title: Angular2: Subscribe valuechanges in formbuilder
body: <p>I'm using ionic2 with angular2 and I have a form built with formbuilder but I want to detect new changes of a certain input.</p>
  
  <pre><code>ionViewDidLoad() {
    this.purchaseDataForm = this.formBuilder.group({
      kms: ['', Validators.required],
      lts: ['', Validators.required],
      price: ['', Validators.required],
      total: ['', Validators.required]
    });
  }
  </code></pre>
  
  <p>So that's how my form is validated but I want to subscribe the valuechanges of the <strong>price</strong> and the <strong>lts</strong> inputs, do you know how to do that?</p>
  
  <p>I'm trying with the following function but it displays <code>Cannot read property 'valueChanges' of undefined</code></p>
  
  <pre><code> ngOnInit() {
    this.purchaseDataForm.price.valueChanges.subscribe(value =&gt; {
      console.log(value);
    });
  }
  </code></pre>
  
  <p>Thanks!</p>