title: Angular Mat Select Multiple selectionchange finding which option was changed
body: <p>I have a <code>&lt;mat select&gt;</code> with the <code>multiple</code> option set (a multi-select). When the <code>selectionChange</code> event fires I need to know which option has been checked or unchecked, however, it only returns the new current list of selected options.</p>
  
  <p>For instance I have a list:</p>
  
  <pre><code>&lt;mat-select (selectionChange)="change($event)" multiple placeholder="Select"&gt;
    &lt;mat-option value="1"&gt;one&lt;/mat-option&gt;
    &lt;mat-option value="2"&gt;two&lt;/mat-option&gt;
    &lt;mat-option value="3"&gt;three&lt;/mat-option&gt;
    &lt;mat-option value="4"&gt;four&lt;/mat-option&gt;
  &lt;/mat-select&gt;
  </code></pre>
  
  <p>If options <code>one</code>, <code>three</code> and <code>four</code> are checked and then the user unchecked option <code>four</code>, in the event handler I need to know which option triggered the event (i.e. option <code>four</code>) and its new state. I currently don't see a way of accessing that information in the <code>selectionChange</code> event.</p>
  
  <p><a href="https://stackblitz.com/edit/angular-1e9gsd?file=app/select-overview-example.ts" rel="noreferrer">https://stackblitz.com/edit/angular-1e9gsd?file=app/select-overview-example.ts</a></p>
  
  <p>I tried putting the event handler <code>(selectionChange)="change($event)"</code> on the <code>&lt;mat-option&gt;</code> but it doesn't seem to be supported.</p>