How to pass variables through Event listeners ('attempt to index a nil value')?

Mells · April 2012

Hi,

I'm having some troubles to pass some variables through an Event listener.
Could someone point me in the right direction?

-- [ scenecontroller.lua ]
function SceneController:init()
        self.testvar = "Test"
end
 
-- [ interface.lua ]
function Interface:init(sceneController)
        print (sceneController.testvar)
        -- Test ok
 
	self:createInterface(sceneController)
end
 
function Interface:createInterface(sceneController)
        print (sceneController.testvar)
        -- Test ok
 
	*******|| self.nextPage:addEventListener("click", self.nextPageClick, sceneController) ||*******
end
 
function Interface:nextPageClick(sceneController)
	print (sceneController.testvar)
        -- Error attempt to index a nil value
end
 
-- [ main.lua ]
sc = SceneController.new()
interface = Interface.new(sc)

How can I give sceneController to nextPageClick through the Event listener?

Thank you,
Mells

ndoss · April 2012

@Mells, one way would be to wrap your function call inside another function:

self.nextPage:addEventListener("click", function() self:nextPageClick(sceneController) end)

Mells · April 2012

@ndoss
Thank you very much (and also for all your contributions to this community).

I have 2 other questions :
1. I was wondering how the third argument of the addEventListener function (the data parameter) could be used?
I thought it was straightforward to use sceneController as an argument (like "self" is often used in that case) and I don't really get why it didn't work.

(Also See the way it's used in the Custom Events section in the Ultimate Guide)

How can one take advantage of the third argument (data) that is passed in the addEventListener function?

2. How do you remove the EventListener that is created the way suggested :

self.nextPage:addEventListener("click", function() self:nextPageClick(sceneController) end)

?

ndoss · April 2012

1. I was wondering how the third argument of the addEventListener function (the data parameter) could be used?
I thought it was straightforward to use sceneController as an argument (like "self" is often used in that case) and I don't really get why it didn't work.

I gave you a crappy answer. The best answer is probably is to call:

self.nextPage:addEventListener("click", self.nextPageClick, self, sceneController)

The call to self:nextPageClick(sceneController) is the same as self.nextPageCLick(self, sceneController).

2. How do you remove the EventListener that is created the way suggested :
self.nextPage:addEventListener("click", function() self:nextPageClick(sceneController) end)
?

I don't know -- don't do it that way

Mells · April 2012

Thank you again for your help @ndoss.

However

-- [ interface.lua ]
function Interface:createInterface(sceneController)
        print (sceneController.testvar)
        -- Test ok
 
        self.nextPage:addEventListener("click", self.nextPageClick, self, sceneController)
end
 
 
function Interface:nextPageClick(sceneController)
	print (sceneController.testvar)
        -- ***** nil *****
end

gives me an error.

Maybe I am missing something?

ndoss · April 2012

Maybe I am missing something?

Looks like I gave you two crappy answers, my apologies for that. I should have tested before I answered. I'm not sure about the best way to do what you want. The following code works, but I'm not sure it's the best way ... hopefully someone else will point out something better ...

EventTest = Core.class()
function EventTest:init(arg)
   local function onEvent(handler, event)
      self:print(arg)
      stage:removeEventListener(Event.MOUSE_DOWN, handler, handler)
   end
   stage:addEventListener(Event.MOUSE_DOWN, onEvent, onEvent)
end
 
function EventTest:print(event, arg)
   print(inspect(event))
end
 
et = EventTest.new("abc")

Mells · May 2012

hopefully someone else will point out something

I will wait patiently, because I don't know much where else I can learn about it and this will be very useful for the future.

....

ar2rsawseen · May 2012

In case, when I need to pass a variable to event listener, I usually hook it up to object that calls listener as property, and use getTarget inside event listener. But I haven't used custom event listeners, so I don't know if there are any specifics there.

Here's a quick example:

local ball = Bitmap.new(Texture.new("ball.png"))
ball.cnt = 1	
ball:addEventListener(Event.MOUSE_DOWN, function(e)
	local target = e:getTarget()
	print(target.cnt)
end)

Caroline · May 2012

I think if I were doing that exact same thing, I would do this:

function Interface:init(sceneController)
        print (sceneController.testvar)
        self.sceneController = sceneController
end

Then it would be available at any time.

