ngrepeat 时注意的地方和一些little tricks

angularjs的一些使用经验总结,此篇文章单谈ng指令之一ngrepeat

1. ngrepeat 时报错 Duplicates in a repeater are not allowed,

正常的时候,我们表达式内的数据是类似这样的(这时不会有任何问题)

<span ng-repeat="answer in ['good answer','perfect answer', 'bad answer','not bad answer'] ">{{answer}} </span>

但一旦当我们表达式的数据取到的是类似这样的

<span ng-repeat="answer in ['aa','bb','cc','cc','bb'] track by $index">{{answer}} </span>

    便会报错 Duplicates in a repeater are not allowed,
   此时他会用英文提示你 use track by 表达式
   发生此错误的原因是 ngrepeat 的数组或者对象数组或者对象的每个迭代项中有重复值
   因为当使用ngrepeat不加track by表达式时,默认的为 item in items is equivalent to item in items track by $id(item)
   所以出现重复值时就会报错页面repeat的那个dom元素不会在视图里显示出来

2.ngrepeat的filter

ng-repeat指令有一个特有的filter 名为filter的filter 用来对数据内容进行具体的过滤

同时当然也可以在使用angularjs框架的一些预先内置filter,如limitto (设置只显示一个)

ng-repeat="item in items | orderBy:'oreder_prop' | filter:query | limitTo:4"  (类似这样的语法使用)

ng-repeat="course in home.my_join_course | limitTo:1"

对这个特别的名为filter的使用

<div class="box">
    <div class="col-xs-12">        
        <div ng-init="output = [{name:'a',addr:'东京',score:22},{name:'b',addr:'西京',score:10},{name:'c',addr:'南京'},{name:'d',addr:'北京',area:'china'}]" >
            
　　　　　　　　<p ng-repeat="print in output | filter: 'china' ">{{print}}</p>  <!-- 只显示属性值里有china的 -->
              <p ng-repeat="print in output | filter: {name:'b'} ">{{print}}</p>  <!-- 只显示name属性值里有b的 -->
              
              <p ng-repeat="print in output | filter: have_score_than_15 ">{{print}}</p>  
              <!-- 只显示score属性大于15的 即只显示name:a,addr:东京,score:22的那项 -->
        </div>
    </div>
</div>

$scope.have_score_than_15 = function(item) {
		return item.score > 15;
	}

另外再简单介绍下排序 (这点直接引用:http://www.geekcome.com/content-10-1906-1.html )如下

<div>{{ childrenArray | orderBy : 'age' }}</div>      //按age属性值进行排序，若是-age，则倒序
<div>{{ childrenArray | orderBy : orderFunc }}</div>   //按照函数的返回值进行排序
<div>{{ childrenArray | orderBy : ['age','name'] }}</div>  //如果age相同，按照name进行排序

此外还可以支持自定义过滤器满足一些复杂的自定义操作如

<div ng-repeat=" depart in departments | child_department "> .... </div>

appFilters.filter('child_department',function () {
    return function (departments) {
        var departmentId = 3;
        var array = [];
        $.each(departments,function(i,n){
            if(n.parent_id == departmentId){
                array.push(n);
            }
        });
        return array;
    }
});

3.活用 $index $first $middle $last $even $odd

　　　　 <ul class="dropdown-menu pull-right dropdown-menu-right" >            
            <li app-pressdark ng-click="course_filter('','depart','全部科室')" class="dropdown-divider">全部科室</li>    
            <li app-pressdark ng-repeat="depart in departs | filter:{parent_id:null}" ng-click="course_filter(depart.id,'depart',depart.name)" class="{{$last?'dropdown-nodivider':'dropdown-divider'}} ">
               {{depart.name}} 
            </li>        
        </ul>

在 class="{{$last?'dropdown-nodivider':'dropdown-divider'}} " 上利用三元表达式对repeat的项进行个别的样式处理

`$index`	number	iterator offset of the repeated element (0..length-1)
`$first`	boolean	true if the repeated element is first in the iterator.
`$middle`	boolean	true if the repeated element is between the first and last in the iterator.
`$last`	boolean	true if the repeated element is last in the iterator.
`$even`	boolean	true if the iterator position `$index` is even (otherwise false).
`$odd`