ng-options屬性可以在表達式中使用數組或對象來自動生成一個select中的option列表。ng-options與ng-repeat很相似，很多時候可以用ng-repeat來代替ng-options。但是ng-options提供了一些好處，例如減少內存提高速度，以及提供選擇框的選項來讓用戶選擇。當select中一個選項被選擇，該選項將會被綁定到ng-model。如果你想設一個默認值，可以像這樣：$scope.selected = $scope.collection[3]。

之前一直在用ng-repeat就見到過track by，沒有去了解它的用法，這次了解了一下。track by主要是防止值有重復，angularjs會報錯。因為angularjs需要一個唯一值來與生成的dom綁定，以方便追蹤數據。例如：items=[“a”,“a”,“b”],這樣ng-repeat=“item in items”就會出錯，而用ng-repeat=“(key,value) in items track by key”就不會出現錯誤了。

ng-options一般有以下用法：

對於數組：

• label for value in array
• select as label for value in array
• label group by group for value in array
• label disable when disable for value in array
• label group by group for value in array track by trackexpr
• label disable when disable for value in array track by trackexpr
• label for value in array | orderBy:orderexpr track by trackexpr(for including a filter with track by)
• 對於對象：

• label for (key , value) in object
• select as label for (key ,value) in object
• label group by group for (key,value) in object
• label disable when disable for (key, value) in object
• select as
  label group by group for(key, value) in object
• select as label disable when disable for (key, value) in object

一個簡單的例子：

<script>
angular.module(‘selectExample‘, [])
  .controller(‘ExampleController‘, [‘$scope‘, function($scope) {
    $scope.colors = [
      {name:‘black‘, shade:‘dark‘},
      {name:‘white‘, shade:‘light‘, notAnOption: true},
      {name:‘red‘, shade:‘dark‘},
      {name:‘blue‘, shade:‘dark‘, notAnOption: true},
      {name:‘yellow‘, shade:‘light‘, notAnOption: false}
    ];
    $scope.myColor = $scope.colors[2]; // red
  }]);
</script>
<div ng-controller="ExampleController">
  <ul>
    <li ng-repeat="color in colors">
      <label>Name: <input ng-model="color.name"></label>
      <label><input type="checkbox" ng-model="color.notAnOption"> Disabled?</label>
      <button ng-click="colors.splice($index, 1)" aria-label="Remove">X</button>
    </li>
    <li>
      <button ng-click="colors.push({})">add</button>
    </li>
  </ul>
  <hr/>
  <label>Color (null not allowed):
    <select ng-model="myColor" ng-options="color.name for color in colors"></select>
  </label><br/>
  <label>Color (null allowed):
  <span  class="nullable">
    <select ng-model="myColor" ng-options="color.name for color in colors">
      <option value="">-- choose color --</option>
    </select>
  </span></label><br/>
 
  <label>Color grouped by shade:
    <select ng-model="myColor" ng-options="color.name group by color.shade for color in colors">
    </select>
  </label><br/>
 
  <label>Color grouped by shade, with some disabled:
    <select ng-model="myColor"
          ng-options="color.name group by color.shade disable when color.notAnOption for color in colors">
    </select>
  </label><br/>
 
  Select <button ng-click="myColor = { name:‘not in list‘, shade: ‘other‘ }">bogus</button>.
  <br/>
  <hr/>
  Currently selected: {{ {selected_color:myColor} }}
  <div style="border:solid 1px black; height:20px"
       ng-style="{‘background-color‘:myColor.name}">
  </div>
</div>

ng-options用法詳解