为什么我无法在AngularJS中的mdDialog控制器中访问`this`的属性？

The code is as follows:

代码如下:

function showVote(ev, poster) {
            $mdDialog.show({
                // var parentDOM = document.getElementById("poster-page");
                parent: angular.element(document.getElementById("poster-page")),
                controller: posterVoteController,
                templateUrl: 'view/poster_vote_tmpl.html',
                targetEvent: ev,
                locals: {
                    poster: poster
                }
            });
        }

function posterVoteController($scope, $mdDialog, poster) {
    var vm = this;
    vm.test = "TEST!";
}
function show