Creating a Responder
Now it's time to create a responder class which will respond to the input the EMController brings in.
  1. Go to File -> New File.
  2. Create a new Objective-C file, and name it EMResponder. Two files will be generated, EMResponder.h and EMResponder.m.
  3. In the EMController.h and EMController.m, add the line #import "EMResponder.h"
  4. Since this program will deal heavily with math (being a calculator and all), add the line #include <math.h> to the top of the EMResponder.h file.