theopensystemslab · johnrees · Sep 29, 2021 · Sep 29, 2021 · Sep 29, 2021 · Sep 29, 2021
diff --git a/index.html b/index.html
@@ -14,7 +14,7 @@
     />
   </head>
   <body>
-    <my-map zoom="18" maxZoom="20" drawMode />
+    <my-map zoom="18" maxZoom="23" drawMode />
 
     <script>
       const map = document.querySelector("my-map");

diff --git a/src/draw.ts → src/drawing.ts b/src/draw.ts → src/drawing.ts
@@ -1,11 +1,18 @@
-import MultiPoint from 'ol/geom/MultiPoint';
+import MultiPoint from "ol/geom/MultiPoint";
 import { Draw, Modify, Snap } from "ol/interaction";
 import { Vector as VectorLayer } from "ol/layer";
 import { Vector as VectorSource } from "ol/source";
-import { Circle as CircleStyle, Fill, RegularShape, Stroke, Style } from "ol/style";
+import {
+  Circle as CircleStyle,
+  Fill,
+  RegularShape,
+  Stroke,
+  Style,
+} from "ol/style";
+import { pointsSource } from "./snapping";
 
 const redLineBase = {
-  color: '#ff0000',
+  color: "#ff0000",
   width: 3,
 };
 
@@ -30,7 +37,7 @@ const drawingPointer = new CircleStyle({
 const drawingVertices = new Style({
   image: new RegularShape({
     fill: new Fill({
-      color: "#fff"
+      color: "#fff",
     }),
     stroke: new Stroke({
       color: "#ff0000",
@@ -57,7 +64,7 @@ export const drawingLayer = new VectorLayer({
       stroke: redLineStroke,
     }),
     drawingVertices,
-  ]
+  ],
 });
 
 export const draw = new Draw({
@@ -71,7 +78,7 @@ export const draw = new Draw({
 });
 
 export const snap = new Snap({
-  source: drawingSource,
+  source: pointsSource, // empty if OS VectorTile basemap is disabled & zoom > 20
   pixelTolerance: 15,
 });
 

diff --git a/src/my-map.ts b/src/my-map.ts
@@ -11,14 +11,19 @@ import { Fill, Stroke, Style } from "ol/style";
 import View from "ol/View";
 import { last } from "rambda";
 
-import { draw, drawingLayer, drawingSource, modify, snap } from "./draw";
-import { scaleControl } from "./scale-line";
+import { draw, drawingLayer, drawingSource, modify, snap } from "./drawing";
 import {
+  getFeaturesAtPoint,
   makeFeatureLayer,
   outlineSource,
-  getFeaturesAtPoint,
 } from "./os-features";
 import { makeOsVectorTileBaseMap, makeRasterBaseMap } from "./os-layers";
+import { scaleControl } from "./scale-line";
+import {
+  getSnapPointsFromVectorTiles,
+  pointsLayer,
+  pointsSource,
+} from "./snapping";
 import { AreaUnitEnum, fitToData, formatArea, hexToRgba } from "./utils";
 
 @customElement("my-map")
@@ -221,7 +226,7 @@ export class MyMap extends LitElement {
       map.getViewport().style.cursor = "grab";
     });
 
-    // add a vector layer to display static geojson if features are provided
+    // display static geojson if features are provided
     const geojsonSource = new VectorSource();
 
     if (this.geojsonData.type === "FeatureCollection") {
@@ -259,15 +264,14 @@ export class MyMap extends LitElement {
 
       // log total area of static geojson data (assumes single polygon for now)
       const data = geojsonSource.getFeatures()[0].getGeometry();
-      this.dispatch(
-        "geojsonDataArea",
-        formatArea(data, this.areaUnit)
-      );
+      this.dispatch("geojsonDataArea", formatArea(data, this.areaUnit));
     }
 
+    // draw interactions
     if (this.drawMode) {
       // check if single polygon feature was provided to load as the initial drawing
-      const loadInitialDrawing = Object.keys(this.drawGeojsonData.geometry).length > 0;
+      const loadInitialDrawing =
+        Object.keys(this.drawGeojsonData.geometry).length > 0;
       if (loadInitialDrawing) {
         let feature = new GeoJSON().readFeature(this.drawGeojsonData, {
           featureProjection: "EPSG:3857",
@@ -313,6 +317,31 @@ export class MyMap extends LitElement {
       });
     }
 
