Example of directive that validate an input in asynchronous.
The source code contain one controller and one directive. The first purposes a method for validate a value. This method is asynchronous: it uses the service: $q for create a object: Promise.
The second asks the controller method for validate the input through the attribute: validator.
When the user enters characters, the directive will execute the controller method. Meanwhile, it give a value: $inProgress to indicate progress. After execution, a value "custom" provides the result: true if input is valid or otherwise, false.