setState = ({mirrorOffset, initialX, initialY, ...args}) => {
      this.mirrorOffset = mirrorOffset;
      this.initialX = initialX;
      this.initialY = initialY;
      this.lastMovedX = initialX;
      this.lastMovedY = initialY;
      return {mirrorOffset, initialX, initialY, ...args};
    }
Mirror created handler
@param {MirrorCreatedEvent} mirrorEvent
@return {Promise}
@private