Getting at class data inside an event listener

Dud · May 2012

I have a question about getting access to class data from within an event listener function...

I have successfully set up an event listener function inside my class and am passing extra event data through to it without problems.

However in addition to the data I am passing through to the listener function, I would also like to access data from the class itself using the self. qualifier however I get the common error 'attempt to index field 'xxx' (a nil value).

Am I wrong to assume that inside an event listener function I would have access to class properties? Do I only have access to event data specifically passed to the listener function?

I can provide a code sample if the above explanation is confusing

Thanks

OZApps · May 2012

you can pass the class as an extra parameter as

obj:addEventListener(Event_Name, Event_Handler, obj)

and in the handler, you need to get that as

function Event_Handler(theObj, theEvent)
end

or am I missing something in your question?

ar2rsawseen · May 2012

Hi, you can check this thread for more examples:

http://www.giderosmobile.com/forum/discussion/926/how-to-pass-variables-through-event-listeners-039attempt-to-index-a-nil-value039

Dud · May 2012

Thanks, I did read that thread but couldn't see an example that seemed to match what I was trying to do. I will re-read as I am new to Lua so am still learning

OzApps - I am already passing data as an extra parameter, my problem is that I want to ALSO get access to data from my class as well. It does not seem possible to pass data from TWO sources through to the event handler.

I've seen several examples where an event handler is declared like so:

function Classname:Event_Handler(eventdata)

local buttonInfo = eventData:getTarget() -- This gives me access to event data

self.xxxx -- But trying to access a property of the class results in nil
end

I suppose my question is - whats the purpose of qualifying an event handler with the name of a class if I can't access class data from within the event handler?

I have already thought of a way around my problem but I am interested to know the answer to the above question.

ar2rsawseen · May 2012

This is the right example for you:

--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)

We define class and in first example we can't access class parameters and in second example we can, all only because we pass self or class instance as a third parameter to addEventListener function

Dud · May 2012

Thanks for the example - I shall work on this tonight and see how I do

OZApps · May 2012

@Dud, if you can pass only one item and you want to access more than one, simple pass a structure (a table or an object)

 local myObj={}
 myObj.myClass = theObj -- your class
 myObj.myVar1 = 100        --Some var you wanted
 myObj.myVar2 = 200       --Some other var you wanted
 
theObj:addEventListener(Event.Name, Event_Handler, myObj)

do you think that could help you?

and if you are using this in a theObj:func type function, then you can replace the theObj with self as it is referencing to the same.

Dud · May 2012

OzApps - yes thanks thats exactly what I did in the end. Re-reading the API docs I saw the example of constructing a more complex event object and that gave me exactly what I needed.

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