尝试索引 upvalue

我今天刚开始学习 Lua。我一直在 coronalabs.com 网站上做教程…… 我尝试将第一个练习中弹跳气球的功能适应到 asteroids 游戏的场景模板中。有人能告诉我我是如何“尝试索引 upvalue”吗？

local composer = require("composer")

local scene = composer.newScene()

-- -----------------------------------------------------------------------------------
-- 以下的代码在场景事件函数之外只会执行一次
-- 除非场景被完全删除（而不是通过“composer.removeScene()”回收）
-- -----------------------------------------------------------------------------------

local physics = require("physics")
physics.start()

local tapCount = 0
local platform
local balloon
local tapText

local function pushBalloon()
     balloon:applyLinearImpulse( 0, -0.75, balloon.x, balloon.y )
     tapCount = tapCount + 1
     tapText.text = tapCount

end

-- -----------------------------------------------------------------------------------
-- 场景事件函数
-- -----------------------------------------------------------------------------------

-- create()
function scene:create( event )

    local sceneGroup = self.view
    -- 当场景初次创建且未显示在屏幕上时，此处的代码运行
physics.pause()
local background = display.newImageRect("background.png", 360, 570)
background.x = display.contentCenterX
background.y = display.contentCenterY
local platform = display.newImageRect("platform.png", 300, 50)
platform.x = display.contentCenterX
platform.y = display.contentHeight-25
local balloon = display.newImageRect("balloon.png", 112, 112)
balloon.x = display.contentCenterX
balloon.y = display.contentCenterY
balloon.alpha = 0.8

local tapText = display.newText(tapCount, display.contentCenterX, 20, native.systemFont, 40)
tapText:setFillColor(0, 0, 0)

physics.addBody(platform, "static")
physics.addBody(balloon, "dynamic", { radius=50, bounce=0.6 })

balloon:addEventListener("tap", pushBalloon)

end

-- show()
function scene:show( event )

    local sceneGroup = self.view
    local phase = event.phase

    if ( phase == "will" ) then
        -- 此时场景仍未显示在屏幕上

    elseif ( phase == "did" ) then
        -- 此时场景完全显示在屏幕上
physics.start()
    end
end

-- hide()
function scene:hide( event )

    local sceneGroup = self.view
    local phase = event.phase

    if ( phase == "will" ) then
        -- 此时场景正在显示在屏幕上

    elseif ( phase == "did" ) then
        -- 此时场景完全从屏幕上消失
physics.pause()
    end
end

-- destroy()
function scene:destroy( event )

    local sceneGroup = self.view
    -- 此处的代码在删除场景视图之前运行

end

-- -----------------------------------------------------------------------------------
-- Scene event function listeners
-- -----------------------------------------------------------------------------------
scene:addEventListener("create", scene)
scene:addEventListener("show", scene)
scene:addEventListener("hide", scene)
scene:addEventListener("destroy", scene)
-- -----------------------------------------------------------------------------------

return scene