In one of my talks at recent NT konferenca I talked about some of the new ways of working with application data in Windows Phone 8.1. With all the exciting additions to the latest Windows Phone 8.1 APIs, the best news around application data in those new APIs is that they are now (mostly) aligned with the Windows Runtime (WinRT), so Windows 8 developers can pick up on them immediately and transition their work to the new platform, share code, etc.
With the aligned data APIs, Windows Phone 8.1 applications (both Silverlight and Runtime) can now store their data into one of the following folders:
- LocalFolder – a local folder, used for storing application data, it’s persistent across application updates and gets backed up to the cloud. This folder was already available in Windows Phone 8 and it’s the same folder as Isolated Storage known from Windows Phone 7.
- TemporaryFolder – also a local folder, but its space is managed by the OS. Whenever the system detects it’s running low on storage space, it will start deleting files in temporary so don’t ever strongly depend on files stored in this folder. It makes a perfect place for storing cached web responses or images that can be recreated any time. This folder does not participate in backups.
- RoamingFolder – files, stored in this folder, will roam across devices, meaning those files will be synchronized over the cloud whenever possible. Perfect for settings and/or small chunks of state (i.e. for continuous clients). When roaming is disabled on the device, those files (and settings) will get backed up as well. The storage space is limited to 100kB – if application goes over that quota, the synchronization stops (until the total usage falls back under roaming storage quota).
But wait - there’s one more folder in Windows Phone 8.1, that’s available in Windows Phone 8.1 API only. Even more, unlike some other APIs that are present on both Windows and Windows Phone and will throw an exception when used from the wrong platform, this one is completely hidden from Windows. Using it from a shared project, you have to #ifdef it out of Windows platform and include in Windows Phone build only:
var localCache = ApplicationData.Current.LocalCacheFolder;
So what is it? Is it used for caching? Well, not exactly (though it might, but I prefer the TemporaryFolder for that).
The LocalCacheFolder is almost exactly like the good old LocalFolder, with one single, but important exception – unlike LocalFoder, the files written to this folder will never be backed up to the cloud and will always stay on the device it was originally written at. Good for storing some (semi) sensitive information or pieces of state you don’t want or need to restore later.
And while I mentioned sensitive data, there’s one more thing worth mentioning - the best place to store sensitive data is into the Credential Locker (represented by the PasswordVault class), a familiar class from WinRT APIs, that is now also available to Windows Phone 8.1 – YAY!
To finish up, here’s a quick recap – use PasswordVault for storing sensitive information, LocalCacheFolder for data you don’t want ever to leave that particular phone, TemporaryFolder for temporary, easy-to-recreate data, RoamingFolder for settings and small state to synchronize across devices, and LocalFolder for main application data that gets backed up to the cloud (and, of course – restored).