Draw a rectangle in Cesium with shift-click-drag

gistfile1.js

var viewer = new Cesium.Viewer('cesiumContainer', {
  targetFrameRate: 60,
  homeButton: false,
  sceneModePicker: false,
  navigationHelpButton: false,
  baseLayerPicker: false,
  clock: new Cesium.Clock({
    startTime: Cesium.JulianDate.fromIso8601('1880-01-01'),
    currentTime: Cesium.JulianDate.fromIso8601('1880-01-01'),
    stopTime: Cesium.JulianDate.fromIso8601("2013-12-01"),
    clockRange: Cesium.ClockRange.CLAMPED,
    canAnimate: false,
    shouldAnimate: false,
    multiplier: 31622400 //Fast forward 1 year a second
  }),
  imageryProvider: new Cesium.ArcGisMapServerImageryProvider({
    url: '//services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer',
    enablePickFeatures: false
  }),
  //Saves GPU memory
  scene3DOnly: true,
  automaticallyTrackDataSourceClocks: false
});

viewer.scene.debugShowFramesPerSecond = true;
viewer.scene.screenSpaceCameraController.enableTranslate = false;
viewer.scene.screenSpaceCameraController.enableTilt = false;
viewer.scene.screenSpaceCameraController.enableLook = false;
viewer.scene.screenSpaceCameraController.enableCollisionDetection = false;
viewer.imageryLayers.get(0).brightness = 0.7;

var selector;
var rectangleSelector = new Cesium.Rectangle();
var screenSpaceEventHandler = new Cesium.ScreenSpaceEventHandler(viewer.scene.canvas);
var cartesian = new Cesium.Cartesian3();
var tempCartographic = new Cesium.Cartographic();
var center = new Cesium.Cartographic();
var firstPoint = new Cesium.Cartographic();
var firstPointSet = false;
var mouseDown = false;
var camera = viewer.camera;


//Draw the selector while the user drags the mouse while holding shift
screenSpaceEventHandler.setInputAction(function drawSelector(movement) {
  if (!mouseDown) {
    return;
  }

  cartesian = camera.pickEllipsoid(movement.endPosition, viewer.scene.globe.ellipsoid, cartesian);

  if (cartesian) {
    //mouse cartographic
    tempCartographic = Cesium.Cartographic.fromCartesian(cartesian, Cesium.Ellipsoid.WGS84, tempCartographic);

    if (!firstPointSet) {
      Cesium.Cartographic.clone(tempCartographic, firstPoint);
      firstPointSet = true;
    }
    else {
      rectangleSelector.east = Math.max(tempCartographic.longitude, firstPoint.longitude);
      rectangleSelector.west = Math.min(tempCartographic.longitude, firstPoint.longitude);
      rectangleSelector.north = Math.max(tempCartographic.latitude, firstPoint.latitude);
      rectangleSelector.south = Math.min(tempCartographic.latitude, firstPoint.latitude);
      selector.show = true;
    }
  }
}, Cesium.ScreenSpaceEventType.MOUSE_MOVE, Cesium.KeyboardEventModifier.SHIFT);

var getSelectorLocation = new Cesium.CallbackProperty(function getSelectorLocation(time, result) {
    return Cesium.Rectangle.clone(rectangleSelector, result);
  }, false);

  screenSpaceEventHandler.setInputAction(function startClickShift() {
    mouseDown = true;
    selector.rectangle.coordinates = getSelectorLocation;
  }, Cesium.ScreenSpaceEventType.LEFT_DOWN, Cesium.KeyboardEventModifier.SHIFT);

  screenSpaceEventHandler.setInputAction(function endClickShift() {
    mouseDown = false;
    firstPointSet = false;
    selector.rectangle.coordinates = rectangleSelector;
  }, Cesium.ScreenSpaceEventType.LEFT_UP, Cesium.KeyboardEventModifier.SHIFT);


//Hide the selector by clicking anywhere
screenSpaceEventHandler.setInputAction(function hideSelector() {
  selector.show = false;
}, Cesium.ScreenSpaceEventType.LEFT_CLICK);


selector = viewer.entities.add({
  selectable: false,
  show: false,
  rectangle: {
    coordinates: getSelectorLocation,
    material: Cesium.Color.BLACK.withAlpha(0.5)
  }
});

what a nice code !