allow anonymous uuid generation to be customizable

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

@@ -199,7 +199,10 @@ public class PostHog private constructor(
             synchronized(anonymousLock) {
                 anonymousId = getPreferences().getValue(ANONYMOUS_ID) as? String
                 if (anonymousId.isNullOrBlank()) {
-                    anonymousId = UUID.randomUUID().toString()
+                    var uuid = UUID.randomUUID()
+                    // when getDeviceId method is available, pass-through the value
+                    config?.getDeviceId?.let { uuid = it(uuid) }
+                    anonymousId = uuid.toString()
                     this.anonymousId = anonymousId ?: ""
                 }
             }

posthog/src/main/java/com/posthog/PostHogConfig.kt

@@ -8,6 +8,7 @@ import com.posthog.internal.PostHogNetworkStatus
 import com.posthog.internal.PostHogPreferences
 import com.posthog.internal.PostHogPrintLogger
 import com.posthog.internal.PostHogSerializer
+import java.util.UUID
 
 /**
  * The SDK Config
@@ -94,6 +95,11 @@ public open class PostHogConfig(
      * The hook is called before the event is cached or sent over the wire
      */
     public var propertiesSanitizer: PostHogPropertiesSanitizer? = null,
+    /**
+     * Hook that allows for modification of the default mechanism for
+     * generating anonymous device id (which as of now is just random UUID v4)
+     */
+    public var getDeviceId: ((UUID) -> UUID) = { it },
 ) {
     // fix me: https://stackoverflow.com/questions/53866865/leaking-this-in-constructor-warning-should-apply-to-final-classes-as-well-as
     @PostHogInternal

posthog/src/test/java/com/posthog/PostHogTest.kt

@@ -10,11 +10,13 @@ import okhttp3.mockwebserver.MockResponse
 import org.junit.Rule
 import org.junit.rules.TemporaryFolder
 import java.io.File
+import java.util.UUID
 import java.util.concurrent.Executors
 import kotlin.test.AfterTest
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertFalse
+import kotlin.test.assertNotEquals
 import kotlin.test.assertNotNull
 import kotlin.test.assertNull
 import kotlin.test.assertTrue
@@ -852,6 +854,18 @@ internal class PostHogTest {
         sut.close()
     }
 
+    @Test
+    fun `allows for modification of the uuid generation mechanism`() {
+        val expected = UUID.randomUUID()
+        val config = PostHogConfig(API_KEY, getDeviceId = {
+            assertNotEquals(it, expected, "Expect two unique UUIDs")
+            expected
+        })
+        // now generate an event
+        val sut = PostHog.with(config)
+        assertEquals(expected.toString(), sut.distinctId(), "It should use the injected uuid instead")
+    }
+
     @Test
     fun `enables debug mode`() {
         val config = PostHogConfig(API_KEY)