iCloud + Storage of media in iPhone Documents folder

Question

I, like many developers, got an email from Apple recently that stated we should move our data from the documents directory into another folder to permit more streamlined backup to iCloud.

In recent testing it appears that [your app] stores a fair amount of data in its Documents folder.

Since iCloud backups are performed daily over Wi-Fi for each user's iOS device, it's important to ensure the best possible user experience by minimizing the amount of data being stored by your app.

Marco Arment, of instapaper fame, has a good take on the issue, which is that the recommended location for storing downloadable files is in /Library/Caches. However, the problem is that both /tmp and /Caches can be 'cleaned' anytime the OS decides that the device is running low on storage. If your app is cleaned then the data downloaded by your app and stored by your user is gone. Naturally, the user will blame you and not Apple.

What to do?

Answer 1

iOS 5.0.1 introduced a flag to address this issue:

https://developer.apple.com/library/ios/#qa/qa1719/_index.html

Their recommendation is to create a folder in /Library/ like /Library/PrivateDocs , and put your files there. However you will also have to set a "do not backup" flag on them as every file in /Library except for those in /Library/Cache or tmp are backed up by default. Set the flag on PrivateDocs folder with this command:

#include <sys/xattr.h>
- (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL
{
    const char* filePath = [[URL path] fileSystemRepresentation];

    const char* attrName = "com.apple.MobileBackup";
    u_int8_t attrValue = 1;

    int result = setxattr(filePath, attrName, &attrValue, sizeof(attrValue), 0, 0);
    return result == 0;
}