Disable the submit button, which takes to a new state by `<a ui-sref="next_state">Submit</a>`. Enable button only when the form is valid .

ui-sref-if.js

App.directive('uiSrefIf', function($compile) {
    return {
        link: function($scope, $element, $attrs) {

            var uiSrefVal = $attrs.uiSrefVal,
                uiSrefIf  = $attrs.uiSrefIf;

            $element.removeAttr('ui-sref-if');
            $element.removeAttr('ui-sref-val');
            $scope.$watch(
                function(){
                    return $scope.$eval(uiSrefIf);
                },
                function(bool) {
                    if (bool) {
                        $element.attr('ui-sref', uiSrefVal);
                    } else {
                        $element.removeAttr('ui-sref');
                        $element.removeAttr('href');
                    }
                    $compile($element)($scope);
                }
            );
        }
    };
})

//Usage:
//<a ui-sref-if="form.$valid" ui-sref-val="next_state" class="btn-next" ng-disabled="form.$invalid">Next</a>

//Attribution: http://stackoverflow.com/a/33276520