public static Scheduler initMainThreadScheduler(Callable<Scheduler> scheduler) {
        if (scheduler == null) {
            throw new NullPointerException("scheduler == null");
        }
        Function<Callable<Scheduler>, Scheduler> f = onInitMainThreadHandler;
        if (f == null) {
            return callRequireNonNull(scheduler);
        }
        return applyRequireNonNull(f, scheduler);
    }
Utility class to inject handlers to certain standard RxAndroid operations.
android/android_framework/base/src/main/java/rx2/android/plugins/RxAndroidPlugins.java