It is certainly possible because a smartphone is a computer.
The UAPP developer can add some related code if he wants to.
He should have a lot of requirements, and he should carry out some prioritization.
Personally, it is not my requirement, because I used to listen my preferred music in shuffle mode.
Personally, a smartphone is superior than a DAP because it is a computer:
. one can use the best music player available at a given time
. one can stream music from a personal DLNA/UPnP cloud, from some Internet cloud, from some online music provider like Spotify, Qobuz ...
. one can store music on some additional standard mass storage device
For an audiophile, a DAP has a lot of useless features like internal DAC and internal amp, because he certainly wants to select an external DAC and an external amp allowing the best matching/synergy with his headphones.
And as everyone has to have a phone, why not using this phone acting as a music-oriented computer to transmit digital USB audio stream to an external DAC?
Why go the boring route with an additional limited device like a DAP?
Most if not all DAPs are computers as they have micro-processors in them. It's just a question of how good or bad the software is. I find the iPod Touch 5G to be quite good, only that it cannot take a micro-SD card so you can get it with up to 64 GB internal. Its built in DAC and Amp are surprisingly good, the software is very good as you can get Neutron (recently) on iOS. There are other good player apps on it as well, in the past not so much. It is small, light weight, easily attaches to external DACs and its USB Audio implementation is far better than anything I've seen on Android and I'm a big Android fan and can admit this. You can also stream from the device as well as listen to streaming services, even Google All Access.
Edited by StanD - 5/11/14 at 5:01pm