In a directive, where you set a method callback through an arg, say:

```
<my-directive callback='showAlert()' />

```

You can retrieve the variable through the scope setting in the return:

```
scope: {
    callback: "&callback"
}

```

If the callback is not set, eg:

```
<my-directive />

```

The value of $scope.callback is still:

```
$scope.callback():function (locals) {
    return parentGet(scope, locals);
}

```

Is there a good way to check that the callback was not set?