how pass dynamic attribute to custom validator

I'm building a custom validator as a directive for a component, and I am trying to pass an attribute, but I am not being able to retrieve the value, just the string of the method I am calling.

Part of the code is:

directive:

return {
    restrict: 'A',
    require: 'ngModel',
 link: (scope, element, attributes, ngModel) => {
            const myAttrib = attributes.myValidation ;
            console.log(myAttrib);
            ...
    }

component:

my-validation="$ctrl.getmyAtt()"

controller:

getmyAtt() { return "blah"; }

so in the console I get "$ctrl.getmyAtt()" instead of blah. How do I get blah?

You'll need to inject the $parse service and use it to get the actual value being passed in.

$parse(attributes.myValidation)($scope);

$parse documentation here.

edit: Actually, that won't work for your callback parameter. Instead you'll want to store that string and run it as javascript at a later time when the callback needs to be run.

var returnValue = scope.$eval(myAttrib);