title: navigationBar change titleView with animation
body: <p>I would like to change the content of my titleView in the navigationBar to display different kinds of information, for instance a UIsegmentedControl or a UILabel.
  The change of the information should be a done with a transition (segmentedControl fades out and label fades in).</p>
  
  <p>I tried to add multiple UIViews in the titleView and animate the alpha value, but it's not working.</p>
  
  <pre><code>[self.navigationItem.titleView addSubview:_mainSegmentedControl];
  </code></pre>
  
  <p>So I tried to change the content during the animation but since the value can't be animated it will be called immediately:</p>
  
  <pre><code>[UIView animateWithDuration:0.2 animations:^{
       _filterSegmentedControl.alpha = 0.0f;} completion:^(BOOL finished) {
              [UIView animateWithDuration:0.3 animations:^{
                  self.navigationItem.titleView = _mainSegmentedControl;
                  _mainSegmentedControl.alpha = 1.0f;
              }];
  }];
  </code></pre>
  
  <p>Any suggestions?</p>