I have an app that allows me to download characters (images), sounds, etc. When the user chooses which character to download can I store that in a directory in my App Bundle? If not, where is the best place to store my content (data)? I was thinking the documents directory, but then I saw somewhere that Apple now wants data stored in the caches directory.
Thanks for the help.
can I store that in a directory in my App Bundle?
No, you can not modify the app bundle.
If not, where is the best place to store my content (data)?
If your app must support iOS 5.0 or earlier, you will need to store your app data in the Caches directory. However, keep in mind the files could be deleted in low space situations so your app will need to degrade gracefully if your app data is deleted.
As of iOS 5.0.1, you can store your data files in the Documents Directory and flag them as "do not back up". By flagging the files, your app complies with the guideline that apps are responsible for ensuring that only user data and not application data is backed up to iCloud and iTunes and avoids a possible rejection by Apple. In this case, the files will not be deleted in low space situations.
As of iOS 5.1, you can store your data files in the Documents Directory and use either NSURLIsExcludedFromBackupKey or kCFURLIsExcludedFromBackupKey file properties to exclude files from backups. Either of these APIs is preferred over flagging them as "do not back up". In this case, your app complies with the App Store guidelines and the files will not be deleted in low space situations.
The iOS Data Storage Guidelines state:
To ensure that backups are as efficient as possible, be sure to store your app’s data according to the following guidelines:
Only documents and other data that is user-generated, or that cannot otherwise be recreated by your application, should be stored in the /Documents directory and will be automatically backed up by iCloud.
Data that can be downloaded again or regenerated should be stored in the /Library/Caches directory. Examples of files you should put in the Caches directory include database cache files and downloadable content, such as that used by magazine, newspaper, and map applications.
Data that is used only temporarily should be stored in the /tmp directory. Although these files are not backed up to iCloud, remember to delete those files when you are done with them so that they do not continue to consume space on the user’s device.
If you have an OS X or Unix background, it’s easy to understand Apple’s position that such directories have no guarantee as to how long the data in them will persist.
The fact that the tmp directory in in the app’s sandbox is not the same as the root /tmp directory, should not make a difference how you think about this directory.
If you have never observed files being removed from these directories in the past, that is not a guarantee that it will not change in the future. Especially when the change is in accordance with documentation. This is a general rule.
Changes/gaps in Apple’s documentation:
As late as June 29, 2011 Apple’s documentation regarding /Documents said:
Use this directory to store user documents and application data files.
This is pretty clear. No wonder developers are unhappy that the rules for the Documents directory have changed in iOS 5 without any suitable alternative.
Regarding /Library/Caches:
Use this directory to write any application-specific support files that you want to persist between launches of the application or during application updates. Your application is generally responsible for adding and removing these files. It should also be able to re-create these files as needed because iTunes removes them during a full restoration of the device.
App review rejections
Developers are reporting that apps that store any/some/much data in the Documents directory are being rejected by App Review.
It’s unlikely that the App Review team has detailed knowledge of which files are being stored in which directory and which of those are user generated vs. data that can be downloaded again or regenerated. Some developers have reported success in responding to the App Review team with an explanation of how their app is storing data and how that is in accordance with the rules.
What is being backed up by iTunes and iCloud
Everything in the app’s home directory is backed up, with the exception of:
Other documentation clearly states that the Application Support directory is also backed up by iTunes (and presumably iCloud). In the discussions some developers have suggested that Application Support directory would be safer (= more permanent) alternative to Caches. To me it seems that App Review would crack down on large amounts of data stored in Application Support with the same fervor as for the Documents directory, since it’s all about iCloud storage.
Changes in behavior in iOS 5
As of iOS 5, /Library/Caches may be purged while your app is not running if the device experiences a low disk space warning.
There is no longer a directory where your app can store files that are:
Downloaded content should not be stored in the Documents directory according to Apple's Guidelines:
Only documents and other data that is user-generated, or that cannot otherwise be recreated by your application, should be stored in the <Application_Home>/Documents
Use the /Library/Caches directory instead
Data that is used only temporarily should be stored in the <Application_Home>/tmp directory.
