title: Swift: how to use PREPROCESSOR Flags (like `#if DEBUG`) to implement API keys?
body: <p>In <code>Objective-C</code> it was sometimes useful to use static string constants to define alternate API keys (for example to differentiate between RELEASE and DEBUG keys for analytics packages, like MixPanel, Flurry or Crashlytics):</p>
  
  <pre><code>#if DEBUG
  static NSString *const API_KEY = @"KEY_A";
  #else
  static NSString *const API_KEY = @"KEY_B";
  #endif
  </code></pre>
  
  <p>and then...</p>
  
  <pre><code>[Analytics startSession:API_KEY];
  </code></pre>
  
  <p>How does this translate to Swift, since the Swift compiler no longer uses a preprocessor?</p>