optionally parse some or all MVT feature properties as JSON

src/sources/mvt.js

@@ -1,9 +1,18 @@
 import DataSource, {NetworkTileSource} from './data_source';
 import Geo from '../utils/geo';
+import log from '../utils/log';
 
 import Pbf from 'pbf';
 import {VectorTile, VectorTileFeature} from '@mapbox/vector-tile';
 
+const PARSE_JSON_TYPE = {
+    NONE: 0,
+    ALL: 1,
+    SOME: 2
+};
+
+const PARSE_JSON_TEST = ['{', '[']; // one-time allocated array/strings
+
 /**
  Mapbox Vector Tile format
  @class MVTSource
@@ -13,6 +22,27 @@ export class MVTSource extends NetworkTileSource {
     constructor (source, sources) {
         super(source, sources);
         this.response_type = 'arraybuffer'; // binary data
+
+        // Optionally parse some or all properties from JSON strings
+        if (source.parse_json === true) {
+            // try to parse all properties (least efficient)
+            this.parse_json_type = PARSE_JSON_TYPE.ALL;
+        }
+        else if (Array.isArray(source.parse_json)) {
+            // try to parse a specific list of property names (more efficient)
+            this.parse_json_type = PARSE_JSON_TYPE.SOME;
+            this.parse_json_prop_list = source.parse_json;
+        }
+        else {
+            if (source.parse_json != null) {
+                let msg = `Data source '${this.name}': 'parse_json' parameter should be 'true', or an array of ` +
+                    `property names (was '${source.parse_json}')`;
+                log({ level: 'warn', once: true }, msg);
+            }
+
+            // skip parsing entirely (default behavior)
+            this.parse_json_type = PARSE_JSON_TYPE.NONE;
+        }
     }
 
     parseSourceData (tile, source, response) {
@@ -55,6 +85,8 @@ export class MVTSource extends NetworkTileSource {
                     properties: feature.properties
                 };
 
+                this.parseJSONProperties(feature_geojson);
+
                 var geometry = feature_geojson.geometry;
                 var coordinates = feature.loadGeometry();
                 for (var r=0; r < coordinates.length; r++) {
@@ -92,6 +124,29 @@ export class MVTSource extends NetworkTileSource {
         return layers;
     }
 
+    // Optionally parse some or all feature properties from JSON strings
+    parseJSONProperties (feature) {
+        if (this.parse_json_type !== PARSE_JSON_TYPE.NONE) {
+            const props = feature.properties;
+            for (const p in props) {
+                // if specified, check list of explicit properties to parse
+                // (otherwise try to parse all properties)
+                if (this.parse_json_type === PARSE_JSON_TYPE.SOME &&
+                    this.parse_json_prop_list.indexOf(p) === -1) {
+                    continue; // skip this property
+                }
+
+                // check if this property looks like JSON, and parse if so
+                if (PARSE_JSON_TEST.indexOf(props[p][0]) > -1) {
+                    try {
+                        props[p] = JSON.parse(props[p]);
+                    } catch (e) {
+                        // continue with original value if couldn't parse as JSON
+                    }
+                }
+            }
+        }
+    }
 }
 
 // Decode multipolygons, which are encoded as a single set of rings