How to use pubsub notifications for cloud storage to trigger dataflow pipeline

I'm trying to integrate a Google Cloud Dataflow pipeline with Google Cloud Pub/Sub Notifications for Google Cloud Storage. The idea is start processing a file as soon it is created. The messages are being published and with PubsubIO.readMessagesWithAttributes() source I manage to extract the file URI:

Pipeline p = Pipeline.create(options);
PCollection<String> uris = p.apply(PubsubIO.readMessagesWithAttributes()
            .withTimestampAttribute(PUBSUB_TIMESTAMP_LABEL_KEY)
            .fromSubscription(options.getPubsubSubscription()))
            .apply(MapElements
                    .into(TypeDescriptors.strings())
                    .via((PubsubMessage msg) -> {
                        String bucket = msg.getAttribute("bucketId");
                        String object = msg.getAttribute("objectId");
                        GcsPath uri = GcsPath.fromComponents(bucket, object);
                        return uri.toString();
                    }));

Which PTransform can be used to start reading/processing each file in the uris PCollection?