ng-src作用

...
<ul class="phones">
  <li ng-repeat="phone in $ctrl.phones | filter:$ctrl.query | orderBy:$ctrl.orderProp" class="thumbnail">
    <a href="#/phones/{{phone.id}}" class="thumb">
      <img ng-src="{{phone.imageUrl}}" alt="{{phone.name}}" />
    </a>
    <a href="#/phones/{{phone.id}}">{{phone.name}}</a>
    <p>{{phone.snippet}}</p>
  </li>
</ul>
...

We also added phone images next to each record using an image tag with the ngSrc directive. That directive prevents the browser 
from treating the Angular {{ expression }} markup literally, and initiating a request to an invalid URL 

為什麼不直接使用<img src="{{phone.imageUrl}}">因為瀏覽器會請求{{phone.imageUrl}}這個url。