Trigger a function inside a directive by watching a variable - angularjs

Question

Assume I have the following directive:
angular.module("myModule", [])
.directive('myDirective', function ($timeout) {
return {
restrict: 'EA',
scope: {
value: '=',
valueIsValid: '='
},
link: function (scope, element, attrs) {
scope.returnValueIsValid = function() {
// return valueIsValid = true or false
};
}
}
});
invoked the following way:
<div my-directive value="value" value-is-valid="valueIsValid"></div>
The value variable is input to the directive, where the directive should return if value is valid or not in the valueIsValid variable. Is there a way to trigger the validation function by just watching valueIsValid ?

Answer 1

I assume valueIsValid will be true when value>0
var myApp = angular.module('myApp',[]);
myApp.controller('MyCtrl',function($scope, $timeout) {
});
myApp.directive('myDirective', function ($timeout) {
return {
restrict: 'EA',
scope: {
value: '=',
valueIsValid: '='
},
link: function (scope, element, attrs) {
scope.$watch("value",function(a,b){
if(a && a != 0){
scope.valueIsValid =true;
}else{
scope.valueIsValid =false;
}
})
}
}
});
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.6/angular.min.js"></script>
<div ng-app="myApp" ng-controller="MyCtrl">
<input type="number" value="0" ng-model="value"/>
<div my-directive value="value" value-is-valid="valueIsValid"></div>
value is valid:- {{valueIsValid}}
</div>