Hi,
You can use openni2 compatible rgbd camera and usb/firewire stereo cameras on MacOS X with the current desktop binaries. See darwin DMG
here and tutorials
here.
The code is written in C++, so yes only an objective c front end would be required for iOS, and maybe camera drivers and arkit integration as well. I just don't own an arkit compatible iphone + structure sensor to try porting the app yet :p
On android, you could have ARCore which is similar to ARkit. I personally prefer Google Tango (even if its future is uncertain) as there is already a depth camera integrated.
Cheers,
Mathieu