title: How to identify which item in FormArray emitted valueChanges event?
body: <p>In <code>Angular</code>, is there a way to identify which <code>FormGroup</code>/<code>FormControl</code> in a <strong>dynamic</strong><code>FormArray</code> emitted the <code>valueChanges</code> event?</p>
  
  <p>My <code>FormArray</code> is dynamic. It starts out empty and users could add a <code>FormGroup</code> to the <code>FormArray</code> by clicking a button.</p>
  
  <p>When valueChanges, I need to re-validate the control. Since I dont know which control emitted the event, I loop through the entire <code>FormArray</code> and validate all <code>FormGroup</code>/<code>FormControl</code> even though only one control changed - and this is every time when anything in the array changes. How can I avoid doing this?</p>
  
  <pre><code>        this.myFormArray
          .valueChanges
          .subscribe(data =&gt; this.onValueChanged(data));
  
      onValueChanged(data?: any): void {
  
      // the data I receive is an entire form array.
      // how can I tell which particular item emitted the event, 
      // so I don’t need to loop through entire array and run validation for all items.
  
      for (let control in this.myFormArray.controls) {
          // run validation on each control.
      }
  }
  </code></pre>