title: What does fillMode do exactly?
body: <p>The documentation is not talking really much about what this technically does.</p>
  
  <pre><code>fadeInAnimation.fillMode = kCAFillModeForwards;
  </code></pre>
  
  <p>I don't really see the effect of this. I have an view with alpha set to 0.0. Then I fade it in. But when I uncomment the line abouth, nothing changes. Same behavior. I'd like to understand what this fillMode really does. </p>