index.html

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body ng-app="app">
<input ng-model="a" dand /><button ng-click="a=a+1">change</button>
<div>{{a}}</div>
<script src="lib/angular/angular.js"></script>
<script>
    angular.module('app', [])
            .directive('dand', function() {
                return {
                    require: 'ngModel',
                    restrict: 'A',
                    link: function(scope, elem, attrs, ngModelCtrl) {
                        ngModelCtrl.$parsers.push(function(v) {
                            return v +1;
                        });
                        ngModelCtrl.$formatters.push(function(v){
                            ngModelCtrl.$setViewValue(v+3);
                            ngModelCtrl.$render();
                            return v+3;
                        })
                    }
                }
            });
</script>
</body>
</html>