Class inheritance and memory

gianmichele · July 2012

Hi all, I have a quick question: I have a class (Class A) that creates three sprite objects through another class (Class

.

How should I approach memory cleaning when I instance Class A in my game loop but then delete all of the children of Class A manually?

zvardin · July 2012

If you're worried about niling out all the sprites within Class A, you could create a destroy function that you call to remove it. This way within that function you could do any necessary cleanup.

gianmichele · July 2012

Better if I post something:

item.lua

Bello = Core.class(Sprite)
 
function Bello:init(texture)
 
	local bitmap = Bitmap.new(Texture.new(texture))
	bitmap:setAnchorPoint(0.5, 0.5) -- center the pivot point
	self:addChild(bitmap) -- aggiunge la bitmap all'oggetto cosi da poterlo manipolare con self
	self.isFocus = false
 
	self:addEventListener(Event.MOUSE_DOWN, self.onMouseDown, self)
	self:addEventListener(Event.MOUSE_MOVE, self.onMouseMove, self)
	self:addEventListener(Event.MOUSE_UP, self.onMouseUp, self)
end
 
function Bello:onMouseDown(event)
	if self:hitTestPoint(event.x, event.y) then
		self.isFocus = true
 
		self.x0 = event.x
		self.y0 = event.y
 
		print("Touched!")
 
		event:stopPropagation()
	end
end
 
function Bello:onMouseMove(event)
	if self.isFocus then
		local dx = event.x - self.x0
		local dy = event.y - self.y0
 
		self:setX(self:getX() + dx)
		self:setY(self:getY() + dy)
 
		self.x0 = event.x
		self.y0 = event.y
 
		event:stopPropagation()
	end
end
 
function Bello:onMouseUp(event)
	if self.isFocus then
		self.isFocus = false
		event:stopPropagation()
		print("Touch is ended")
	end
end

mechanic.lua

 
-- Multi elements + death timer
 
Trio = Core.class(Sprite)
 
function Trio:init(texture, n)
 
	self.elements = n
	self.isAlive = true
 
	for j = 1,n do
		gameobj[j] = Bello.new(texture)
		gameobj[j]:setPosition(math.random(20,420), math.random(20, 320))
		self:addChild(gameobj[j])
	end
 
	local timer = Timer.new(5000, 1)
	local function onTimerComplete(event)
		print("Timer has ended!")
 
		for j = self:getNumChildren(), 1,-1 do
			self:removeChildAt(j)
		end
 
		self.isAlive = false 
		print(self.isAlive)
		print(self:getNumChildren())
 
	end
	timer:addEventListener(Event.TIMER_COMPLETE, onTimerComplete)
	timer:start()
 
	print(self:getNumChildren())
 
end

main.lua

 
-- Test meccanica di gioco
 
gameStage = Sprite.new() -- Game group
 
 
gameobj = {} -- game objects
 
 
myMech = Trio.new("images/penguin.png", 3)
gameStage:addChild(myMech)
 
 
stage:addChild(gameStage)

zvardin · July 2012

I'm not really sure to this extent would be necessary, but to show the concept of what I meant with a destroy function to be used as cleanup:

function Bello:destroy()
        local bitmap = self:getChildAt(0) -- Since it seems to only have one child
        self:removeChild(bitmap)
        bitmap = nil
	self.isFocus = nil
 
	self:removeEventListener(Event.MOUSE_DOWN, self.onMouseDown, self)
	self:removeEventListener(Event.MOUSE_MOVE, self.onMouseMove, self)
	self:removeEventListener(Event.MOUSE_UP, self.onMouseUp, self)
 
end

And then in your Trio class, you could change the one loop to add in a destroy call before removing them:

local function onTimerComplete(event)
		print("Timer has ended!")
 
		for j = self:getNumChildren(), 1,-1 do
                        self:getChildAt(j):destroy()
			self:removeChildAt(j)
		end
 
		self.isAlive = false 
		print(self.isAlive)
		print(self:getNumChildren())
 
	end

ar2rsawseen · July 2012

Hello,

Here is the way I understand it

Let's look at this hierarchy:

Rectangles representing objects and arrows representing reference.

As you may know, when object has no references it can be garbage collected.

So let's see:

If you remove gameStage from stage, then gameStage does not have any references left to it, so it will be garbage collected.

stage:removeChild(gameStage)

Variable myTech still will be referencing to Trio instance, and since myTech is aglobal object (I didn't see any definition of local myTech) thus Trio will remain in the memory and everything else also.

So next step, you nil out the myTech variable

myTech = nil

And here goes the domino effect:

It means there are no references to Trio instance, so it will also be garbage collected.

Which means that there are no references left to all Bello instances, so they also will be garbage collected.
Same goes to bitmap. We had a local variable inside Bello class and we also added same bitmap as a child of Bello instance. You could think that it's two references, but, when Bello instance is garbage collected, local variable bitmap will also be removed automatically.

That is why it's important to use local variables and not global ones, unless you know what you're doing

So all that is left is a Bitmap instance without parent (Bello was removed), which means there are no references to it, which means it will also be garbage collected

So basically minimum what you need to do is to remove gameStage from stage and nil out the myTech variable

Hope that clears something up

And please correct me if I'm wrong

P.S hopefully that will make a nice post for my 600th forum post

gianmichele · July 2012

Thanks everyone, this makes perfectly sense now (and you know, I'm an animator not a programmer!)

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Here is the way I understand it