关于ng-option

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

一个具体工作中的例子：

  <div class="input-field col s6" ng-class="{ 'has-error':change_newStorage_carForm.arrive_city.$invalid&&submitted }">
             <select material-select watch id="arrive_city" name="arrive_city" ng-change="get_received(arrive_city.id)" ng-options="arrive_city.city_name for arrive_city in get_city"  ng-model="arrive_city" required>
                 <option value="" disabled selected>请选择</option>
             </select>

<label for="arrive_city">目的地城市</label>
</div>

     // 城市
        _basic.get($host.api_url+"/city").then(function (data) {
            if(data.success==true){
                $scope.get_city=data.result;
            }
        });
例如：
[
    {id: 100, city_name: "大连", city_status: 1, created_on: "2017-06-29T06:20:25.000Z"},
    {id: 101, city_name: "沈阳", city_status: 1, created_on: "2017-07-05T06:45:17.000Z"},
    {id: 102, city_name: "鞍山", city_status: 1, created_on: "2017-07-05T07:52:20.000Z"},
    {id: 103, city_name: "铁岭", city_status: 1, created_on: "2017-07-06T08:43:20.000Z"}
]

下拉选择的时候，

ng-model中的

arrive_city是一个选中的对象。

  {id: 100, city_name: "大连", city_status: 1, created_on: "2017-06-29T06:20:25.000Z"}

ng-options="arrive_city.city_name for arrive_city in get_city"，中显示city.name的字段，value则是一个对象：{id: 100, city_name: "大连", city_status: 1, created_on: "2017-06-29T06:20:25.000Z"}

如果后台返回数据中有个默认的选中值，取出默认值的含有id的字段，假如100，


则先循环遍历

  $scope.get_city中的数据，

$scope.get_city.forEach(function (val) {
    if(val.id==100){
        $scope.select_city_start=val;
    }
});

通过id 来确认默认选中的那个对象，然后把这个选中的对象赋值给ng-model

$scope.start_city= $scope.select_city_start;