BaseLayerPicker，其下拉面板打开。
BaseLayerPicker是一个单按钮小部件，显示可用图像和面板地形提供者。选择图像后，将创建并插入相应的图像层作为图像集合的基础层;删除现有的基地。选择地形时它取代了当前的地形提供者。可用提供者列表中的每个项目都包含一个名称，代表图标和工具提示，以便在悬停时显示更多信息。该列表最初是为空，必须在使用前配置，如下例所示。

Throws:

• DeveloperError :文档中不存在具有id’container’的元素。

Example:

// In HTML head, include a link to the BaseLayerPicker.css stylesheet,
// and in the body, include: <div id="baseLayerPickerContainer"
//   style="position:absolute;top:24px;right:24px;width:38px;height:38px;"></div>

//Create the list of available providers we would like the user to select from.
//This example uses 3, OpenStreetMap, The Black Marble, and a single, non-streaming world image.
var imageryViewModels = [];
imageryViewModels.push(new Cesium.ProviderViewModel({
     name : 'Open\u00adStreet\u00adMap',
     iconUrl : Cesium.buildModuleUrl('Widgets/Images/ImageryProviders/openStreetMap.png'),
     tooltip : 'OpenStreetMap (OSM) is a collaborative project to create a free editable \
map of the world.\nhttp://www.openstreetmap.org',
     creationFunction : function() {
         return Cesium.createOpenStreetMapImageryProvider({
             url : 'https://a.tile.openstreetmap.org/'
         });
     }
 }));

 imageryViewModels.push(new Cesium.ProviderViewModel({
     name : 'Earth at Night',
     iconUrl : Cesium.buildModuleUrl('Widgets/Images/ImageryProviders/blackMarble.png'),
     tooltip : 'The lights of cities and villages trace the outlines of civilization \
in this global view of the Earth at night as seen by NASA/NOAA\'s Suomi NPP satellite.',
     creationFunction : function() {
         return new Cesium.IonImageryProvider({ assetId: 3812 });
     }
 }));

 imageryViewModels.push(new Cesium.ProviderViewModel({
     name : 'Natural Earth\u00a0II',
     iconUrl : Cesium.buildModuleUrl('Widgets/Images/ImageryProviders/naturalEarthII.png'),
     tooltip : 'Natural Earth II, darkened for contrast.\nhttp://www.naturalearthdata.com/',
     creationFunction : function() {
         return Cesium.createTileMapServiceImageryProvider({
             url : Cesium.buildModuleUrl('Assets/Textures/NaturalEarthII')
         });
     }
 }));

//Create a CesiumWidget without imagery, if you haven't already done so.
var cesiumWidget = new Cesium.CesiumWidget('cesiumContainer', { imageryProvider: false });

//Finally, create the baseLayerPicker widget using our view models.
var layers = cesiumWidget.imageryLayers;
var baseLayerPicker = new Cesium.BaseLayerPicker('baseLayerPickerContainer', {
    globe : cesiumWidget.scene.globe,
    imageryProviderViewModels : imageryViewModels
});

See:

• TerrainProvider
• ImageryProvider
• ImageryLayerCollection