title: Text jumps when animating position of UITextField subclass
body: <p>When I animate the location of my <code>UITextField</code> subclass, the text in the field jumps out of step with the location of the <code>UITextField</code> holding it, which is very jarring and I can't figure out why it happens:
  <img src="https://i.stack.imgur.com/iPQMD.png" alt="before">
  <img src="https://i.stack.imgur.com/0AtIt.png" alt="during">
  <img src="https://i.stack.imgur.com/mIDOX.png" alt="during">
  <img src="https://i.stack.imgur.com/PtvF0.png" alt="during">
  <img src="https://i.stack.imgur.com/jeDdP.png" alt="end"></p>
  
  <p>My subclass adds a button to the right hand side of the text field that when tapped calls a method on the view controller, which changes the textfield border colour and moves the text field up the screen by changing the top space constraint of the text field, and a couple of other views located above it.</p>
  
  <pre><code>[UIView animateWithDuration:0.3
                       animations:^{
                           self.tableViewBottomSpaceConstraint.constant = 0;
                           self.mainLabelHeightConstraint.constant = 0;
                           self.subLabelTopSpaceConstraint.constant = 0;
                           self.postCodeFieldTopSpaceConstraint.constant = 12;
                           [self.view layoutIfNeeded];
  
                       }];
  </code></pre>