title: Based on whether checkbox is ticked, show another form question
body: <p>I'm creating a form in angular and one of the questions is a multi-select question. The subsequent question should only be visible if an option is selected in the previous question.</p>
  
  <p>Previously, I had been doing this:</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;mat-form-field class="selector"&gt;
        &lt;mat-select placeholder="Q1"
          formControlName="Q1" multiple&gt;
          &lt;mat-option *ngFor="let option of options" [value]="option.value" (click)="setBool(true)"&gt;
            {{ option.value }}
          &lt;/mat-option&gt;
        &lt;/mat-select&gt;
      &lt;/mat-form-field&gt;
  
  &lt;mat-form-field class="selector" *ngIf="bool"&gt;
        &lt;mat-select placeholder="Q2"
          formControlName="q2"&gt;
          &lt;mat-option *ngFor="let option of options2" [value]="option.value"&gt;
            {{ option.value }}
          &lt;/mat-option&gt;
        &lt;/mat-select&gt;
      &lt;/mat-form-field&gt;
  
  </code></pre>
  
  <p>.ts:</p>
  
  <pre><code>setBool(bool: boolean) {
          this.bool = bool;
      }
  </code></pre>
  
  <p>This worked in that when I clicked an option, bool was set to true so Q2 was then also shown. However, I have come across an issue in that when I unselect the options (so that none are selected), the question is obviously not hidden again.</p>
  
  <p>Is there a solution so that if any of the options are selected, bool = true and if none are selected bool = false?</p>