OnBeginContact - Object activated (local variable)

peach303 · January 2013

Hi,

when a collision occures i want to check if one of the bodys is "activated". Activated is a local variable in the Object class:

 self:body.activated = true -- for example

Here is the OnBeginContact function:

function onBeginContact(event)
	local fixtureA = event.fixtureA
	local fixtureB = event.fixtureB
	local bodyA = fixtureA:getBody()
	local bodyB = fixtureB:getBody()
 
	print("begin contact: " .. bodyA.type .. " & " .. bodyB.type)
 
	if bodyA.type and bodyA.type == "Trigger" and bodyB.aktiviert == true then 
		bodyA.aktiviert = true 
	end
	if bodyB.type and bodyB.type == "Trigger" and bodyA.aktiviert == true then 
		bodyB.aktiviert = true 
	end
 
end

bodyA.aktiviert (or BodyB.aktiviert) is nil because its a local in the object class. But how to solve this??

An object shall only be activated if the touching object is already activated...

daniel303

zvardin · January 2013

Well, you can either move that variable to the body OR add a reference to the containing object in the body.

-- pseudo code
SomeClass = Core.class(Sprite)
 
function SomeClass:init(world)
          local body = world:createBody()
          body.parent = self
end

peach303 · January 2013

Thx. Hm, i dont get it. In your code body.parent is a local variable of SomeClass right? So i cant use it outside of SomeClass. The same problem like with body.aktiviert.

ar2rsawseen · January 2013

@peach303 can you show your Object's class?

peach303 · January 2013

Sure!

This is the init of the PuckClass in puck.lua:

function Puck:init(xp, yp, world)
	self.puckArrow = Bitmap.new(Texture.new("puckArrow.png"))
	self.puckHalo = Bitmap.new(Texture.new("puckHalo.png"))
	self.puckBody = Bitmap.new(Texture.new("puck.png"))
	self.puckLight = Bitmap.new(Texture.new("puckLight.png"))
 
	self.dir = 0
	self.pow = 0
	self.illumination = 0
 
	self.puckArrow:setAnchorPoint(0.5, 1)
	self.puckArrow:setVisible(false)
 
	self.puckHalo:setAlpha(0.3)
 
	self.puckLight:setAlpha(0)
 
	self:addChild(self.puckArrow)
	self:addChild(self.puckBody)
	self:addChild(self.puckHalo)
	self:addChild(self.puckLight)
 
	self.puckBody:setPosition(-50,-50)
	self.puckHalo:setPosition(-100,-100)
	self.puckLight:setPosition(-50,-50)
 
	self:setPosition(xp,yp)
 
	local body = world:createBody{type = b2.DYNAMIC_BODY}
    body:setPosition(xp, yp)
    body:setAngle(self:getRotation() * math.pi/180)
    local circle = b2.CircleShape.new(0, 0, 50)
    local fixture = body:createFixture{shape = circle, density = 1.0, friction = 0, restitution = 1}
    self.body = body
    self.body.type = "Puck"
	self.parent = self
 
	self.body:setLinearDamping(damping)
 
	self.body.aktivert = true
 
	self:addEventListener(Event.MOUSE_DOWN, self.onTouchesBegin, self)
	self:addEventListener(Event.MOUSE_MOVE, self.onTouchesMove, self)
	self:addEventListener(Event.MOUSE_UP, self.onTouchesEnd, self)
	--self:addEventListener(Event.TOUCHES_CANCEL, self.onTouchesCancel, self)
 
	self:addEventListener(Event.ENTER_FRAME, updateLight, self)
end

And the onBeginContact function (see above) is in scene3.lua

zvardin · January 2013

My apologies, I think I read your first post too fast and saw you needed to access something from your class, thus my first example.

However, looking at it again it looks like your problem is trying to compare aktiviert to true since it might be nil maybe? The class you showed, shows aktiviert always being initialized and the type is always "Puck" for the body. Does its type get changed to "Trigger" at some point or is there a different class/body?

If aktiviert being nil is what throws the error then I suggest changing any check on aktiviert, to the following:

-- by removing the == true on the aktiviert check, nil will count as false and not throw an error
 
if bodyB.type and bodyB.type == "Trigger" and bodyA.aktiviert then 
   bodyB.aktiviert = true 
end

If this isn't what's going on, then we may need to see more as I said above, we can't see where a body becomes a "Trigger" etc so I suspect there's another class.

peach303 · January 2013

Ah, i'm sorry. I should explain a little more.. There are 2 objects. "Puck" is allways aktiviert = true. And "Trigger" is the secound object (initialised as aktiviert = false. When hit by Puck or another activated (aktiviert) object, it should get activated too.

Yes, the problem is that aktiviert is nil. But it should be true in case Puck is the collision partner.

Should i post more of my code?

zvardin · January 2013

Are there walls it can hit? I still suspect the error happens when you hit something else or there's a small mistake initializing the aktiviert property. If it's nil then it still seems like it shouldn't be either of those classes causing the problem if you initialize the value.

If you change the code similar to like what I said in my last post your code should function even if the value IS nil however because nil will be treated as a false if you take away the "== true".

atilim · January 2013

@peach303 your code seems ok. Maybe printing the values as:

print(bodyA.type, bodyA.aktiviert, bodyB.type, bodyB.aktiviert)

can help understanding the problem?

ar2rsawseen · January 2013

@peach303 the problem is that in your class you have aktivert while on contact you are checking for aktiviert (mistyped?)

But all in all you should be able to access it like that, there should be no problem

peach303 · January 2013

If you change the code similar to like what I said in my last post your code should function even if the value IS nil however because nil will be treated as a false if you take away the "== true".

@ zvardin: thx a lot for this advise! I forgot the walls they are there too.

@atilim: THX! This brought the answer! It was the small mistake initializing the aktiviert property.. i wrote "aktivert" 8-|

I still dont understand why i can work with aktiviert here. Isn't it a local in the object class? But it is working. Thx a lot!!

peach303 · January 2013

thx ar2rsawseen!

ar2rsawseen · January 2013

@peach303

local var = "somevalue" -- this is local
 
function MyClass:init()
    self.somevalue -- this is a property and it is public (thus can be accessed anywhere where instance is)
end

zvardin · January 2013

Anything defined locally in ar2sawseen's example essentially works as a static/constant variable. Anything defined in the case of self.somevalue is a property given to each instance of the class.

OZApps · January 2013

interesting fun when variables get localised

it is easier to use words that can be spelled, so aktiv or active would have been easier/simpler.

just my $0.02 on naming variables

peach303 · January 2013

Thx a lot for the tipps. I have to get used to this local public stuff.

@OZApps: You're right. "aktiviert" is just the german word for "is active" $:\$ ">

I'm really happy to work with gideros now. I used corona before. No forum access and the APK was very slow on device. I've read a lot about this problem with corona. But maybe it was my fault. Anyway gideros is better.

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