angular中为什么会使用track by 实现ng-repeat嵌套

slides是一个二维数组，我以上的代码会报错Error: [ngRepeat:dupes]

<div ng-repeat="links in slides"><div ng-repeat="link in links track by $index">{{link.name}}</div>
</div> $http.get('index.php?option=com_mtree&task=ajax.load').success(function(response) {
    if(response.status) {
        $scope.links = response.links;

        if(typeof response.links != 'undefined') {
            var slides = [],slide;
            for(var i=0; i<response.links.length;) {
                slide = [];
                var c=0; c<3&&c<response.links.length; c++,i++) {
                    slide.push(response.links.indexOf(i));
                }
                slides.push(slide);
            }
            $scope.slides = slides;
        }

        /* setTimeout(function(){ jQuery('.saved-list .slideshow').cycle('destroy'); jQuery('.saved-list .slideshow').cycle(); },0); */
    }
});

Error: [ngRepeat:dupes]这个出错提示具体到题主的情况，意思是指数组中有2个以上的相同数字。ngRepeat不允许collection中存在两个相同Id的对象

For example:item in itemsis equivalent toitem in items track by $id(item). This implies that the DOM elements will be associated by item identity in the array.

对于数字对象来说，它的id就是它自身的值，因此，数组中是不允许存在两个相同的数字的。为了规避这个错误，需要定义自己的track by表达式。例如：item in items track by item.id或者item in items track by fnCustomId(item)。嫌麻烦的话，直接拿循环的索引变量$index来用item in items track by $index