如何在Corona SDK中使用id来加载精灵和图像文件？

标题听起来很复杂，但我需要的是类似于这样的东西：https://www.youtube.com/watch?v=GhI98-HJk6o

只需观看第一分钟，注意当玩家点击底部的正方形时，适当的单位会生成。我正在尝试在Corona SDK中实现这一点，但最近卡住了很多，希望你们能帮忙。

我尝试做的是这样的：

local m = {} -- characters.lua

m.units = {
   {img = "sprites/girl.png", timeBetweenAtk = 2350, name = girl, cost = 75, respawnTime = 3750, state = active}
}

m.girl = {}
m.girl.walking = {}
local data = {width = 309, height = 494, numFrames = 10, sheetContentWidth = 1545, sheetContentHeight = 988}
m.girl.walking.sheet = graphics.newImageSheet("sprites/walking-girl.png", data )
m.girl.walking.seq = {name = "walking", start = 1, count = 10, loopCount = 0, loopDirection = "forward"}

m.girl.primaryAtk = {}
local data = {width = 385, height = 477, numFrames = 10, sheetContentWidth = 1925, sheetContentHeight = 954}
m.girl.primaryAtk.sheet = graphics.newImageSheet("sprites/melee-girl.png", data)
m.girl.primaryAtk.seq = {name = "melee", start = 1, count = 10, loopCount = 0, loopDirection = "forward"}

m.girl.idle = {}
local data = {width = 271, height = 473, numFrames = 10, sheetContentWidth = 813, sheetContentHeight = 1892}
m.girl.idle.sheet = graphics.newImageSheet("sprites/idle-girl.png", data)
m.girl.idle.seq = {name = "idle", start = 1, count = 10, loopCount = 0, loopDirection = "forward"}

return m

然后我在main.lua中写了一个函数，调用正确的精灵：

local characters = require "characters"

for k = 1, #imgSet do
  local function spawnCharacter(event)
     local identifier = characters.units[k].name
     if event.phase == "began" then
       if moneyCount < characters.units[k].cost then
          print("资金不足")
       else
          moneyCount = moneyCount - characters.units[k].cost
          moneyText.text = moneyCount .. " / " .. moneyWallet
          local unit = display.newSprite(characters.identifier.walking.sheet, characters.identifier.walking.seq)
       end
     end
     return true
  end
  imgSet[k]:addEventListener("touch", spawnCharacter)
end

但现在我知道无法连接显示对象，那么有人知道是否有另一种方法来做到这一点吗？