在AngularJS自定义验证指令中调用异步服务

为了使它起作用，我需要添加“返回值”；在异步调用之外。下面的代码。

commonModule.directive("usernameVerify", [    'userSvc', function(userSvc) {        return { require: 'ngModel', scope: false, link: function(scope, element, attrs, ctrl) {     ctrl.$parsers.unshift(checkForAvailability);     ctrl.$formatters.unshift(checkForAvailability);     function checkForAvailability(value) {         if (value.length < 5) {  return value;         }         userSvc.userExists(value)  .success(function(alreadyUsed) {      var valid = alreadyUsed === 'false';      if (valid) {          ctrl.$setValidity('usernameVerify', true);          return value;      }      ctrl.$setValidity('usernameVerify', false);      return undefined;  });         // Below is the added line of pre.         return value;     } }        }    }]);