TopResumedActivityTracker.kt

@HiltAndroidApp
class MyApp : Application(), ImageLoaderFactory {
  @Inject
  internal lateinit var activityTracker: TopResumedActivityTracker

  override fun onCreate() {
    super.onCreate()
    registerActivityLifecycleCallbacks(activityTracker)
  }
}

class RealAuthRepository @Inject constructor(
  private val activityTracker: TopResumedActivityTracker,
) : AuthRepository {
  fun signIn() {
    val activity = activityTracker.topResumedActivity ?: return
    // start other activity for signing in ... (via browser / chrome custom tab intent, eg. Auth0)
  }
}

@Singleton
class TopResumedActivityTracker @Inject constructor(
  appCoroutineScope: AppCoroutineScope,
) : Application.ActivityLifecycleCallbacks {
  @Retention(value = AnnotationRetention.BINARY)
  @Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION, AnnotationTarget.TYPEALIAS, AnnotationTarget.PROPERTY)
  @RequiresOptIn(level = RequiresOptIn.Level.ERROR)
  annotation class Experimental

  /**
   * Keep the top resumed activity and its identity hashcode.
   */
  private val _topResumedActivityStateFlow = MutableStateFlow<Pair<Activity, Int>?>(null)

  init {
    if (BuildConfig.DEBUG) {
      _topResumedActivityStateFlow
        .onEach { Timber.d("[topResumedActivityStateFlow] $it") }
        .launchIn(appCoroutineScope)
    }
  }

  @get:AnyThread
  val topResumedActivity: Activity?
    get() = _topResumedActivityStateFlow.value?.first

  @Experimental
  val topResumedActivityStateFlow: StateFlow<Activity?> =
    _topResumedActivityStateFlow
      .asStateFlow()
      .map { it?.first }
      .stateIn(
        scope = appCoroutineScope,
        started = SharingStarted.Eagerly,
        initialValue = null
      )

  @Suppress("NOTHING_TO_INLINE")
  private inline fun clearTopResumedActivity(activity: Activity) {
    Timber.d("[clearMainActivity] activity=$activity, ref=${_topResumedActivityStateFlow.value}")

    _topResumedActivityStateFlow.update { old ->
      old?.let {
        if (System.identityHashCode(activity) == it.second) {
          null
        } else {
          it
        }
      }
    }
  }

  override fun onActivityCreated(activity: Activity, savedInstanceState: Bundle?) = Unit
  override fun onActivityStarted(activity: Activity) = Unit
  override fun onActivitySaveInstanceState(activity: Activity, outState: Bundle) = Unit

  override fun onActivityResumed(activity: Activity) {
    Timber.d("[onActivityResumed] activity=$activity, ref=${_topResumedActivityStateFlow.value}")

    _topResumedActivityStateFlow.update { it ?: (activity to System.identityHashCode(activity)) }
  }

  override fun onActivityPaused(activity: Activity) = clearTopResumedActivity(activity)
  override fun onActivityStopped(activity: Activity) = clearTopResumedActivity(activity)
  override fun onActivityDestroyed(activity: Activity) = clearTopResumedActivity(activity)
}