Question

In my iPhone app I’m caching the raw data of some compressed files to save loading time. But those files may change in an update.

Will iOS clear /Library/Caches for me when the app is updated or I need to clear it myself?

Answer 1

Short:

Will iOS clear /Library/Caches for me when the app is updated
No

Is it possible that iOS does clear everything or parts of Application_Home/Library/Caches during an update? Yes

or I need to clear it myself? You need to ensure what you want to be cleared is cleared.

Be prepared for both situations.

How do you know whether your app got updated? See iOS Application Update Test

Long:

Files Saved During Application Updates When a user downloads an
application update, iTunes installs the update in a new application
directory. It then moves the user’s data files from the old
installation over to the new application directory before deleting the
old installation. Files in the following directories are guaranteed to
be preserved during the update process:

• Application_Home/Documents
• Application_Home/Library

Although files in other user directories may also be moved over, you
should not rely on them being present after an update.

From Apples Documentation: The Application Runtime Environment – Backup and Restore

Application_Home/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.
In iOS 2.2 and later, the contents of this directory are not backed up
by iTunes.

From Apples Documentation: The Application Runtime Environment – The File System (daveoncode)

Here an example of when it does clear the cache during an “update”:
You install app X V1.0. Lunch it once and the start doing something else. iOS malfunctions and needs to be restored. Developer releases V1.1. You update the app in iTunes on your Mac. You restore your device via iTunes. iTunes installs V1.1. User lunches your app. Your app does not get notified that any of that stuff happened but all files in Application_Home/Library/Cache are gone (iTunes removes them during a full restoration of the device).

Really important information in there: It should also be able to re-create these files as needed. Like Kendall also pointed out it is a cache, so files could be deleted from it by the operating system at any point. So every time you load any file from Application_Home/Library/Caches you have to account for the case that the file isn’t there anymore (Kendall touched on this).

NSFileManager *fileManager = [NSFileManager defaultManager];
NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) objectAtIndex:0];
if(![fileManager fileExistsAtPath:[cachePath stringByAppendingPathComponent:@"myfile.zip"]]) {
    //create it, copy it from app bundle, download it etc.
}
//start using it