蛇不動，蛇身和頭重疊在中央不動，代碼應(yīng)該和視頻里的一樣，不知怎么破，請教老師求解

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

蛇就是不動！！