public void addCallback(ResourceCallback cb) {
        Util.assertMainThread();
        if (hasResource) {
            cb.onResourceReady(engineResource);
        } else if (hasException) {
            cb.onException(exception);
        } else {
            cbs.add(cb);
        }
    }
A class that manages a load by adding and removing callbacks for for the load and notifying callbacks when the
load completes.
core/src/main/java/com/bumptech/glide/load/engine/EngineJob.java