Fixes [CSDK-16].
See
https://docs.google.com/document/d/1DDyed98RFpIvUwUPJcetc7X4fBhDzf-r1trC4g3zf4c/edit#
for reasoning and documentation.
### Motivation
The RevenueCat SDK has always used `UserDefaults.standard` as the
default value. The advantage of this was simplicity, however, this
brings a few challenges.
### Issues
- While `UserDefaults` is itself thread-safe, our users might be
observing `UserDefaults.defaults` changes via `KVO`,
`NotificationCenter`, or `@AppStorage` (see
https://github.com/RevenueCat/purchases-ios/issues/1527). We update
`UserDefaults` from background threads, and users might be getting
updates on those threads without handling thread-safety correctly.
- Users can accidentally delete data from the shared `UserDefaults` (see
https://rev.cat/userdefaults-crash).
Using a custom `UserDefaults` solves all these issues.
### Change
The new `UserDefaults.revenueCatSuite` is used when:
- Only if users aren’t overriding the `UserDefaults` parameter
- Only if `UserDefaults.standard` has no value for
`com.revenuecat.userdefaults.appUserID.new` (the user hasn't used the
SDK in that app before).
#### Logged during app start:
> PurchaseTester[35110:6323140] DEBUG: ℹ️ Configuring SDK using
RevenueCat's UserDefaults suite.
### Alternatives Considered
See
https://docs.google.com/document/d/1c2B4MNAFKs_DcFJ3OE_KnmryHCArBkjlZbAD9_kSXu0/edit#
We considered also migrating data for everyone into the new
`UserDefaults`.
This new approach is much simpler, less error prone, easier to
implement, and maintain.
The only downside is that existing users won’t benefit from this change,
but @aboedo correctly pointed out: if the app is working for them, no
reason to change anything.
So in this way, this becomes a purely forward-looking improvement.
[CSDK-16]:
https://revenuecats.atlassian.net/browse/CSDK-16?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
