How to use the esp-js/src.observeEvent function in esp-js

} else if (currencyPairUpdate.updateType === UpdateType.Removed && _this._workspaceItemsById.hasOwnProperty(key)) {
        let workspaceItem = _this._workspaceItemsById[key];
        delete _this._workspaceItemsById[key];
        
        let removeAtIndex = this.workspaceItems.indexOf(workspaceItem);
        if (removeAtIndex > -1) {
          this.workspaceItems.splice(removeAtIndex, 1);
        }
        // fire an event at the spot tile model telling it that it's been closed
        _this.router.publishEvent(workspaceItem.modelId, 'tileClosed', {});
      }
    });
  }

  @observeEvent('tearOffWorkspaceItem')
  _onTearoff(e:{itemId:string}) {
    _log.debug(`Tearing off workspace item with id [${e.itemId}].`);

  }
}

_referenceDataService:ReferenceDataService;
  _spotTileFactory:SpotTileFactory;
  _workspaceItemsById:Object;
  _isInitialised:boolean;
  workspaceItems:Array;

  constructor(router:Router, referenceDataService:ReferenceDataService, spotTileFactory:SpotTileFactory) {
    super('workspaceModelId', router);
    this._referenceDataService = referenceDataService;
    this._spotTileFactory = spotTileFactory;
    this._workspaceItemsById = {};
    this._isInitialised = false;
    this.workspaceItems = [];
  }

  @observeEvent('init')
  _onInit() {
    let _this = this;
    if (!_this._isInitialised) {
      _this._isInitialised = true;
      _this.addDisposable(
        _this._referenceDataService.getCurrencyPairUpdatesStream().subscribeWithRouter(
          _this.router,
          _this.modelId,
          (referenceData:CurrencyPairUpdates) => {
            _this._processCurrencyPairUpdate(referenceData.currencyPairUpdates);
          }
        )
      );
    }
  }