This is an example iOS application that demonstrates SSL pinning.

The application consists of a simple master/detail application that
displays the first page of results when searching Twitter for #SSL
tweets.

The Twitter search API is accessed over HTTPS. The built-in iOS
APIs are used to ensure that the SSL certificate presented by the
server is signed by a specific certificate authority. This is in
addition to the normal certificate validation that the platform
performs when making HTTPS requests.

The starting place for looking at how this works is the delegate
responsible for handling HTTP authentication challenges:
  XBURLConnectionAuthenticationDelegate

Disclaimer:
  This project is published as-is and is intended as a proof-of-concept
  to demonstrate a technique. Appropriate security techniques and
  their implementation depend on the application context and require
  careful consideration and review.