Function setMask

src/openlayers/core/Util.js:408–449

* @function Util.setMask * @description 为图层设置掩膜。 * @version 10.1.0 * @param {ol.layer.Layer|Array. } layers 图层 * @param {ol.geom.Geometry|ol.Feature} polygon 掩膜矢量要素，支持面类型的要素。

(layers, polygon)

    * @param {ol.geom.Geometry|ol.Feature} polygon 掩膜矢量要素，支持面类型的要素。
    */
   setMask(layers, polygon) {
     if (!polygon) {
       return;
     }
     const geo = polygon instanceof Feature ? polygon.getGeometry() : polygon;
     if (!(geo instanceof Geometry) && ['MultiPolygon', 'Polygon'].indexOf(polygon.getType()) < 0) {
       return;
     }
     const feature = polygon instanceof Feature ? polygon : new Feature(polygon);
     const style = new Style({
       fill: new Fill({
         color: 'black'
       })
     });

     const clipLayer = new VectorLayer({
       source: new VectorSource({
         features: [feature],
         wrapX: false
       })
     });

     const clipRender = function (e) {
       const vectorContext = getVectorContext(e);
       e.context.globalCompositeOperation = 'destination-in';
       clipLayer.getSource().forEachFeature(function (feature) {
         vectorContext.drawFeature(feature, style);
         e.context.globalCompositeOperation = 'source-over';
       });
     };
     const todoLayers = Array.isArray(layers) ? layers : [layers];
     unsetMask(todoLayers);
     todoLayers.forEach((layer) => {
       layer.classNameBak_ = layer.className_;
       layer.className_ = `ol_mask_layer_${layer.ol_uid}`;
       layer.clipRender = clipRender;
       layer.extentBak_ = layer.getExtent();
       layer.setExtent(clipLayer.getSource().getExtent());
       layer.on('postrender', clipRender);
       layer.changed();
     });
   },
   /**
    * @function Util.unsetMask
    * @description 取消图层掩膜。