title: How to observe property changed in Swift with ReactiveCocoa
body: <p>I am writing Swift with the new ReactiveCocoa + ReactiveSwift. I am trying to do something like the following (in ReactiveCocoa 2.5) with the new ReactiveCocoa framework:</p>
  
  <pre><code>[[RACObserve(user, username) skip:1] subscribeNext:^(NSString *newUserName) {
      // perform actions...
  }];
  </code></pre>
  
  <p>After some researches, I still could not figure out how to do it. Please help! Thank you very much!</p>