如何在ngFor循环外得到索引

Question

angular2，使得下方的p标签随着上方slider的变化而变化    <div class="slider">        <ul class="slides">            <li *ngFor="let uploadedPhoto of uploadedPhotos;let idx=index">                <img [src]="uploadedPhoto.photo">            </li>        </ul>    </div>    <p class="light grey-text text-darken-3 center-align ">{{uploadedPhotos[0].caption}}</p>//如何得到idx