how to set an interpolated value in angular directive?

Question

How do I set the interpolated value in a directive? I can read the correct value from the following code, but I have not been able to set it.

js:

app.directive('ngMyDirective', function () {
    return function(scope, element, attrs) {
        console.log(scope.$eval(attrs.ngMyDirective));

        //set the interpolated attrs.ngMyDirective value somehow!!!
    }
});

html:

<div ng-my-directive="myscopevalue"></div>

where myscopevalue is a value on my controller's scope.

Answer 1

Whenever a directive does not use an isolate scope and you specify a scope property using an attribute, and you want to change that property's value, I suggest using $parse. (I think the syntax is nicer than $eval's.)

app.directive('ngMyDirective', function ($parse) {
    return function(scope, element, attrs) {
        var model = $parse(attrs.ngMyDirective);
        console.log(model(scope));
        model.assign(scope,'Anton');
        console.log(model(scope));
    }
});

fiddle

$parse works whether or not the attribute contains a dot.

Answer 2

If you want to set a value on the scope but don't know the name of the property (ahead of time), you can use object[property] syntax:

scope[attrs.myNgDirective] = 'newValue';

If the string in the attribute contains a dot (e.g. myObject.myProperty), this won't work; you can use $eval to do an assignment:

// like calling  "myscopevalue = 'newValue'"
scope.$eval(attrs.myNgDirective + " = 'newValue'");

[Update: You should really use $parse instead of $eval. See Mark's answer.]

If you're using an isolate scope, you can use the = annotation:

app.directive('ngMyDirective', function () {
    return {
        scope: {
            theValue: '=ngMyDirective'
        },
        link: function(scope, element, attrs) {
            // will automatically change parent scope value
            // associated by the variable name given to `attrs.ngMyDirective`
            scope.theValue = 'newValue';
        }
    }
});

You can see an example of this in this Angular/jQuery color picker JSFiddle example, where assigning to scope.color inside the directive automatically updates the variable passed into the directive on the controller's scope.