Detect ng-model change into directive - validation

Question

I'm doing a custom validation (directive) to compare two dates and show error if start_date is greater than end_date...
I'm passing start_date through ng-model
ng-model="start_date"
and end_date with my directive:
lower-than-date="{{end_date.toISOString()}}" //ignore the toISOString()
The input where I'm using my directive...
<input type="text" ng-model="start_date"
datepicker-popup="yyyy-MM-dd" min="today" show-weeks="false"
lower-than-date="{{end_date.toISOString()}}"/>
The directive...
.directive("lowerThanDate", ['$parse', function($parse) {
return {
require: 'ngModel',
link: function (scope, element, attrs, ctrl) {
var lowerThan = null;
function revalidate(_val) {
var valid = _val && lowerThan ? _val < lowerThan : true;
ctrl.$setValidity('lowerThanDate', valid);
}
attrs.$observe('lowerThanDate', function(_value) {
//Here I'm detecting when end_date change
lowerThan = new Date(_value);
revalidate(ctrl.$modelValue);
});
ctrl.$parsers.unshift(function(_value) {
revalidate(_value);
return _value;
});
}
};
}])
This code is working fine, but the validation is triggered only when I change the end_date. I want to validate when I change start_date too.
So, the question is: How can I "observe" the ng-model value to trigger the validation when start_date change.
Note:
This is a generic directive to compare dates. Keep this on mind.

Answer 1

Set up binding to ngModel inside your link function:
var modelValue = $parse(attr.ngModel);
scope.$watch(modelValue, function(value) {
// validate value against endDate.
});

Answer 2

You should a formatter: "Array of functions to execute, as a pipeline, whenever the model value changes".
So just do something like:
function revalidate(_val) {
var valid = _val && lowerThan ? _val < lowerThan : true;
ctrl.$setValidity('lowerThanDate', valid);
return _val;
}
....
ctrl.$formatters.unshift(revalidate);
ctrl.$parsers.unshift(revalidate);
As the reason could be that the model changed from some other place, not directly in DOM element.