If I have an AngularJS directive without a template and I want it to set a property on the current scope, what is the best way to do it?

For example, a directive that counts button clicks:

<button twoway=counter>Click Me</button>

<p>Click Count: {{ counter }}</p>

With a directive that assigns the click count to the expression in the two way attribute:

.directive('twoway', [

'$parse',

  function($parse) {

    return {

      scope: false,

      link: function(scope, elem, attrs) {

        elem.on('click', function() {

          var current = scope.$eval(attrs.twoway) || 0;

          $parse(attrs.twoway).assign(scope, ++current);

          scope.$apply();

        });

      }

    };

  }

])

Is there a better way to do this? From what I've read, an isolated scope would be overkill, but do I need a child scope? And is there a cleaner way to write back to a scope variable defined in the directive attribute other than using $parse. I just feel like I'm making this too difficult.

Full Plunker here.