More refactoring: using a decent, configurable datapipeline now

Logic/FeatureSource/RememberingSource.ts

@@ -0,0 +1,24 @@
+/**
+ * Every previously added point is remembered, but new points are added
+ */
+import FeatureSource from "./FeatureSource";
+import {UIEventSource} from "../UIEventSource";
+
+export default class RememberingSource implements FeatureSource{
+    features: UIEventSource<{feature: any, freshness: Date}[]> = new UIEventSource<{feature: any, freshness: Date}[]>([]);
+    
+    constructor(source: FeatureSource) {
+        const self = this;
+        source.features.addCallbackAndRun(features => {
+            if(features === undefined){
+                return;
+            }
+            const ids = new Set<string>( features.map(f => f.feature.properties.id+f.feature.geometry.type));
+            const newList = features.concat(
+                self.features.data.filter(old => !ids.has(old.feature.properties.id+old.feature.geometry.type))
+            )
+            self.features.setData(newList);
+        })
+    }
+    
+}