title: How to use stream of change events in mat-select?
body: <p>I am trying to use the optionSelectionChanges observable property in <a href="https://material.angular.io/components/select/api" rel="nofollow noreferrer">Angular Materials mat-select</a>. It gives a combined stream of all of the child options' change events.</p>
  
  <p>I want to get the previous value when a user changes the option to update validators on it. I don't know how to handle observable streams from the template to the component or if this property is best to do it? </p>
  
  <p>html:</p>
  
  <pre><code>&lt;mat-form-field&gt;
    &lt;mat-select (optionSelectionChanges)='getStream' formControlName='value1'&gt;
      &lt;mat-option value='1'&gt;Option1&lt;/mat-option&gt;
      &lt;mat-option value='2'&gt;Option2&lt;/mat-option&gt;   
    &lt;/mat-select&gt;
  &lt;/mat-form-field&gt;
  </code></pre>
  
  <p>Component:</p>
  
  <pre><code>import { Component, OnInit } from '@angular/core';
  import {FormGroup, FormControl, Validators, FormBuilder} from '@angular/forms';
  import { MatOptionSelectionChange } from '@angular/material';
  
  @Component({
    selector: 'test',
    templateUrl: './test.component.html',
    styleUrls: ['./test.component.css']
  })
  
  export class TestComponent implements OnInit {  
    form: FormGroup;
    getStream: Observable&lt;MatOptionSelectionChange&gt;;
  
    constructor(private _fb: FormBuilder) { }
  
    ngOnInit() {
      this.getStream.subscribe(res =&gt; {console.log(res)})
  
      this.form = this._fb.group({
        'value1': [null, Validators.compose([])]
      });
    }
  }
  </code></pre>
  
  <p>Thanks</p>