feat: ability to read geojson polygon into initial drawing layer

src/my-map.ts

@@ -72,6 +72,15 @@ export class MyMap extends LitElement {
   @property({ type: Boolean })
   drawMode = false;
 
+  @property({ type: Object })
+  drawGeojsonData = {
+    type: "Feature",
+    geometry: {},
+  };
+
+  @property({ type: Number })
+  drawGeojsonDataBuffer = 100;
+
   @property({ type: Boolean })
   showFeaturesAtPoint = false;
 
@@ -254,11 +263,24 @@ export class MyMap extends LitElement {
     }
 
     if (this.drawMode) {
-      // ensure we start from an empty array of features
-      drawingSource.clear();
+      // check if single polygon feature was provided to load as the initial drawing
+      const loadInitialDrawing = Object.keys(this.drawGeojsonData.geometry).length > 0;
+      if (loadInitialDrawing) {
+        let feature = new GeoJSON().readFeature(this.drawGeojsonData, {
+          featureProjection: "EPSG:3857",
+        });
+        drawingSource.addFeature(feature);
+        // fit map to extent of intial feature, overriding zoom & lat/lng center
+        fitToData(map, drawingSource, this.drawGeojsonDataBuffer);
+      } else {
+        drawingSource.clear();
+      }
+
       map.addLayer(drawingLayer);
 
-      map.addInteraction(draw);
+      if (!loadInitialDrawing) {
+        map.addInteraction(draw);
+      }
       map.addInteraction(snap);
       map.addInteraction(modify);