The biggest problem here is that Apple unfortunately doesn't provide any API for peer-to-peer wi-fi access [1], as far as I know.
This means that all solutions will either require sender and receiver being connected to the same wi-fi network (possibly provided by one of the devices involved, which is pretty awkward and also has its downsides like unintentionally sharing the internet connection [2]), or resorting to Bluetooth LE (resulting in extremely slow connection speeds not really suitable for file transfer).
Finally, any solution requiring installing a third-party app has an inherent growth impediment given that sharing the app itself is all but impossible when offline.
[1] Wi-Fi Direct would seem to be ideal for this use case but is Android only, and even there does not seem to reliably work on all devices. Even Google's native AirDrop alternative resorts to using mobile data in some circumstances/device combinations.
Apple do provide two APIs for peer-to-peer Wi-Fi access — one of them is the Multipeer Connection Framework and the other is the NetService API with the “includesPeerToPeer” parameter. This only really helps us when talking to other Apple devices though as the AWDL protocol is proprietary.
The Wi-Fi Alliance have been trying to standardise something not dissimilar to AWDL under the name “Wi-Fi Aware” (NAN/Neighbour Awareness Networking), but it is not widely implemented and will probably never be supported by Apple either, limiting cross-platform potential.
This means that all solutions will either require sender and receiver being connected to the same wi-fi network (possibly provided by one of the devices involved, which is pretty awkward and also has its downsides like unintentionally sharing the internet connection [2]), or resorting to Bluetooth LE (resulting in extremely slow connection speeds not really suitable for file transfer).
Finally, any solution requiring installing a third-party app has an inherent growth impediment given that sharing the app itself is all but impossible when offline.
[1] Wi-Fi Direct would seem to be ideal for this use case but is Android only, and even there does not seem to reliably work on all devices. Even Google's native AirDrop alternative resorts to using mobile data in some circumstances/device combinations.
[2] It seems possible to avoid this given that the Android device is providing the network: https://developer.android.com/reference/android/net/wifi/Wif...