Is it good practice to call the following "purge" methods at the beginning of each Scene? And if not when should I call them and is there any tutorial in explaining when to use each call? Am I missing something?
[CCTextureCache purgeSharedTextureCache];
[CCSpriteFrameCache purgeSharedSpriteFrameCache];
[CCAnimationCache purgeSharedAnimationCache];
[CCShaderCache purgeSharedShaderCache];
[[CCFileUtils sharedFileUtils] purgeCachedEntries];
(I am using Cocos2d 2.0 and ARC is enabled, don't think it is relevant but still thought that was worth mentioning)
IMO it's bad practice to purge cocos2d's caches at all! Purging caches is as useful a tool as a hammer is in repairing electronic devices.
It's contraproductive to purge everything only to have the characters reload their purged animations and textures in the next frame because they frickin' need them! If you purge caches unconditionally, you haven't done your asset memory-management job (well).
There can still be uses to purge everything but they should be few and far between (like shutting down cocos2d altogether in a UIKit app) - because why would you ever want to remove all of the cached resources? You remove what you've added, and if that isn't enough, you've done as much as you could.
As an app developer you should be aware of your app's total memory usage and what assets are in use and which aren't. Including the resources handled by cocos2d. Release only those assets you know aren't in use currently and will also not be needed in the near future. It's really that simple, yet it's more work than simply purging caches - which seems to do the job but really is just a terrible thing to do.
One problem with purging caches specifically during memory warnings is that memory warnings may occur when you're currently preloading assets. Now when you purge the caches while you're loading assets, you're shooting yourself in the foot because the already preloaded assets will be removed and then need to be loaded again as soon as they're needed. At worst this can actually cause an unrecoverable memory warning if the reload happens instantly due to the additional memory needed to load assets in the first place (ie textures use 2x memory while loading for a short period of time!).
In most cases purging caches will only delay the memory warning related termination, while adding lag to the game in the meantime. In the remaining cases it will simply make for a bad experience as the game stutters to recover, possibly over a longer period of time.
Cocos2D purges the caches during memory warnings only as a last resort measure, and mainly for developers who won't concern themselves with such nonsense as memory usage. It's a solution that works for taking the first few app development steps, perhaps even for a developer's first app, but is really pretty much useless for any serious/ambitious efforts.
As an ambitious app developer with an eye on quality you should react to memory warnings in a more graceful manner. Meaning you first and foremost work hard to minimize memory warnings altogether (here are some tips) and when they do occur, you need to make sure of two things:
Regarding #2: If you're programming close to the edge, you may want to fall back to some sort of "memory safety" mode where you tune down your app's memory usage altogether, for example by displaying fewer sprites, particles, combining the use of two textures, removing certain textures at the expense of additional loading times, etc.
If you can't free up enough memory with step #2 then #1 will happen almost inevitably, regardless of whether you purge cocos2d's caches or not. Purging the "unused" textures of CCTextureCache can help, but since sprite frames retain the texture it usually doesn't do much (or nothing) if you're using texture atlases without first releasing the corresponding sprite frames. Keep that in mind.
The process for handling a memory warning thus is:
Do not purge cocos2d's caches unconditionally! It's not helping, it will probably only make things worse.
Considering all of the cocos2d caches, 99% of the memory will be retained by CCTextureCache. So it's pretty much pointless to purge any of the other caches anyway, just ignore them.
You really only need to be looking at which texture atlas you're currently using, and those you don't you remove the sprite frames and their textures.
If you're using .pvr.ccz textures to begin with, you can even ignore "caching to reduce load times" altogether and remove from memory every texture atlas whenever you stop using it - because .pvr.ccz load so fast it makes barely any difference as far as switching scenes is concerned. That also helps to avoid memory warnings in the first place.