feat: add spawner utility

Introduce `createSpawner` and `spawnerEffect` to manage the lifecycle
of side effects based on a collection of IDs. `spawnerEffect` uses
Preact signals to automatically trigger spawners when IDs are added
to an iterable and cleanup functions when they are removed.

src/index.ts

@@ -71,4 +71,6 @@ export { createRNG, Mulberry32RNG } from "./utils/rng";
 export type { MiddlewareChain } from "./utils/middleware";
 export { createMiddlewareChain } from "./utils/middleware";
 
+export * from "./utils/spawner";
+
 export * from "@preact/signals-core";

src/utils/spawner.ts

@@ -0,0 +1,39 @@
+import { effect } from "@preact/signals-core";
+
+export type SpawnerFunction = (id: string) => () => void;
+
+export function createSpawner(spawner: SpawnerFunction) {
+  const entries = new Map<string, () => void>();
+
+  function update(set: Set<string>) {
+    for (const key of set) {
+      if (!entries.has(key)) {
+        entries.set(key, spawner(key));
+      }
+    }
+
+    for (const [key, entry] of entries) {
+      if (!set.has(key)) {
+        entry();
+        entries.delete(key);
+      }
+    }
+  }
+
+  return update;
+}
+
+export function spawnerEffect(
+  iterable: () => Iterable<string>,
+  spawner: SpawnerFunction,
+) {
+  const update = createSpawner(spawner);
+
+  return effect(() => {
+    const set = new Set<string>();
+    for (const each of iterable()) {
+      set.add(each);
+    }
+    update(set);
+  });
+}