Best place for save image downloaded from web in iOS devices?

Question

My application download images from web, and for obviously performance reason i need to cache it on my devices. So my question is: where is the best place for store that pictures? I saw a lot of people use NSDocumentDirectory but apple say:

Critical data should be stored in the /Documents directory. Critical data is any data that cannot be recreated by your app, such as user documents and other user-generated content.

so i think the image are not in this category. apple also say:

Support files include files your application downloads or generates and that your application can recreate as needed. The location for storing your application’s support files depends on the current iOS version.

so maybe the NSApplicationSupportDirectory is the best place for store it.

but i saw also NSDownloadsDirectory and NSPicturesDirectory. The cache directory is not good for me because it is deleted after app update. All of this stuff make me a lot of confusion. What is the right place and why?

Answer 1

Good question! Since you don't want to use NSCachesDirectory, NSApplicationSupportDirectory would be a better option, I think.

According to Apple's File System Programming Guide:

Where You Should Put Your App’s Files

To prevent the syncing and backup processes on iOS devices from taking a long time, be selective about where you place files inside your app’s home directory. Apps that store large files can slow down the process of backing up to iTunes or iCloud. These apps can also consume a large amount of a user's available storage, which may encourage the user to delete the app or disable backup of that app's data to iCloud. With this in mind, you should store app data according to the following guidelines:

• Put user data in the <Application_Home>/Documents/. User data is any data that cannot be recreated by your app, such as user documents and other user-generated content.

• Handle support files—files your application downloads or generates and can recreate as needed—in one of two ways:

  • In iOS 5.0 and earlier, put support files in the <Application_Home>/Library/Caches directory to prevent them from being backed up

  • In iOS 5.0.1 and later, put support files in the <Application_Home>/Library/Application Support directory and apply the com.apple.MobileBackup extended attribute to them. This attribute prevents the files from being backed up to iTunes or iCloud. If you have a large number of support files, you may store them in a custom subdirectory and apply the extended attribute to just the directory.

• Put data cache files in the <Application_Home>/Library/Caches directory. Examples of files you should put in this directory include (but are not limited to) database cache files and downloadable content, such as that used by magazine, newspaper, and map apps. Your app should be able to gracefully handle situations where cached data is deleted by the system to free up disk space.

• Put temporary data in the <Application_Home>/tmp directory. Temporary data comprises any data that you do not need to persist for an extended period of time. 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.

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NSApplicationSupportDirectory: Location of application support files (Library/Application Support).

NSDownloadsDirectory: Location of the user’s downloads directory.

NSPicturesDirectory: Location of user's Pictures directory (~/Pictures)

My understanding of the question is that the user won't deal with the photos you're caching. So they're the files that are supporting your app, rather than user's photos or downloads. Those folders would be suitable to use when let's say you had a download manager app where you categorized user's downloaded files into pictures, music, videos, etc.