title: How to change and animate NSLayoutConstraints programmatically?
body: <p>I'm defining a complex layout via NSLayout constraints, programmatically. Now I found myself in a situation in which I need to animate one of the elements.</p>
  
  <p>I need to animate view crossFadeControlView changing it's height. This will obviously imply that the other will need to dynamically shrink. Is that possible?</p>
  
  <p>If not... what are my options?</p>
  
  <p>Below are my constraints: </p>
  
  <pre><code>    [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|[clipGrid]    [sceneLaunchView(==117)][meter(==9)][tempoControl(==58)]|" options:0 metrics:nil views:views]];
      [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|[tracksView][meter(==9)][tempoControl(==58)]|" options:0 metrics:nil views:views]];
      [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|[crossFadeControlView]|" options:0 metrics:nil views:views]];
      [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[clipGrid]-3-[tracksView(==143)][crossFadeControlView]|" options:0 metrics:nil views:views]];
      [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[sceneLaunchView]-3-[tracksView][crossFadeControlView]|" options:0 metrics:nil views:views]];
      [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[tempoControl][crossFadeControlView]|" options:0 metrics:nil views:views]];
      [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[meter][crossFadeControlView]|" options:0 metrics:nil views:views]];
  </code></pre>