allow optional reuse of anonymous id

[beradeep]

💡 Motivation and Context

Fix #224

💚 How did you test it?

Added unit test.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[marandaneto]

@beradeep thanks
i will review later this week

[marandaneto]

I dont think this will work.

posthog-android/posthog/src/main/java/com/posthog/PostHog.kt

Lines 552 to 557 in 6185d73

	val anonymousId = this.anonymousId
	if (anonymousId.isNotBlank()) {
	props["\$anon_distinct_id"] = anonymousId
	} else {
	config?.logger?.log("identify called with invalid anonymousId: $anonymousId.")
	}

once you identify an user, the given distinctId and the anonymousId are merged within a single user.

if you reset and keep the anonymousId, once you identify another user, now the older distinctId, the anonymousId and the new distinctId will also be merged within the same user.

Otherwise there will never be a "guest mode".
I think the approach we should take is that there's a specific anonymousId guest mode, similar to a$device_id actually, its always the same ID when the user is logged out.

for example:

val guestId = 123
// this is also the current anonymousId
posthog.identify(456)
// this should not merge user 123 and 456 (but it is), so we should not set `props["\$anon_distinct_id"] = anonymousId`
nor pass `featureFlags?.loadFeatureFlags(distinctId, anonymousId = anonymousId, groups, onFeatureFlags)` if `reuseAnonymousId` is enabled.
posthog.reset()
// now yes we don't delete the cached anonymousId, this is correct.
posthog.identify(789)

with this approach, the anonymousId will never be merged with any user, and it will always be the "guest user".

does that make sense?

[beradeep]

Yes, it does make sense. I missed out on that part; we shouldn't merge all users on a device. Thanks for such a nice explanation.

[marandaneto]

@beradeep i think we should not allow this as well

posthog-android/posthog/src/main/java/com/posthog/PostHog.kt

Line 577 in 6185d73

this.anonymousId = previousDistinctId

when the new flag is enabled, otherwise we will lose the anon id (guest mode).

[marandaneto]

@beradeep looks good, i added a more detailed explanation here
can you add unit test for the last changes? right now we are only testing the reset changes.

[marandaneto]

missing 2 tests otherwise LGTM