If the proximity requirement is on the order of a few centimeters, then NFC is a good choice. NFC is still a new technology adopted by mobile phones, so the current installation base is limited.
If the proximity requirement is on the order of a few feet, Bluetooth is the best choice. Bluetooth is available on a large number of devices. Bluetooth has an initial request / discovery process, which is a beacon for discovering nearby devices that you can connect to, with this you can implement and detect the proximity of devices.
. API- iPhone Bluetooth Gamekit, Gamekit . , , , , iPhone Bluetooth GameKit Apis.
iPhone MFi, Blueototh Bluetooth iPhone Blueooth/apis, , Apple MFi . . MFi