title: Angular-formly custom Validation on multiple fields on a repeat section
body: <p>I was trying to create a custom validator for angular-formly repeated section.</p>
  
  <p>The form should be valid only if the sum of the percentage inputs is 100. So for example if repeat section has 2 fields 50 and 50 should be a valid option or 25 and 75 and so on.</p>
  
  <p>While I was working on a <a href="https://jsbin.com/mekeca/edit?js,output" rel="noreferrer">JSbin</a> in order to do that I found out that the repeated model is not actually updated <code>onKeydown</code>. Therefore iterating though all the repeat section values and calculating their sum is not possible.</p>
  
  <p>I also tried with <code>modelOptions: { updateOn: 'Keydown' }</code> with no success. It actually makes the validator not to get called at all.</p>
  
  <hr>
  
  <p><strong>UPDATE</strong></p>
  
  <p>I came up with the following solution from the matching fields example.
  Unfortunately it appears that the example its self has a problem.</p>
  
  <p>Play with the following JSbin and see that there are many cases where the validator gets called and returns true but the field/fields still remain red (indicating they have a problem).</p>
  
  <p>Here is the <a href="https://jsbin.com/weyoco/edit?js,console,output" rel="noreferrer">JSBin</a>.</p>