Function addController

packages/reactive-element/src/reactive-element.ts:1070–1079

* Registers a `ReactiveController` to participate in the element's reactive * update cycle. The element automatically calls into any registered * controllers during its lifecycle callbacks. * * If the element is connected when `addController()` is called, the * controller's `hostConne

(controller: ReactiveController)

   * @category controllers
   */
  addController(controller: ReactiveController) {
    (this.__controllers ??= new Set()).add(controller);
    // If a controller is added after the element has been connected,
    // call hostConnected. Note, re-using existence of `renderRoot` here
    // (which is set in connectedCallback) to avoid the need to track a
    // first connected state.
    if (this.renderRoot !== undefined && this.isConnected) {
      controller.hostConnected?.();
    }
  }

  /**
   * Removes a `ReactiveController` from the element.