angularjs – 将文字属性值传递给angular js指令

在我的HTML中说我引用了这样的指令：

<div ng-controller="MyCtrl">
    <my-directive reference-attr='referenceVal' literal-attr='literalVal'></my-directive>
</div>

referenceVal和literalVal都在我的控制器中定义,如下所示：

var myApp = angular.module('myApp',[]);

function MyCtrl($scope) {
    $scope.referenceVal = 'This was passed by reference';
    $scope.literalVal = 'This was also passed by reference,but should have been literal';
}

我试着在我的指令中接受它们：

myApp.directive("myDirective",function () {

    return {
        restrict: "E",replace: true,scope: {
            referenceAttr: "=",literalAttr: "@"
        },template: '<div><p>Reference:{{referenceAttr}}</p><p>Literal:{{literalAttr}}</p></div>',controller: function ($scope) {
            alert("ref: " + $scope.referenceAttr + ",lit: " + $scope.literalAttr);
        }
    };
});

输出看起来很好：

Reference: This was passed by reference
Literal: literalVal

但警告信息是废话：

ref: This was passed by reference,lit: undefined

您可以使用this JSFiddle自己尝试一下

知道为什么literalAttr在这一点上在指令范围内是未定义的？