isValid.js

angular.module('isValid')
.directive('isValid', function() {
	return {
		link: linker,
		require: '^form'
	};

	function linker($scope, $element, $attrs, form) {
		var expression = $attrs.isValid,
			name = $attrs.error;

		$scope.$watch(expression, function(value) {
			form.$setValidity(name, !!value);
		});
	}
});