public static void registerLottieTaskIdleListener(LottieTaskIdleListener listener) {
    taskIdleListeners.add(listener);
    listener.onIdleChanged(taskCache.size() == 0);
  }
Use this to register a callback for when the composition factory is idle or not.
This can be used to provide data to an espresso idling resource.
Refer to FragmentVisibilityTests and its LottieIdlingResource in the Lottie repo for
an example.
lottie/src/main/java/com/airbnb/lottie/LottieCompositionFactory.java