+    // show snapping points when in drawMode, with vector tile basemap enabled, and at zoom > 20
+    if (
+      this.drawMode &&
+      Boolean(this.osVectorTilesApiKey) &&
+      !this.disableVectorTiles
+    ) {
+      map.addLayer(pointsLayer);
+      drawingLayer.setZIndex(1001); // display draw vertices on top of snap points
+
+      map.on("moveend", () => {
+        if (map.getView().getZoom() < 20) {
+          pointsSource.clear();
+          return;
+        }
+
+        // extract snap-able points from the basemap, and display them as points on the map
+        setTimeout(() => {
+          pointsSource.clear();
+          const extent = map.getView().calculateExtent(map.getSize());
+          getSnapPointsFromVectorTiles(osVectorTileBaseMap, extent);
+        }, 200);
+      });
+    }
+
+    // OS Features API & click-to-select interactions
     if (this.showFeaturesAtPoint && Boolean(this.osFeaturesApiKey)) {
       getFeaturesAtPoint(
         fromLonLat([this.longitude, this.latitude]),
@@ -339,7 +368,7 @@ export class MyMap extends LitElement {
         ) {
           // fit map to extent of features
           fitToData(map, outlineSource, this.featureBuffer);
-          
+
           // write the geojson representation of the feature or merged features
           this.dispatch(
             "featuresGeojsonChange",
@@ -348,12 +377,17 @@ export class MyMap extends LitElement {
             })
           );
 
-          // calculate the total area of the feature or merged features
-          const data = outlineSource.getFeatures()[0].getGeometry();
+          // write the geojson representation of the feature or merged features
           this.dispatch(
-            "featuresAreaChange",
-            formatArea(data, this.areaUnit)
+            "featuresGeojsonChange",
+            new GeoJSON().writeFeaturesObject(outlineSource.getFeatures(), {
+              featureProjection: "EPSG:3857",
+            })
           );
+
+          // calculate the total area of the feature or merged features
+          const data = outlineSource.getFeatures()[0].getGeometry();
+          this.dispatch("featuresAreaChange", formatArea(data, this.areaUnit));
         }
       });
     }

diff --git a/src/snapping.ts b/src/snapping.ts
@@ -0,0 +1,51 @@
+import { Feature } from "ol";
+import Point from "ol/geom/Point";
+import { Vector as VectorLayer } from "ol/layer";
+import VectorTileLayer from "ol/layer/VectorTile";
+import VectorSource from "ol/source/Vector";
+import { Fill, Style } from "ol/style";
+import CircleStyle from "ol/style/Circle";
+import { splitEvery } from "rambda";
+
+export const pointsSource = new VectorSource({
+  features: [],
+  wrapX: false,
+});
+
+export const pointsLayer = new VectorLayer({
+  source: pointsSource,
+  style: new Style({
+    image: new CircleStyle({
+      radius: 3,
+      fill: new Fill({
+        color: "black",
+      }),
+    }),
+  }),
+});
+
+/**
+ * Extract points that are available to snap to when a VectorTileLayer basemap is displayed
+ * @param basemap - a VectorTileLayer
+ * @param extent - an array of 4 points
+ * @returns - a VectorSource populated with points within the extent
+ */
+export function getSnapPointsFromVectorTiles(
+  basemap: VectorTileLayer,
+  extent: number[]
+) {
+  const points = basemap
+    .getSource()
+    .getFeaturesInExtent(extent)
+    .filter((feature) => feature.getGeometry().getType() !== "Point")
+    .flatMap((feature: any) => feature.flatCoordinates_);
+
+  return (splitEvery(2, points) as [number, number][]).forEach((pair, i) => {
+    pointsSource.addFeature(
+      new Feature({
+        geometry: new Point(pair),
+        i,
+      })
+    );
+  });
+}
diff --git a/tsconfig.json b/tsconfig.json
@@ -6,7 +6,7 @@
     "emitDeclarationOnly": true,
     "experimentalDecorators": true,
     "forceConsistentCasingInFileNames": true,
-    "lib": ["es2017", "dom", "dom.iterable"],
+    "lib": ["es2019", "dom", "dom.iterable"],
     "module": "es2020",
     "moduleResolution": "node",
     "noFallthroughCasesInSwitch": true,