I’m finding out that Lua is not as forgiving a language as I
had first thought. These were the changes
I made in level0 to get rid of the various memory leaks I had created.
Destroying
Objects
This was the most obvious thing to do, and so became the
first order of business. We have been
inserting objects into a self.view group – and that order of removal is a very
important thing in Lua.
Using this article on removing
objects properly as a guide I found a number of things. First just removing an object from a group isn’t
enough for the garbage collector – the reference the variable is pointing to
must also get removed. So there are two
steps to destroying an object – first is to remove it from a group by calling
the group:remove( object name or index ) or object:removeSelf() methods. Then you have to set the object = nil to
remove the reference.
But wait – there’s more!
The order of removal from a group is also important. In fact a common pitfall is to iterate thru a
group in a normal forware fashion – which may in fact skip every other item in the
group. I’m going to copy and paste the
relevant section from the above link here:
Common Pitfalls
A common mistake is to improperly remove
all objects from a group. This typically happens when you write code that
iterates through a group attempting to remove each child from the display
hierarchy. It’s natural to iterate through the group in the forward
direction. However, this can cause no end of confusion.
Continuing with our solar system example,
consider the following where we attempt (incorrectly) to remove all the objects
from the solar system.
for i=1,solarSystem.numChildren do
local child = solarSystem[i]
child.parent:remove( child )
end
The problem here is that we are modifying a
collection (i.e. the group’s children array) as we iterate through that same
collection. The result is we remove every other child. The easiest way to illustrate
this is with a parallel example involving an array of integers:
local array = {1,2,3,4,5,6,7,8,9,10}
print( table.concat( array, " " )
) --> 1 2 3 4 5 6 7 8 9 10
for i=1,#array do
table.remove( array, i )
end
print( table.concat( array, " " )
) --> 2 4 6 8 10
The fix is to iterate backwards.
for i=solarSystem.numChildren,1,-1 do
local child = solarSystem[i]
child.parent:remove( child )
end
Of course, this only ensures that all
children have been removed from the display hierarchy; you till have to set all
references to these display objects to nil. So in this example, we were
merely trying to illustrate the highlight the pitfalls of iterating forward
through the children of a group and modifying the group at the same time. A
good implementation would also set the corresponding properties in solarSystem
to nil for proper cleanup.
So as part of my exitScene function I’ve added the
following:
for i=screenGroup.numChildren,1,-1 do
local child = screenGroup[i]
child.parent:remove( child )
end
Then I do this:
screenGroup:removeSelf()
And then set the various objects equal to nil.
We aren’t done yet tho…
Forward Referencing
This part comes from reading this handy article on The
Significance of Local Variables in Lua.
We’ve all seen this:
local myFunction = function()
callback() -- ERROR: function 'callback' does not exist
yet
end
local callback()
print( "Hello World" )
end
myFunction() -- will produce error
Refactored
this way, the code works:
local callback -- forward reference (for later use)
local myFunction = function()
callback() -- output: "Hello World"
end
-- function below is still local,
because of forward reference
callback = function()
print( "Hello World" )
end
myFunction() -- no error; happy code :-)
So I took
every function that was not part of the scene template and moved it above function scene:createScene( event ). This took care of a few ambiguous runtime
errors. This includes the addListeners()
and removeListeners functions.
At this
point things were supposed to be better, but I started to get screen freezes –
holy cow things are getting worse!
Runtime Listeners
I next turned to this handy guide, Corona
SDK Memory Leak Prevention 101. And
found this handy scrap if information:
When you remove a display object, listeners
that are attached to it are also freed from memory. However, when you add
listeners to Runtime (e.g. enterFrame listeners), those never get freed until
you manually remove them.
A common leak that occurs with
Runtime/enterFrame listeners is when a developer adds a Runtime listener to the
particular screen they’re in, but they forget to remove the event listener
whenever the user leaves the screen. So what happens is, the user leaves the screen,
and then when they come back to it, there are two identical Runtime
listeners running on top of one another.
Now, we do add a Runtime enterFrame
listener with the gameloop code. I make
extra sure that the removeListeners function has the Runtime:removeEventListener('enterFrame', gameloop ) and that it
gets called.
Timers and Transitions
Again back to this handy guide, Corona
SDK Memory Leak Prevention 101. I’m
just going to copy and paste from the guide:
Timers and Transitions are probably one of
the most common causes of crashes, mostly due to the fact that countless
(unaccounted for) things could happen between the time the timer/transition
starts, and the time it ends. What’s worse, if you forget to cancel these and nil out
their variables when no longer needed, things could get stuck in memory and
cause more leaks.
One way you could manage these is to store
ALL of your timers and transitions in a table, so that when you know for sure
no timers or transitions should be running, you can cancel all of them at once.
If you add the following code to your
main.lua file (or another module and require it in), you can easily keep track
of your timers/transitions and also cancel them all at once if and when needed:
timerStash = {}
transitionStash = {}
function cancelAllTimers()
local k, v
for k,v in pairs(timerStash) do
timer.cancel( v )
v = nil; k = nil
end
timerStash = nil
timerStash = {}
end
--
function cancelAllTransitions()
local k, v
for k,v in pairs(transitionStash) do
transition.cancel( v )
v = nil; k = nil
end
transitionStash = nil
transitionStash = {}
end
Then, whenever you start a new timer or transition, you assign it to the
respective table first, like so:
timerStash.newTimer = timer.performWithDelay( ...
transitionStash.newTransition = transition.to( myObject { ...
Then you can use the cancelAllTimers() and
cancelAllTransitions() functions to stop all timers and transitions all at
once. If you have timers you don’t want to be cancelled, then don’t add them to
the “stash” tables in the first place (but be sure not to forget to keep track
of them).
So I added the above cancel functions to level0, and then
call them after the removeListeners call.
I found I could do this in the gameloop function and it would work:
transitionStash.newTransition1
= bg1:translate(0,2)
transitionStash.newTransition2
= bg2:translate(0,2)
Things are better but I still was getting some runtime errors
– not as many, and no crashes but still annoying.
Proper Placement of Cleanup Code
I was experimenting with the right place to put all this
garbage collection code – and in desperation decided to go back to the
documentation. One last link – storyboard.gotoScene():
Description:
Used to transition to a specific scene
(determined by scene's module name). Internally, this will handle all
loading/unloading of the scenes. Scenes may also be explicitly unloaded/removed
by the user.
When this function is called, the
"exitScene" event will be dispatched for the currently loaded scene
(if one exists), followed by the "createScene" event (for the
specified scene—sceneName—if no "view" property is present), and then
the "enterScene" event (also for the specified scene—sceneName).
It dawned on me – keep the code in the exitScene and it
would always get called in correct order.
At this point I was about 99.5% there.
Zombie Scenes
Yes, the dead still inhabited the program. I was seeing calls to level0 when in level2 –
no crashes but still getting some runtime errors. So I found this reference to storyboard.purgeAll(). In cutscene1, the scene:createScene( event )function
I replaced the call to storyboard.purgeScene( priorScene ) with storyboard.purgeAll(). Inside the Lua command box I was seeing calls
to scene:destroyScene( event ) in both level0 AND train3. More importantly it got rid of the last
runtime errors.
