Edit in JSFiddle

var myapp = angular.module('sampleapp', [ ]);

myapp.controller('samplecontoller', function ($scope) {
	$scope.model = {
  	name:'',
    lname:''
  }
  $scope.submitForm = function()
	{
  	alert('Success fully updated : ' + $scope.model.name +' '+$scope.model.lname);
  }
});
angular.module('sampleapp').directive('disableBtn',
	function() {
		return {
			restrict : 'A',
			link : function(scope, element, attrs) {
				var $el = $(element);
				var submitBtn = $el.find('button[type="submit"]');
				var fName = attrs.name;
				
				scope.$watch(fName + '.$valid', function(val) {
					if (val) {
						submitBtn.removeAttr('disabled');
					} else {
						submitBtn.attr('disabled', 'disabled');
					}
				});
			}
		};
	}
);


 

<div ng-app="sampleapp">
    <div ng-controller="samplecontoller" ng-init="showData()">
     <form name="form" disable-btn ng-submit="submitForm()">
       <div>
       <label>First Name:</label>
         <input type="text" placeholder="First Name" ng-model="model.name" required></input>
      </div>
      <div>
        <label>Last Name:</label>
        <input type="text"placeholder="Last Name"  ng-model="model.lname" required></input>
       </div>
      <div>
        <button class="btn btn-primary" type="submit">Submit</button>
      </div>
    </form>  
    </div>
</div>