Emergency communication system
According to early reports, the suspect first went into the theater as a paying customer. He then exited and held open an emergency exit, while he went to his car to get his armor and weapons. He then attacked after coming back in through the open door.  With different systems, this could have provided a warning.  Technology exists that can warn about open doors (such as an emergency door that should not normally be open), and can send clear warning and instructions to any types of device, including a movie screen projector.