Angular2: Subscribe valuechanges in formbuilder
<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>