我正在尝试实现自定义sortBy指令，以使html表中的列可排序。

HTML：

<thead>
   <tr>
    <sort-by-directive
      ng-repeat="header in headers"
      onsort="onSort"
      sortdir="filterCriteria.sortDir"
      sortedby="filterCriteria.sortedBy"
      sortvalue="{{ header.value }}">{{ header.title }}
    </sort-by-directive>
  </tr>
</thead>

JS：

angular.module('mainApp.directives').directive('sortByDirective', function () {

        return {
            templateUrl: 'SortHeaderTemplate',
            restrict: 'E',
            transclude: true,
            replace: true,
            scope: {
                sortdir: '=',
                sortedby: '=',
                sortvalue: '@',
                onsort: '='
            },
            link: function (scope, element, attrs) {
                scope.sort = function () {
                    if (scope.sortedby == scope.sortvalue)
                        scope.sortdir = scope.sortdir == 'asc' ? 'desc' : 'asc';
                    else {
                        scope.sortedby = scope.sortvalue;
                        scope.sortdir = 'asc';
                    }
                    scope.onsort(scope.sortedby, scope.sortdir);
                }
            }
        };
    });

指令模板：

<script id="SortHeaderTemplate" type="text/ng-template">
<th ng-click="sort(sortvalue)">
  <span ng-transclude=""></span>
  <span ng-show="sortedby == sortvalue">
    <i ng-class="{true: 'sorting_asc', false: 'sorting_desc'}[sortdir == 'asc']"></i>
  </span>
  <span ng-show="sortedby != sortvalue">
    <i ng-class="{true: 'sorting', false: 'sorting'}[sortdir == 'asc']"></i>
  </span>
</th>
</script>

因此，当我将其th用作指令模板的根标记时，会检索到错误：

Error: [$compile:tplrt] Template for directive 'sortByDirective' must have exactly one root element. SortHeaderTemplate

但是当我更改th为a或span标记时，一切正常。

我究竟做错了什么？