Add caching into local storage for a faster map experience

Logic/FeatureSource/LocalStorageSource.ts

@@ -0,0 +1,22 @@
+import FeatureSource from "./FeatureSource";
+import {UIEventSource} from "../UIEventSource";
+import LocalStorageSaver from "./LocalStorageSaver";
+
+export default class LocalStorageSource implements FeatureSource {
+    public features: UIEventSource<{ feature: any; freshness: Date }[]>;
+
+    constructor() {
+        this.features = new UIEventSource<{ feature: any; freshness: Date }[]>([])
+        try {
+            const fromStorage = localStorage.getItem(LocalStorageSaver.storageKey);
+            if (fromStorage == null) {
+                return;
+            }
+            const loaded = JSON.parse(fromStorage);
+            this.features.setData(loaded);
+        } catch (e) {
+            console.log("Could not load features from localStorage:", e)
+        }
+
+    }
+}