Improve example with a background layer, intersect and comments

examples/layer-clipping-vector.js

@@ -1,39 +1,56 @@
 import GeoJSON from '../src/ol/format/GeoJSON.js';
 import Map from '../src/ol/Map.js';
-import OSM from '../src/ol/source/OSM.js';
-import VectorSource from '../src/ol/source/Vector.js';
 import View from '../src/ol/View.js';
 import {Fill, Style} from '../src/ol/style.js';
+import {OSM, Stamen, Vector as VectorSource} from '../src/ol/source.js';
 import {Tile as TileLayer, Vector as VectorLayer} from '../src/ol/layer.js';
 import {fromLonLat} from '../src/ol/proj.js';
 import {getVectorContext} from '../src/ol/render.js';
+import {intersects} from '../src/ol/extent.js';
+
+//A distinct className is required to use another canvas for the background
+const background = new TileLayer({
+  className: 'stamen',
+  source: new Stamen({
+    layer: 'toner',
+  }),
+});
 
 const base = new TileLayer({
   source: new OSM(),
 });
 
 const clipLayer = new VectorLayer({
-  style: new Style({
-    fill: new Fill({
-      color: 'black',
-    }),
-  }),
+  style: null,
   source: new VectorSource({
     url: './data/geojson/switzerland.geojson',
     format: new GeoJSON(),
   }),
 });
 
-clipLayer.on('prerender', function (e) {
-  e.context.globalCompositeOperation = 'destination-in';
+const style = new Style({
+  fill: new Fill({
+    color: 'black',
+  }),
 });
 
-clipLayer.on('postrender', function (e) {
+base.on('postrender', function (e) {
+  const vectorContext = getVectorContext(e);
+  e.context.globalCompositeOperation = 'destination-in';
+  //Checking interect is required so the base layer is totally hidden when not clipped
+  if (intersects(e.frameState.extent, clipLayer.getSource().getExtent())) {
+    clipLayer.getSource().forEachFeature(function (feature) {
+      vectorContext.drawFeature(feature, style);
+    });
+  } else {
+    e.context.fillStyle = 'transparent';
+    e.context.fillRect(0, 0, 1, 1);
+  }
   e.context.globalCompositeOperation = 'source-over';
 });
 
 const map = new Map({
-  layers: [base, clipLayer],
+  layers: [background, base, clipLayer],
   target: 'map',
   view: new View({
     center: fromLonLat([8.23, 46.86]),