title: ionic 2 3, why after scrolling, value of variable doesn't change?
body: <p>I was wrote this code for showing button after scrolling is more than 500px, but "showButton" didn't get new value.</p>
  
  <pre><code>&lt;ion-content (ionScroll)="onScroll($event)"&gt;
      &lt;button *ngIf="showButton"&gt;Scroll Top&lt;/button&gt;
  &lt;/ion-content&gt;
  </code></pre>
  
  <p>my.ts file:</p>
  
  <pre><code>showButton= false;
  
  onScroll($event) {    
    if ($event.scrollTop &gt; 500) {
      console.log(this.showButton);
      this.showButton= true;
    }
  }
  </code></pre>
  
  <p>This console.log shows change of "showButton", but in html it doesn't change.</p>
  
  <p>"showButton" for first time get value "false" but when value change to "true" it can not listen to change, how I can solve this?</p>