Require ObservableScopes of state holders to be specified explicitly

Previously we had a ViewModel class which was responsible for little more than creating an ObservableScope. However, since this ObservableScope would be created implicitly upon view model construction, it became a tad bit harder for callers to remember to eventually end the scope (as you wouldn't just have to remember to end ObservableScopes, but also to destroy ViewModels). Requiring the scope to be specified explicitly by the caller also makes it possible for the caller to reuse the scope for other purposes, reducing the number of scopes mentally in flight that need tending to, and for all state holders (not just view models) to be handled uniformly by helper functions such as generateKeyed$.

src/utils/observable.ts

@@ -23,6 +23,7 @@ import {
 } from "rxjs";
 
 import { type Behavior } from "../state/Behavior";
+import { ObservableScope } from "../state/ObservableScope";
 
 const nothing = Symbol("nothing");
 
@@ -117,3 +118,56 @@ export function pauseWhen<T>(pause$: Behavior<boolean>) {
       map(([value]) => value),
     );
 }
+
+/**
+ * Maps a changing input value to an output value consisting of items that have
+ * automatically generated ObservableScopes tied to a key. Items will be
+ * automatically created when their key is requested for the first time, reused
+ * when the same key is requested at a later time, and destroyed (have their
+ * scope ended) when the key is no longer requested.
+ */
+export function generateKeyed$<In, Item, Out>(
+  input$: Observable<In>,
+  project: (
+    input: In,
+    createOrGet: (
+      key: string,
+      factory: (scope: ObservableScope) => Item,
+    ) => Item,
+  ) => Out,
+): Observable<Out> {
+  return input$.pipe(
+    scan<
+      In,
+      {
+        items: Map<string, { item: Item; scope: ObservableScope }>;
+        output: Out;
+      },
+      { items: Map<string, { item: Item; scope: ObservableScope }> }
+    >(
+      (state, data) => {
+        const nextItems = new Map<
+          string,
+          { item: Item; scope: ObservableScope }
+        >();
+        const output = project(data, (key, factory) => {
+          let item = state.items.get(key);
+          if (item === undefined) {
+            const scope = new ObservableScope();
+            item = { item: factory(scope), scope };
+          }
+          nextItems.set(key, item);
+          return item.item;
+        });
+        for (const [key, { scope }] of state.items)
+          if (!nextItems.has(key)) scope.end();
+        return { items: nextItems, output };
+      },
+      { items: new Map() },
+    ),
+    finalizeValue((state) => {
+      for (const { scope } of state.items.values()) scope.end();
+    }),
+    map(({ output }) => output),
+  );
+}