angularjs创建弹出框实现拖动效果

.directive('draggable', ['$document', function($document) {        return function(scope, element, attr) {        var startX = 0, startY = 0, x = 0, y = 0;        element= angular.element(document.getElementsByClassName("modal-dialog"));         element.css({          position: 'relative',          cursor: 'move'        });        element.on('mousedown', function(event) {          // Prevent default dragging of selected content          event.preventDefault();          startX = event.pageX - x;          startY = event.pageY - y;          $document.on('mousemove', mousemove);          $document.on('mouseup', mouseup);        });        function mousemove(event) {          y = event.pageY - startY;          x = event.pageX - startX;          element.css({          top: y + 'px',          left: x + 'px'          });        }        function mouseup() {          $document.off('mousemove', mousemove);          $document.off('mouseup', mouseup);        }        };  }]);