angularjs – 在指令中嵌套指令

>为什么我不能在我的链接功能中引用scope.bindValue？有没有办法可以从scope.bindValue计算一个值并将其设置为范围？
>为什么在范围内使用“@”表示子对象绑定：{}但是在链接函数中不使用scope.value = attrs.value？

以下所有内容均可在http://jsfiddle.net/sdg9/AjDtt/13/看到

HTML：

<directive bind-value="12" value="7"></directive>

JS：

var myApp = angular.module('myApp',[]);
var commonTemplate = '<div>{{name}} bind-value is: {{Bindvalue}} </div><div>{{name}} value is: {{value}} </div><div>{{name}} add one to bind-value is: {{addOneBindvalue}} </div><div>{{name}} add one to value is: {{addOnevalue}} </div><br/>';

myApp.directive('directive',function () {
    return {
        scope: {
            bindValue: "@",},template: commonTemplate + '<br/><sub-directive bind-value="{{value}}" value="{{value}}"></sub-directive>',reStrict: 'E',link: function (scope,element,attrs) {
            scope.name = "Directive";
            scope.value = attrs.value;
            scope.addOneBindValue = parseInt(scope.bindvalue) + 1;
            scope.addOneValue = parseInt(scope.value) + 1;
        }
    };
});


myApp.directive('subDirective',function () {
    return {
        scope: {
            bindValue: "@"
        },template: commonTemplate,attrs) {   
            scope.name = "SubDirective";
            scope.value = attrs.value;
            scope.addOneBindValue = parseInt(scope.bindvalue) + 1;
            scope.addOneValue = parseInt(scope.value) + 1;
        }
    };
});

输出：

Directive bind-value is: 12
Directive value is: 7
Directive add one to bind-value is: null    <--- why?
Directive add one to value is: 8    

SubDirective bind-value is: 7
SubDirective value is:                      <--- why?
SubDirective add one to bind-value is: null
SubDirective add one to value is: null