title: Angular material stepper before-selection-change & prevent step-change for some condition
body: <p>I am using linear mat-stepper.</p>
  
  <p>It works fine with next.  I do api call &amp; if it's success then i call stepper-next event instead of using matStepperNext directive.</p>
  
  <p>Now when user fills all data on step 1 &amp; <strong>directly click</strong> on some other <strong>step in heade</strong>r of next(or any other on edit record mode) then it will redirect to next step since form is valid.</p>
  
  <p>I want to do <strong>service call before</strong> that &amp; <strong>prevent step-change until service call is successful</strong> then only it should redirect to next step otherwise it shouldn't.</p>
  
  <p>I can't find any method to implement this functionality in doc.</p>
  
  <p>Can anyone suggest me what to do. I don't want to disable header click.</p>