title: Observe a FormControl value to disable a mat-slide-toggle
body: <p>I try to control a mat-slide-toggle, with a form, controlled by a FormControl.
  It means that the <code>disabled</code> attribute of my mat-slide-toggle depends on the FormControl value. More precisely, I want to disable the toggle if the FormControlValue is not set.</p>
  <p>The form is as follow:</p>
  <pre><code>&lt;mat-form-field&gt;
    &lt;mat-label i18n=&quot;@@main.level&quot;&gt;Level&lt;/mat-label&gt;
    &lt;mat-select [formControl]=&quot;levelControl&quot; &gt;
       &lt;mat-option *ngFor=&quot;let level of levelList&quot; [value]=&quot;level.value&quot;&gt;
         {{level.viewValue | titlecase}}
       &lt;/mat-option&gt;
    &lt;/mat-select&gt;
  &lt;/mat-form-field&gt;
  </code></pre>
  <p>I cannot directly use levelControl.value to set disabled or not my toggle, because this toggle generates a dynamic component, and this methods generates ChangedAfterItHasBeenChecked Errors.</p>
  <p>So I defined an Observable in my component:</p>
  <pre><code>disableToggle: Observable&lt;boolean&gt;;
  
  ngOnInit() {
    this.disableToggle = this.levelControl.valueChanges.pipe(filter(next =&gt; next.level === ''));
  }
  </code></pre>
  <p>I use it into the toggle as it follows:</p>
  <pre><code>&lt;mat-slide-toggle [disabled]=&quot;(disableToggle | async)&quot;&gt; &lt;/mat-slide-toggle&gt;
  </code></pre>
  <p>But nothing happens, the toggle is always enabled.
  Has anybody a solution ?
  Thanks for your help !</p>