iOS 5 does not allow to store downloaded data in Documents directory? ios5.0及以后的版本对于下载的文件存储路径有了改变

I have made an application for my client by keeping target iOS as 4.
But since the application still not submitted to Apple store, my client is planning to upgrade it for iOS 5.0.

For this I read the guideline from Apple and found that "Only user-generated data or that cannot otherwise be recreated by your application, should be stored in the /Documents directory and rest should be stored to /Library/Caches directory"

In my application, I am using server model of in-app purchase for non-consumable product. For this I am storing all my downloaded data (which are basically books or magazines) to Documents directory. The Database is also present in the same directory which contains the details about the downloaded products.

My question is,
1. Should I have to change my code to store the downloaded data to Library/Caches directory instead of to the Documents directory?
2. Where should my database file be placed (to Documents or Caches)?

If I put it products in the Caches then I have to change the logic of retrieval also, since it is considered that if record is present in database, there is no need change the existence of the file and it directly opens it when user clicks on the magazine.

Kindly guide me on this issue.
Thanks in advance.

UPDATED:
I am updating this for those who are still not sure about this problem.
Using the guideline of accepted answer, I have implemented this in 2 of my applications and submitted them to Apple Store. Both were approved in review.
This may promote that the solution suggested in the accepted answer is correct.

Here are the trade-offs:

• If you put your files in the Documents directory then they are backed up to iTunes or iCloud but if they are too big and it's possible to download the files again then Apple may reject your app
• If you put your files in the Cache directory then they won't be backed up and Apple won't reject your app. However, when iOS 5 gets low on space it may delete all the files in there.

However, with iOS 5.0.1 there is a third option:

• Put files in Documents but flag them so that they are not backed up. There's a technote (QA1719) on how to do this.

I think this is probably the best answer for you.

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How do I prevent files from being backed up to iCloud and iTunes?

Technical Q&A QA1719

How do I prevent files from being backed up to iCloud and iTunes?

Q:  My app has a number of files that need to be stored on the device permanently for my app to function properly offline. However, those files do not contain user data and don't need to be backed up. How can I prevent them from being backed up?

A: On iOS, apps are responsible for ensuring that only user data and not application data is backed up to iCloud and iTunes. The exact steps necessary vary between iOS version, so this QA will describe the process for each version of iOS. For more information on exactly what data should or should not be backed up, see the App Backup Best Practices section of the iOS App Programming Guide.

Important: Apps should avoid mingling app data and user data in the same file. Doing so will unnecessarily increase backup sizes and can be considered a violation of the iOS Data Storage Guidelines.

iOS 5.1 and later

Starting in iOS 5.1, apps can use either NSURLIsExcludedFromBackupKey or kCFURLIsExcludedFromBackupKey file properties to exclude files from backups. Either of these APIs is preferred over the older, deprecated approach of directly setting an extended attribute. All apps running on iOS 5.1 should use these APIs to exclude files from backups.

Listing 1  Excluding a File from Backups on iOS 5.1

- (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL

{

    assert([[NSFileManager defaultManager] fileExistsAtPath: [URL path]]);

    NSError *error = nil;

    BOOL success = [URL setResourceValue: [NSNumber numberWithBool: YES]

                                  forKey: NSURLIsExcludedFromBackupKey error: &error];

    if(!success){

        NSLog(@"Error excluding %@ from backup %@", [URL lastPathComponent], error);

    }

    return success;

}

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iOS 5.0.1

If your app must support iOS 5.0.1, you can use the following method to set the "do not back up" extended attribute. Whenever you create a file or folder that should not be backed up, write the data to the file and then call this method, passing in a URL to the file.

Warning: The code that follows has been deprecated and should only be used on iOS 5.0.1 or earlier. When running in iOS 5.1, apps should use the NSURL and CFURL keys described above.

Listing 2  Setting the Extended Attribute on iOS 5.0.1

#import <sys/xattr.h>

- (BOOL)addSkipBackupAttributeToItemAtURL:(NSURL *)URL

{

    assert([[NSFileManager defaultManager] fileExistsAtPath: [URL path]]);

    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;

}

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iOS 5.0

It is not possible to exclude data from backups on iOS 5.0. If your app must support iOS 5.0, then you will need to store your app data in Caches to avoid that data being backed up. iOS will delete your files from the Caches directory when necessary, so your app will need to degrade gracefully if it's data files are deleted.

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Document Revision History

Date	Notes
2012-04-23	Updated for iOS 5.1
2011-11-10	-Fixed critical bug in code snippet.
	New document that describes how an app can prevent files from being backed up to iCloud and iTunes.