A Japanese company by the name of Elecom is one of the first out the gate to introduce a projection keyboard to market. The TK-PBL042BK wireless projection keyboard simply requires nothing more than a flat surface and its host to carry either USB or Bluetooth connectivity, which means iOS and Android will natively support it.
When connecting the device to a host over USB it will operate as a USB HID keyboard while simultaneously getting a battery recharge.
The device itself measures only 38.0 x 29.0 x 75.0 mm and weighs in at a mere 77g without the rechargeable battery (said to provide up to two hours of continuous use).