<parent>
        <label>Username: </label>
        <child></child>
        <button id="login" type="submit">Login</button>
    </parent>

angular.module('sampleModule', [])
        .directive("parent",function(signOnService){return {
            restrict : 'E',
            transclude: true,
            scope: {},
            controller: function($scope, $element, $attrs){
                this.username = 'InitU';
                this.password = 'InitP';
                this.setUsername = function(name){
                    this.username = name;
                };
                this.getUsername = function(){
                    return this.username;
                };
                this.getPassword = function(){
                  return this.password
                };
                this.setPassword = function(password){
                    this.password = password;
                };
                this.check = function() {
                 //TODO something on form submission
                };
            },
            controllerAs: 'diff',
            link: function(scope, element, attrs, ctrl){
                scope.user = ctrl.username;
                element.find('button').on('click', function(event){
                   alert('Username: '+ ctrl.username  + 'nPassword: '+ ctrl.password);
                });
            },
             template: '<form name="myForm" ng-submit="diff.check()"><ng-transclude></ng-transclude></form>'
        }})
        .directive("child",function(){
            return {
                restrict : 'E',
                require: '^parent',
                scope: false,
                link: function(scope, element, attrs, ctrl){
                    scope.username = ctrl.getUsername();
                    console.log('INI: '+ scope.username);

                    scope.$watch('username', function(newVal, oldVal){
                        if(newVal!=oldVal)
                            //alert('Value Changed: '+ newVal);
                        ctrl.setUsername(scope.username);
                    });
                },
                template: '<input name="username" type="text" ng-model="username" required>'
            }
        });

<parent>
        <label>Username: </label>
        <child></child>
        <button id="login" type="submit" ng-disabled="myForm.$invalid">Login</button>
    </parent>