Class MutationController

packages/labs/observers/src/mutation-controller.ts:63–167

 * The controller's `value` is usable during the host's update cycle.
 */
export class MutationController<T = unknown> implements ReactiveController {
  private _host: ReactiveControllerHost;
  private _targets = new Set<Element>();
  private _config: MutationObserverInit;
  private _observer!: MutationObserver;
  private _skipInitial = false;
  /**
   * Flag used to help manage calling the `callback` when observe is called
   * in addition to when a mutation occurs. This is done to help setup initial
   * state and is performed async by requesting a host update and calling
   * `handleChanges` once by checking and then resetting this flag.
   */
  private _unobservedUpdate = false;
  /**
   * The result of processing the observer's changes via the `callback`
   * function.
   */
  value?: T;
  /**
   * Function that returns a value processed from the observer's changes.
   * The result is stored in the `value` property.
   */
  callback?: MutationValueCallback<T>;
  constructor(
    host: ReactiveControllerHost & Element,
    {target, config, callback, skipInitial}: MutationControllerConfig<T>
  ) {
    this._host = host;
    // Target defaults to `host` unless explicitly `null`.
    if (target !== null) {
      this._targets.add(target ?? host);
    }
    this._config = config;
    this._skipInitial = skipInitial ?? this._skipInitial;
    this.callback = callback;
    if (isServer) {
      return;
    }
    // Check browser support.
    if (!window.MutationObserver) {
      console.warn(
        `MutationController error: browser does not support MutationObserver.`
      );
      return;
    }
    this._observer = new MutationObserver((records: MutationRecord[]) => {
      this.handleChanges(records);
      this._host.requestUpdate();
    });
    host.addController(this);
  }

  /**
   * Process the observer's changes with the controller's `callback`
   * function to produce a result stored in the `value` property.
   */
  protected handleChanges(records: MutationRecord[]) {
    this.value = this.callback?.(records, this._observer);