蛇不动，蛇身和头重叠在中央不动，代码应该和视频里的一样，不知怎么破，请教老师求解

mainscene.lua:

local Snake = require"app.Snake"

local MainScene = class("MainScene", function()

    return display.newScene("MainScene")

end)

--[[function MainScene:ctor()

    cc.ui.UILabel.new({

            UILabelType = 2, text = "Hello, World", size = 64})

        :align(display.CENTER, display.cx, display.cy)

        :addTo(self)

end]]--

local cMoveSpeed = 0.3

function MainScene:onEnter()

 self.snake = Snake.new(self)

  local tick = function()

   self.snake:Update()

  end

 cc.Director:getInstance():getScheduler():scheduleScriptFunc(tick,cMoveSpeed,false)

end

function MainScene:onExit()

end

return MainScene

Snake.lua:

local Snake = class("Snake")

local Body = require("app.Body")

local cInitLen = 3

function Snake:ctor(node)

 self.BodyArray = {}

 self.node = node

 self.MoveDir = "left"

 for i = 1,cInitLen do

  self:Grow(i == 1)

 end

end

function Snake:GetTailGrid()

 if#self.BodyArray == 0 then

  return 0,0

 end

 local tail = self.BodyArray[#self.BodyArray]

 return tailX,tailY

end

function Snake:Grow(isHead)

 local tailX,tailY = self:GetTailGrid()

 local body = Body.new(self,tailX,tailY,self.node,isHead)

 table.insert(self.BodyArray,body)

end

local function OffsetGridByDir(x,y,dir)

 if dir == "left" then

  return x-1,y

 elseif dir == "right" then

  return x+1,y

 elseif dir == "up" then

  return x,y-1

 elseif dir == "down" then

  return x,y+1

 end

 print("unknown dir",dir)

 return x,y

end

function Snake:Update()

 if#self.BodyArray == 0 then

  return 

 end

 --YXXX

 for i = #self.BodyArray,1,-1 do

  local body = self.BodyArray[i]

   if i == 1 then

   body.X,body.Y = OffsetGridByDir(body.X,body.Y,self.MoveDir)

   else

    local front = self.BodyArray[i-1]

    body.X,body.Y = front.X,front.Y

   end

   body:Update()

  end

end

return Snake

蛇就是不动！！