title: how to know which field is change in formbuilder angular2
body: <p>how to know which field is change in form builder angular2,
  i want to add some checking if field a is change the do what if field b is change do this</p>
  
  <pre><code>this.comparisonForm = this.formBuilder.group({
          city: ['', [Validators.required]],
          brand: ['', [Validators.required]],
          productTemplate: ['', [Validators.required]],
          productAttributeValue: ['', [Validators.required]],
      });
  
  this.comparisonForm.valueChanges
      .debounceTime(500)
      .subscribe(keyword =&gt; 
          console.log(keyword));
  </code></pre>