cesium动态绘制矩形

要实现cesium动态绘制矩形，可以使用Cesium的Primitive API和MouseEvents。具体步骤如下：

1. 创建矩形entity并添加到场景中。

var entity = viewer.entities.add({
  rectangle: {
    coordinates: Cesium.Rectangle.fromDegrees(-100.0, 20.0, -90.0, 30.0),
    material: Cesium.Color.RED.withAlpha(0.5),
    outline: true,
    outlineColor: Cesium.Color.BLACK
  }
});

2. 获取鼠标左键按下和移动事件，并更新矩形的坐标。

var startPosition;
var mouseHandler = new Cesium.ScreenSpaceEventHandler(viewer.scene.canvas);
mouseHandler.setInputAction(function(movement) {
  startPosition = movement.position;
}, Cesium.ScreenSpaceEventType.LEFT_DOWN);

mouseHandler.setInputAction(function(movement) {
  if (Cesium.defined(startPosition)) {
    var endPosition = movement.position;

    var startCartesian = viewer.scene.camera.pickEllipsoid(startPosition, viewer.scene.globe.ellipsoid);
    var endCartesian = viewer.scene.camera.pickEllipsoid(endPosition, viewer.scene.globe.ellipsoid);

    if (Cesium.defined(startCartesian) && Cesium.defined(endCartesian)) {
      var startCartographic = Cesium.Cartographic.fromCartesian(startCartesian);
      var endCartographic = Cesium.Cartographic.fromCartesian(endCartesian);

      var west = Math.min(startCartographic.longitude, endCartographic.longitude);
      var east = Math.max(startCartographic.longitude, endCartographic.longitude);
      var south = Math.min(startCartographic.latitude, endCartographic.latitude);
      var north = Math.max(startCartographic.latitude, endCartographic.latitude);

      entity.rectangle.coordinates = Cesium.Rectangle.fromDegrees(west, south, east, north);
    }
  }
}, Cesium.ScreenSpaceEventType.MOUSE_MOVE);

3. 获取鼠标左键松开事件，结束矩形绘制。

mouseHandler.setInputAction(function(movement) {
  startPosition = undefined;
}, Cesium.ScreenSpaceEventType.LEFT_UP);

这样，当用户按下鼠标左键并移动时，就可以动态绘制矩形。