title: Angular4 Retrigger a subscription after a ViewChild is rendered again
body: <p>I have the following code in my <code>component</code> :</p>
  
  <pre><code>@ViewChild('form') form: NgForm
  isFormShown = true
  
    ngAfterViewInit(): void {
      this.form.valueChanges
          .subscribe(() =&gt; {
              console.log("form has changed")
          })
    }
  </code></pre>
  
  <p>and in my template :</p>
  
  <pre><code>&lt;button type="button" (click)="isFormShown = !isFormShown"&gt;CLICK&lt;/button&gt;
  &lt;form #form="ngForm" *ngIf="isFormShown"&gt;
     ...
  &lt;/form&gt;
  </code></pre>
  
  <p>if I click on my button twice, (so my form will disappear and reappear). It doesn't seem like my subscription is still existing. Indeed, I am fiddling with my form's inputs, and I have nothing logged.
  What do I need to do to make it work again ?</p>