onPinch(event) {
    const pointer = this.getPointer(event.center);

    this.drag.pinched = true;
    if (this.pinch["scale"] === undefined) {
      this.pinch.scale = 1;
    }

    // TODO: enabled moving while pinching?
    const scale = this.pinch.scale * event.scale;
    this.zoom(scale, pointer);
  }
Handle pinch event
@param {Event}  event   The event
@private