You cannot just return something in Objective C  to Unity expect it to appear in Unity / C#. 
It's basically a two way communication.If you look at the asset store you can get several plugins to achieve this functionality.

In order to exchange message between Objective C and Unity,we need to follow the steps below.

What is Unity 3D ?
​Answer:
It is a  multiplatform game engine which gives outof-box functionality to developing best 2d and 3D  games as well as non-game interactive simulations and visualizations.