Caroline · May 2012

However, I think I have done this exact same thing, but it is quite convoluted going between classes. I will have to work it out...

Caroline · May 2012

I hope I am understanding what you want. This is distilled from what I have done in an app:

main.lua

EventClass = gideros.class(EventDispatcher)
eventDispatcher = EventClass.new()
 
dispatcher = Dispatcher.new()
receiver = Receiver.new()
 
eventDispatcher:addEventListener("myEvent", receiver.doEvent, receiver)
 
dispatcher:sendEvent("Hello World")

dispatcher.lua

Dispatcher = gideros.class()
 
function Dispatcher:init()
 
end
 
function Dispatcher:sendEvent(withData)
	local event = Event.new("myEvent")
	event.data1 = withData
	print("Dispatching event with data:", withData)
	eventDispatcher:dispatchEvent(event)
end

receiver.lua

Receiver = gideros.class()
 
function Receiver:init()
end
 
function Receiver:doEvent(event)
	print("Event done:", event.data1)
end

I don't know whether this is good practice, to have a global event dispatcher, but it worked for me

.

Mells · May 2012

@ar2rsawseen & @Caroline
Those answers go a little bit beyond my current level but I will study it a few hours and with no doubt learn a lot.
Thank you for taking the time to answer

I was still curious about the use of "data" in :
EventDispatcher:addEventListener(type, listener, data)
I will try to find more examples on top of the ebook/game templates to get an understanding of this parameter.

ar2rsawseen · May 2012

In system events, parameter data represents the scope, the context of function.
For example

--let's create our own class based on Sprite
Scene = Core.class(Sprite)
 
function Scene:init()
	--variable self is referencing to instance of the class scene
	--so if we do this
	self.somevar = "somevalue"
end
 
--we will be able to access it like this:
local scene = new Scene()
print(scene.somevar)
 
--now if we add a system event to this scene like this
scene:addEventListener(Event.MOUSE_DOWN, function()
	--we won't be able to access somevar
	print(self.somevar) -- results nil
	--because function we defined has it's own scope
	--and variable self is now referencing to it's scope
	--and note scene's scope
end)
 
--but if we pass scene as data parameter
scene:addEventListener(Event.MOUSE_DOWN, function()
	--we can access somevar
	print(self.somevar) -- results "somevalue"
	--because self is now referencing to whatever we passed as data parameter
	--which was scene
end, scene)

I don't know if the same applies to custom events

But all in all, it's really similar to whether you use . or :

--let's create our own class based on Sprite
Scene = Core.class(Sprite)
 
function Scene:init()
	--variable self is referencing to instance of the class scene
	--so if we do this
	self.somevar = "somevalue"
end
 
--create instance
local scene = new Scene()
 
--let's define function using :
--meaning we will use same scope
function scene:samescope()
	--somevar is still accessible
	print(self.somevar) --results "somevalue"
end
 
--now let's use .
--meaning we will use different scope
scene.differentcope = function()
	--somevar is NOT accessible
	print(self.somevar) -- results nil
end

fajarnugroho23 · July 2013

Hmm, what about "self" reference at onTimer function?
Suppose there are many enemy instance that each has a timed behaviour like this:

-- Enemy class
Enemy = gideros.class(Sprite)
 
local enemyX, enemyY
local moveSpeed
 
local enemyTimer
 
function Enemy:init(enemyX, enemyY)	
	self.enemyX = enemyX
	self.enemyY = enemyY
	self:moveSpeed = 10
 
	self:setSpeed()
 
	self.enemyTimer = Timer.new(math.random(2000, 5000), 1)
 
	-- "self" reference?
	self.enemyTimer.parent = self
 
	self.enemyTimer:addEventListener(Event.TIMER, self.onTimer, self.enemyTimer)
	self.enemyTimer:start()
end
 
function Enemy:setSpeed() 	
	if math.random(1, 6) <= 3  then
		self.moveSpeed = 10
	else 
		self.moveSpeed = -10
	end
end
 
function Enemy:onTimer(enemyTimer)
	enemyTimer.parent:setSpeed() -- nil?
 
	enemyTimer = Timer.new(math.random(2000, 5000), 1)
	enemyTimer:addEventListener(Event.TIMER, enemyTimer.parent.onTimer, enemyTimer)
	enemyTimer:start()
end

I've tried to have "self" reference (parent) on enemyTimer, but it became nil on Enemy:onTimer..

