As you mentioned, MATLAB Coder can generate C code from your MATLAB function. The header (.h) and source (.c) files can be plugged into your XCode project. The key part of the work is to decide what the input and output of the function should be and how to interface the rest of the XCode project with the Coder-generated function. Also, existing MATLAB code may not be completely compatible with the Coder, so that some modifications to your MATLAB code may be necessary. The Coder has interactive processes that guide you through this process.
If you haven't done so yet, you should start by looking at some examples of MATLAB Coder provided by MathWorks, e.g.,