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AnimatedConstraints

Version 1.1  21 May 2015    Switch to changing priorities rather than
                            adding/removing constraints.
Version 1.0  18 May 2015    Initial Version
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An example on how to animate changes made to autolayout constraints.
For further details see the post:

useyourloaf.com/blog/2015/05/18/animating-autolayout-constraints.html