Can someone explain this? I'm still new with this lua thing..

Thanks~

Mells · July 2013

@fajarnugroho23

Quickly drafted on my ipod and modified later (but didn't test) but should work as a basis :

=================
Enemy.lua
=================

Enemy = Core.class(Sprite)
function Enemy:init(parent, x, y)
     self.parent = parent
 
     -- Vars
     self.originX = x
     self.originY = y
     self.speedLimitMin = 0
     self.speedLimitMax = 6
     self.speedValue = 10
     self.timerLimitMin = 2000
     self.timerLimitMax = 5000
     self.timerLimitNbRepeat = 1
 
     -- Position
     -- self:setPosition(x, y) ? It is not part of your code but might be what you want to achieve?
 
     -- Speed
     self:setSpeed(self.speedValue, self.speedLimitMin, self.speedLimitMax)
 
     -- Timer
     self.enemyTimer = self:loopTimer(self.timerLimitMin, self.timerLimitMax, self.timerNbRepeat)
end
 
 
function Enemy:setSpeed(limitValue, limitMin, limitMax)
        -- Set defaults so you can use without parameters if you want
 	local limitMin = limitMin or self.speedLimitMin
 	local limitMax = limitMin or self.speedLimitMax
 	local limitValue = limitValue or self.speedLimitValue
	if math.random(limitMin, limitMax) <= ((limitMax-limitMin)*0.5)  then
		self.moveSpeed = limitValue
	else 
		self.moveSpeed = -limitValue
	end
end
 
 
function Enemy:loopTimer(limitMin, limitMax, nbRepeat, prevTimer)
 
      -- Clean and nil current Timer
      if not(prevTimer == nil) then
            prevTimer:removeEventListener(Event.TIMER, self.onTimer, self)
            prevTimer = nil
      end
 
      -- Create new
      local timer = Timer.new(math.random(limitMin, limitMax), nbRepeat)
      timer:addEventListener(Event.TIMER, self.onTimer, self)      
      timer:start() -- If you want to play it by default
      return timer
end
 
function Enemy:onTimer()
 
     self:setSpeed() -- Should work without parameters
 
     -- ####################
     -- I don't get what you are trying to do here, recreating another timer? Want it to loop?
     -- if yes, then
     --[[
     self.enemyTimer = self:loopTimer(self.timerLimitMin, self.timerLimitMax, self.timerNbRepeat, self.enemyTimer)
     ]]--
end

=================
Scene.lua
=================

Scene = Core.class(Sprite)
function Scene:init()
     local enemy = Enemy.new(self, 100, 100)
     self:addChild(enemy)
end

=================
main.lua
=================

local scene = Scene.new()
stage:addChild(scene)

Make sure that Scene.lua is loaded before main.lua (code dependencies).

I hope it helps.

ar2rsawseen · July 2013

What @Mells posted should work
but just to explain the difference, if you define method through colon (:), as you did with function Enemy:onTimer(enemyTimer), then it means that it converts first parameter to self variable inside function. thus if you add event listener like this:

self.enemyTimer:addEventListener(Event.TIMER, self.onTimer, self.enemyTimer)

Then inside your onTimer method you should use it like:

function Enemy:onTimer()
	--now what you passed as self.enemyTimer is self here
	self.parent:setSpeed() -- would do what you want
end

fajarnugroho23 · July 2013

@Mells @ar2rsawseen

whoa, thank you!
i'll try it this night~

Mells · July 2013

@fajarnugroho23
as @ar2rsawseen just did there are many little subtleties that you will discover if you take time to look at the code. I don't have time to comment but it should get you started.

Good luck

(ps : when I joined this forum I had no experience with lua at all; so no doubt you'll be able to get used to it fast.)

fajarnugroho23 · July 2013

yep it works!

now i have a better understanding about variable passing in lua..

thanks again @Mells

Mells · July 2013

@fajarnugroho23
you're welcome

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