Class PerformanceController

packages/labs/observers/src/performance-controller.ts:54–164

 * The controller's `value` is usable during the host's update cycle.
 */
export class PerformanceController<T = unknown> implements ReactiveController {
  private _host: ReactiveControllerHost;
  private _config: PerformanceObserverInit;
  private _observer!: PerformanceObserver;
  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?: PerformanceValueCallback<T>;
  constructor(
    host: ReactiveControllerHost,
    {config, callback, skipInitial}: PerformanceControllerConfig<T>
  ) {
    this._host = host;
    this._config = config;
    this._skipInitial = skipInitial ?? this._skipInitial;
    this.callback = callback;
    if (isServer) {
      return;
    }
    // Check browser support.
    if (!window.PerformanceObserver) {
      console.warn(
        `PerformanceController error: browser does not support PerformanceObserver.`
      );
      return;
    }
    this._observer = new PerformanceObserver(
      (entryList: PerformanceObserverEntryList) => {
        this.handleChanges(entryList.getEntries(), entryList);
        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(
    entries: PerformanceEntryList,
    entryList?: PerformanceObserverEntryList
  ) {
    this.value = this.callback?.(entries, this._observer, entryList);