title: Add method selector into a dictionary
body: <p>I want to add a selector into a dictionary (the main purpose is for identifying the callback method and delegate after finish doing something)</p>
  
  <p>But I find that I can not do that, the program will get an error "EXC_BAD_ACCESS".
  Are there any other way for me to add that method selector to a dictionary?
  Thanks for your help.</p>