ENTER_FRAME listener and garbage collection

bowerandy · September 2012

Can I just get confirmation that including a listener for the ENTER_FRAME event in a class init() method will not prevent the object from being collected if no other references exist? e.g:

function MyClass:init()
    self:addEventListener(Event.ENTER_FRAME, self.onEnterFrame, self)
end
 
function MyClass:onEnterFrame()
end
 
MyClass.new()

Will that instance get collected correctly without first removing the listener.

best regards

atilim · September 2012

Yes it'll be collected. Here is an example based on yours:

MyClass = Core.class(Sprite)
 
function MyClass:init()
	self.frame = 0
	self:addEventListener(Event.ENTER_FRAME, self.onEnterFrame, self)
end
 
function MyClass:onEnterFrame()
	self.frame = self.frame + 1
	if self.frame == 60 then
		print("tick")
		self.frame = 0
	end
 
end
 
MyClass.new()
 
stage:addEventListener("mouseDown", function() 
	collectgarbage()
end)

Here this example prints "tick" at every 60th frame until you press the mouse.

bowerandy · September 2012

@atilim, thanks. I guess I should have thought about testing it like that with a print statement. Sorry to trouble you.

atilim · September 2012

np

Also, you can use unofficial newproxy function to detect when a Lua table is collected:

MyClass = Core.class(Sprite)
 
function MyClass:init()
	self.proxy = newproxy(true)
	getmetatable(self.proxy).__gc = function() print("collected!") end
 
	self.frame = 0
	self:addEventListener(Event.ENTER_FRAME, self.onEnterFrame, self)
end
 
function MyClass:onEnterFrame()
	self.frame = self.frame + 1
	if self.frame == 60 then
		print("tick")
		self.frame = 0
	end
 
end
 
MyClass.new()
 
stage:addEventListener("mouseDown", function() 
	collectgarbage()
end)

bowerandy · September 2012

Hmm.. now that could be very useful, thanks.

john26 · September 2012

@atilim I tried your two examples but the "ticks" do not stop when I press the mouse. I am using an older version, however, 2012.2.2.2.

atilim · September 2012

Yes It was fixed with 2012.08.2. (In release notes, it's mentioned as Bug: Event objects shouldn't store target object internally (late GC of target object))

For example, if you test this code:

MyClass = Core.class(Sprite)
 
function MyClass:init()
	self.proxy = newproxy(true)
	getmetatable(self.proxy).__gc = function() print("collected!") end
 
	self:addEventListener(Event.ENTER_FRAME, self.onEnterFrame, self)
end
 
function MyClass:onEnterFrame()
end
 
-- create 4 objects
MyClass.new()
MyClass.new()
MyClass.new()
MyClass.new()
 
stage:addEventListener("mouseDown", function() 
	collectgarbage()
end)

With 2012.2.2.2, you see 3 objects are GCed, but with latest version all 4 objects are GCed.

bowerandy · September 2012

@atilim, yes I remembered there being some problem like this - hence the paranoid original question.

best regards

john26 · September 2012

@atilim Thanks for your reply! So the bug that previously existed, is it a serious memory leak or just that collection is late? eg:

function doNothing()
end
 
foo=Bitmap.new(Texture.new("foo.png"))
foo:addEventListener(Event.MOUSE_DOWN,doNothing)
stage:addChild(foo)
 
-- destroy level
foo:removeFromParent()                            
 
--start new level
foo=Bitmap.new(Texture.new("bar.png"))   -- garbage collected?

will the old sprite be elligible for GC on "foo=Bitmap.new()" command?

bowerandy · September 2012

@john26, I think in that older version it was a genuine leak. The recommendation back then was to ensure that ENTER_FRAME handlers were removed when an object was taken off the stage. Hence, I always used to use ADDED_TO_STAGE and REMOVED_FROM_STAGE handlers to install/remove the ENTER_FRAME handlers (if that makes sense).

Anyway, as @atilim has confirmed the bug has been fixed in the latest download so there should be no need to jump through these hoops now.

best regards

atilim · September 2012

Also, I think it's not very serious. Only 1 sprite (the last one) remains uncollected.

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