title: Swift extension for selected class instance
body: <p>In Objective-C category, you can bring in the extended capability introduced by the category methods by including the header of the category in your class. </p>
  
  <p>It seems like all Swift extensions are automatically introduced without import. How do you achieve the same thing in Swift? </p>
  
  <p>For example:</p>
  
  <pre><code>extension UIView {
    // only want certain UIView to have this, not all
    // similar to Objective-C, where imported category header
    // will grant the capability to the class
    func extraCapability() {
  
    }
  }
  </code></pre>