title: "_OBJC_CLASS_$_" referenced from error xcode when trying to access subproject files
body: <p>I am using a subproject within my xcode project with files I want to access in my main project. I have put the directories to these files in my "Header Search Paths" in my build settings, so when I import them I don't get any compiler errors. I have the subproject files added to the the compile sources in the Build Phases of my subproject. I have my main project's files added to the compile sources of my main project. However, when I try to build, I get the following errors:</p>
  
  <pre><code>Undefined symbols for architecture x86_64:
  "_OBJC_CLASS_$_LoginService", referenced from:
  objc-class-ref in GettingStartedViewController.o
  "_OBJC_CLASS_$_ProfileService", referenced from:
  objc-class-ref in GettingStartedViewController.o
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>
  
  <p>Any ideas on fixing this? Is there a good way to add the subproject's files to the compile sources of the main project? When I do this, it creates a separate group in xcode without actually adding the files to the compile sources of the main project.<br>
  Thanks!</p>