ng-options属性可以在表达式中使用数组或对象来自动生成一个select中的option列表。ng-options与ng-repeat很相似，很多时候可以用ng-repeat来代替ng-options。但是ng-options提供了一些好处，例如减少内存提高速度，以及提供选择框的选项来让用户选择。当select中一个选项被选择，该选项将会被绑定到ng-model。如果你想设一个默认值，可以像这样：$scope.selected =$scope.collection[3]。

之前一直在用ng-repeat就见到过track by，没有去了解它的用法，这次了解了一下。track by主要是防止值有重复，angularjs会报错。因为angularjs需要一个唯一值来与生成的dom绑定，以方便追踪数据。例如：items=[“a”,“a”,“b”],这样ng-repeat=“item in items”就会出错，而用ng-repeat=“(key,value) in items track by key”就不会出现错误了。

ng-options一般有以下用法：

对于数组：

• 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)

对于对象：

• 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

  一个简单的例子：

<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>

转载：http://www.ncloud.hk/%E6%8A%80%E6%9C%AF%E5%88%86%E4%BA%AB/ng-options-usage/