Corona SDK（LUA） - 尝试调用upvalue'spawnEnemy'（零值）

Question

I just trying to add eventListener to a object, which should disappear when I tap on it. But I get error mentioned in the title. Here is my whole code at this point :

-- housekeeping stuff

display.setStatusBar(display.HiddenStatusBar)

local centerX = display.contentCenterX
local centerY = display.contentCenterY

-- set up forward references

local spawnEnemy

-- preload audio

-- create play screens

local function createPlayScreen()

    local bg = display.newImage("background.png")
    bg.y = 130
    bg.x = 100
    bg.alpha = 0

    local planet = display.newImage("planet.png")
    planet.x = centerX
    planet.y = display.contentHeight +60
    planet.alpha = 0

    transition.to( bg,  { time = 2000, alpha = 1,  y = centerY, x = centerX } )

    local function showTitle()
        local gametitle = display.newImage("gametitle.png")
        gametitle.alpha = 0
        gametitle:scale (4, 4)
        transition.to( gametitle, { time = 500, alpha = 1, xScale = 1, yScale = 1 })
        spawnEnemy()
    end 
    transition.to( planet,  { time = 2000, alpha = 1,  y = centerY, onComplete = showTitle } )
end

-- game functions

local function shipSmash(event)

    local obj = event.target
    display.remove( obj )

end

local function spawnEnemy()

    local enemy = display.newImage("beetleship.png")
    enemy.x = math.random(20, display.contentWidth - 20)
    enemy.y = math.random(20, display.contentHeight - 20)
    enemy:addEventListener ( "tap", shipSmash )

end


local function startGame()

end


local function planetDamage()

end


local function hitPlanet(obj)

end




createPlayScreen()
startGame()

And here is how error window looks like :

I'm kinda new in this area(LUA programming) so sorry for maybe dumb syntax error or something, but what I saw is that this error shows up after I write this line of code: enemy:addEventListener ( "tap", shipSmash )

Answer 1

Change local function spawnEnemy() to function spawnEnemy() as this variable was already declared earlier. Yes, this is typical Lua pitfall for beginners.

Answer 2

You've declared spawnEnemy as a local variable twice. That's allowed (the second one hides the first where ever the second one is in scope) but that's not what you wanted.

You have correctly declared a local variable and captured it in showTitle . To set that very same variable later, use an assignment statement without prefixing it with local . You can assign it a function definition using the "anonymous" function syntax:

spawnEnemy = function() 
   ...
end 

Actually, in Lua, all functions are anonymous since they are just values. But, for debugging, it is helpful to have a name associated with a function. In stack traces, the name of the variable used to call the function is used, where possible.