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