在 Corona SDK 中，我如何将我的局部变量转换为全局变量？

我想把 local line 转换为全局变量，这样我以后就可以引用它了。这是我的代码:

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

local line
lineGroup = display.newGroup()
local prevX,prevY
local isDrawing = false
local i = 0

local function distanceBetween(x1, y1, x2, y2)
local dist_x = x2 - x1
local dist_y = y2 - y1
local distanceBetween = math.sqrt((dist_x*dist_x) + (dist_y*dist_y))
return distanceBetween
end

local function drawLine(e)
  if(e.phase == "began") then
    if(line) then
        lineGroup:remove(1)
        line = nil
    end
    prevX = e.x
    prevY = e.y
    isDrawing = true
  elseif(e.phase == "moved") then
    local distance = distanceBetween(prevX, prevY, e.x, e.y)
    if(isDrawing and distance < 100) then
        if(line) then lineGroup:remove(1) end
        line = display.newLine(prevX, prevY, e.x, e.y)
        line:setStrokeColor( 0.5,0,1 )
        line.strokeWidth = 5

        local dist_x = e.x - prevX
        local dist_y = e.y - prevY
        physics.addBody(line, "static", { density = 1,
                                          friction = 0.5,
                                          bounce = 2,
                                          shape = {0,     0, dist_x, dist_y, 0, 0} } )
        lineGroup:insert(line)
    end
  elseif(e.phase == "ended") then
    isDrawing = false
  end
end

Runtime:addEventListener("touch",drawLine)

每当我尝试在下一个函数中引用 line 时，都会收到错误消息，指出:

尝试对全局变量 'line' 进行索引(值为nil):

function onCollision(e)
  audio.play(bounceSnd)
  score.text = tostring(tonumber(score.text) + 1)
  score.x = 300
end

gameListeners('add')
end

function gameListeners(action)
  if(action == 'add') then
    line:addEventListener( 'collision', onCollision )
  else
     line:addEventListener( 'collision', onCollision)
  end
end

如果有人能帮忙解决，我将非常感激。