public function register() {
		/*
		 * Look up and set the value in the container request cache to allow building the same Controller
		 * with a **different** container. (e.g. in tests).
		 */
		if ( static::is_registered() ) {
			return;
		}

		// Register the controller as a singleton in the container.
		// @todo remove when the Container is updated to bind Providers as singletons by default.
		$this->container->singleton( static::class, $this );

		if ( ! $this->is_active() ) {
			return;
		}

		$this->container->setVar( static::class . '_registered', true );

		$this->do_register();
	}