@Override
    public void onTrimMemory(int level) {
        // If an activity is re-created, onTrimMemory may be called before a manager is ever set.
        // See #329.
        if (requestManager != null) {
            requestManager.onTrimMemory(level);
        }
    }
Returns true if the fragment is a descendant of our parent.
core/src/main/java/com/bumptech/glide/manager/RequestManagerFragment.java