Setting a two-way bound directive attribute in the HTML with AngularJS

Question

I'm just starting out with AngularJs and have run into an issue that might be the result of not understanding directives / controllers /isolated scope properly. I'm trying to find a way to set a directive attribute to true/false from the html, and keep a property on the controller consistent with that attribute. What I'm working with is:

1. A controller (LoginController) with the property signUpEnabled

2. A directive (myLogin) that returns this:

       var directive = {
       bindToController: true,
       controller: 'LoginController',
       controllerAs: 'loginVm',
       templateUrl: 'login/my-login.directive.html',
       restrict: 'E',
       scope: {
           signUpEnabled: '='
       }
   };
   

3. HTML where the directive is used like so:

   <my-login sign-up-enabled="true">

I get the error: Error: [$compile:nonassign] Expression 'true' used with directive 'frintLogin' is non-assignable!

It works ok when I set signUpEnabled to false in the HTML - possibly because that's what the property is initialised to in the controller. Am I going about this wrong, or have I missed something (like watching signUpEnabled in the directive's link function?)

[edit] Thanks for your help. Not sure if this is a good way to do things but I think I can get the behaviour I want by adding this link function to the directive (after changing the binding of signUpEnabled on the isolate scope to '@'):

    function link(scope, element, attrs, ctrl) {
        $timeout(function() {
            ctrl.signUpEnabled = scope.$eval(attrs.signUpEnabled)
        });            ;
    }

Answer 1

A two way binding implies that you can do scope.signUpEnabled = something as well as something = scope.signUpEnabled. Since you're setting sign-up-enabled="true" in the directive, you're getting that error because you can't possibly set "true" to anything.

It sounds like you need a @ binding instead. That'll let you set the sign-up-enabled attribute to a string.

EDIT: Oops, just realized you're trying to keep the value consistent with a controller variable. In that case, keep the binding as = but set sign-up-enabled=scopeFieldFromController instead. You'll also want to initialize $scope.scopeFieldFromController=true in the controller.

Hope that wasn't too confusing..