_x=Math.abs(dog_left-box_left);
        _y=Math.abs(dog_top-box_top);
    
        if(obj.value=="上" && !(_y==0))
        {
            dog.style.top=(dog_top-10)+"px";
            dog_top=dog_top-10;
        }else if(obj.value=="下" && !(_y==(box_height-dog_height)))//这里要越界
        {
            dog.style.top=(dog_top+10)+"px";
            dog_top=dog_top+10;
        }else if(obj.value=="左" && !(_x==0))
        {
            dog.style.left=(dog_left-10)+"px";
            dog_left=dog_left-10;    
        }else if(obj.value=="右" && !(_x==(box.width-dog_width)))//这里要越界
        {
            dog.style.left=(dog_left+10)+"px";
            dog_left=dog_left+10;
        }