title: Angular 7 ngOnDestroy unsubscribe not removing subscription
body: <p>So on one of my pages I am subscribing to the back button press in order to prompt the user with something. I then unsubscribe in <code>ngOnDestroy</code> in order to remove this as I don't want it on any other pages. However when I navigate to another page it doesn't remove the subscription so when I hit back on the next page it goes back then triggers the subscription contents.</p>
  
  <p>How can I stop this from happening.</p>
  
  <p>Start sub:</p>
  
  <pre><code>backBtnSub;
  
  ngOnInit() {    
      this.backBtnSub = this.platform.backButton.subscribe(() =&gt; {
          this.showAlert();
      });
  }
  </code></pre>
  
  <p>Remove sub:</p>
  
  <pre><code>ngOnDestroy(): void {
      this.backBtnSub.unsubscribe();
  }
  </code></pre>
  
  <p>Where I navigate away:</p>
  
  <pre><code>&lt;div class="option" routerLink="/main/documents"&gt;
      &lt;img class="optionImg" src="../../../assets/images/docs.png" alt="Option Image"&gt;
      &lt;p class="optionText"&gt;Docs&lt;/p&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Does <code>routerLink</code> not trigger ngOnDestroy? My only solution I can think of would be to create my own "navigate" function where I unsubscribe then navigate away but I don't think that should be required.</p>