为什么我的蛇只能左右 ，而不能上下移动，运行没有报错，求大神解答

local Snake = require "app.Snake"

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

    return display.newScene("MainScene")

end)

local cMoveSpeed = 0.3

function MainScene:onEnter()

self.Snake = Snake.new(self)

self:ProcessInput()

local tick = function()

self.Snake:Update()

end

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

end

-------------------------------------------------------------

local function vector2Dir(x,y)

if math.abs(x) > math.abs(y) then

if x < 0 then

return "left"

else

return "right"

end

else

if y < 0 then

return "up"

else

return "down"

end

end

end

-------------------------------------------------------

function MainScene:ProcessInput()

local function onTouchBegan(touch,event)

local location = touch:getLocation()

local visibleSize = cc.Director:getInstance():getVisibleSize()

local origin = cc.Director:getInstance():getVisibleOrigin()

local finalX = location.x - (origin.x + visibleSize.width/2)

local finalY = location.y - (origin.y + visibleSize.height/2)

local dir = vector2Dir(finalX,finalY)

self.Snake:SetDir(dir)

end

local listener = cc.EventListenerTouchOneByOne:create()

listener:registerScriptHandler(onTouchBegan,cc.Handler.EVENT_TOUCH_BEGAN)

local eventDispatcher = self:getEventDispatcher()

eventDispatcher:addEventListenerWithSceneGraphPriority(listener,self)

end