title: Angular: Trigger a form control validation on change of another form control
body: <p>Please refer the project in a stackblitz
  <a href="https://stackblitz.com/edit/angular-qapfbd?file=src%2Fapp%2Fapp.component.html" rel="nofollow noreferrer">here</a>.</p>
  
  <p>As can be seen, I have reactive form with controls as <code>firstNumber</code>, <code>secondNumber</code> and <code>thirdNumber</code>. I need to have validation for form control <code>thirdNumber</code> such that its value should not be greater than the value of the form control which is having minimum value among <code>firstNumber</code> and <code>secondNumber</code>.  </p>
  
  <p>The custom validator <code>validateThirdNumber</code> which is there in the component works fine whenever form control <code>thirdNumber</code> changes, but I need 
  fire its validation on changes of form controls <code>firstNumber</code> and <code>secondNumber</code> too as the validation logic can change on changes of form controls
  <code>firstNumber</code> and <code>secondNumber</code>.  </p>
  
  <p>For that purpose, I have added an event on change of form controls <code>firstNumber</code> and <code>secondNumber</code> where I am marking the form control <code>thirdNumber</code> as <code>touched</code> but its validation did not seem to be triggered.  </p>
  
  <p>So, how to fire the validation for form control <code>thirdNumber</code> on changes of form controls <code>firstNumber</code> and <code>secondNumber</code>?</p>
  
  <p>Also, why is <code>this.myFormGroup</code> is undefined some times in custom validator (refer logs in <code>app.componen.ts</code> at <code>line:22</code>) even after binding <code>this</code> to its form control declaration and <code>this.myFormGroup</code> is defined in the <code>constructor</code>? </p>