这段！

// Collision detection
// For now we have a 1-square wall around the entire arena
if ((newHead.x < 1) || (newHead.y < 1) || (newHead.x > mXTileCount - 2)|| (newHead.y > mYTileCount - 2))
{//撞墙
setMode(LOSE);
return;
}

原来的版本是发现撞墙时就直接判定为失败，我这里做个小小的修改，让它可以穿墙而去:

private void updateSnake()
    {
        boolean growSnake = false;

        // grab the snake by the head
        Coordinate head = mSnakeTrail.get(0);
        Coordinate newHead = new Coordinate(1, 1);

        mDirection = mNextDirection;

        switch (mDirection)
        {
        case EAST:
        {
            newHead = new Coordinate(head.x + 1, head.y);
            break;
        }
        case WEST:
        {
            newHead = new Coordinate(head.x - 1, head.y);
            break;
        }
        case NORTH:
        {
            newHead = new Coordinate(head.x, head.y - 1);
            break;
        }
        case SOUTH:
        {
            newHead = new Coordinate(head.x, head.y + 1);
            break;
        }
        }

        //穿墙的处理
        if (newHead.x == 0)
        {//穿左边的墙
            newHead.x = mXTileCount - 2;
        }
        else if (newHead.y == 0)
        {//穿上面的墙
            newHead.y = mYTileCount - 2;
        }
        else if (newHead.x == mXTileCount - 1)
        {//穿右边的墙
            newHead.x = 1;
        }
        else if (newHead.y == mYTileCount - 1)
        {//穿下面的墙
            newHead.y = 1;
        }
        // 判断是否撞到自己
        int snakelength = mSnakeTrail.size();
        for (int snakeindex = 0; snakeindex < snakelength; snakeindex++)
        {
            Coordinate c = mSnakeTrail.get(snakeindex);
            if (c.equals(newHead))
            {
                setMode(LOSE);
                return;
            }
        }
        // 判断是否吃掉“苹果”
        int applecount = mAppleList.size();
        for (int appleindex = 0; appleindex < applecount; appleindex++)
        {
            Coordinate c = mAppleList.get(appleindex);
            if (c.equals(newHead))
            {
                mAppleList.remove(c);
                addRandomApple();
                mScore++;
                mMoveDelay *= 0.9;
                growSnake = true;
            }
        }
        // push a new head onto the ArrayList and pull off the tail
        mSnakeTrail.add(0, newHead);
        // except if we want the snake to grow
        if (!growSnake)
        {
            mSnakeTrail.remove(mSnakeTrail.size() - 1);
        }
        int index = 0;
        for (Coordinate c : mSnakeTrail)
        {
            if (index == 0)
            {
                setTile(YELLOW_STAR, c.x, c.y);
            }
            else
            {
                setTile(RED_STAR, c.x, c.y);
            }
            index++;
        }
}

其实修改后的代码非常简单，就是把新节点的值做些处理，让它移动到对应的行/列的头部或尾部即可。

下面就是修改后的“穿墙”贪食蛇的运行截图：