Event Listener attributes

aditya · January 2014

Have a lua file and a declared Core.class in it.

pScreen = Core.class(Sprite)

Now when I do this, I get "attempt to index field '?' (a nil value)"

1) function pScreen:backButtonClicked(data,event)
2) self.backButton:addEventListener(Event.MOUSE_UP, self.backButtonClicked,{self.backButton})

But this works

1) function backButtonClicked(data,event)
2) self.backButton:addEventListener(Event.MOUSE_UP, backButtonClicked,{self.backButton})

Why?

ar2rsawseen · January 2014

The problem is, that:

function pScreen:backButtonClicked(data,event)

is actually a

function pScreen.backButtonClicked(self, data,event)

and when you add it as

self.backButton:addEventListener(Event.MOUSE_UP, self.backButtonClicked,{self.backButton})

what it receives is:

function pScreen.backButtonClicked({self.backButton},event)

while it expects:

function pScreen.backButtonClicked(self, data, event)

So to make it work, you should do:

--1) 
function pScreen:backButtonClicked(event)
    --self.backButton was clicked
end
 
--2) some where in other method
self.backButton:addEventListener(Event.MOUSE_UP, self.backButtonClicked,self)

Hope that helps

aditya · January 2014

Got it. Thanks!

aditya · January 2014

Am still a tad confused. Sorry again.

Using your solution and event:getTarget() works. But how do you pass external data in the addEventListener above?

ar2rsawseen · January 2014

@aditya you don't, you simple make it as a property of a class, like this:

function MyClass:init()
    self.custom = "myCustom data"
    self:addEventListener(Event.MOUSE_UP, self.onMouseUp, self)
end
 
function MyClass:onMouseUp(e)
    if self:hitTestPoint(e.x, e.y) then
        print(self.custom) -- prints myCustom data
    end
end

aditya · January 2014

Question, if a function is declared as pScreen.backButtonClicked instead of pScreen:backButtonClicked, will the scenemanager still be able to dereference it if the scene is changed?

ar2rsawseen · January 2014

You can think of it as a static function and yes it could be referenced even if the scene was changed, cause it would actually work without any instance.
And in this case, what ever you pass to the adEventListener as a third argument will be received in this function as first, for example:

function MyClass:init()
    self.custom = "myCustom data"
    self:addEventListener(Event.MOUSE_UP, self.onMouseUp, self)
end
 
function MyClass.onMouseUp(self, e)
    if self:hitTestPoint(e.x, e.y) then
        print(self.custom) -- prints myCustom data
        --but only because we passed instance as argument, not because it is a part of the class
    end
end

So you could actually make it work with any instance, even from completely other class, by passing reference to instance like this

aditya · January 2014

Okay.

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