angular.js watch 监听不到input的值的变化

<input type="text" name="name" ng-model="inputname" ng-init="inputname='hello'">
       <span>{{inputname}}</span>
    <button type="button" name="button" ng-click="change()">改变值</button>

$scope.change=function(){
 $scope.inputname="world";
}
$scope.$watch("$scope.inputname",function(newValue,oldValue){
   console.log(newValue);
   console.log(oldValue);
   console.log($scope.inputname);
})

---

$watch方法不对

$scope.$watch('inputname', function(newValue,oldValue){
   console.log(newValue);
   console.log(oldValue);
   console.log($scope.inputname);
})