I use iphone UUID for checking user's device is unique.
I already know UUID will be changed when reinstall app.
But, I have a question. Then, when app updated, UUID for my app will be changed?
If UUID changed when app updated, where can I save UUID in iphone permanently(or same effect)?
Save out your created UUID into user preferences for the app, which should be backed up - if they restore the app from a backup you'll get back the UUID.
If you want to be even more sure you can get back the same UUID, save the generated UUID into the keychain - the keychain is kept intact even after you delete an app (not sure if that's guaranteed forever but it is the effect).
UDID doesn't change,but it's deprecated. Device ID might change. One can use MAC address if wifi is on, which is pretty much given.
来源：https://stackoverflow.com/questions/14699981/iphone-uuid-change-when-app-updated