Add app package

packages/app/src/collection.ts

@@ -0,0 +1,56 @@
+import {readable, derived, type Readable} from 'svelte/store'
+import {indexBy, type Maybe, now} from '@welshman/lib'
+import {withGetter} from '@welshman/store'
+import {getFreshness, setFreshness} from './freshness'
+
+export const collection = <T>({
+  name,
+  store,
+  getKey,
+  load,
+}: {
+  name: string
+  store: Readable<T[]>
+  getKey: (item: T) => string
+  load: (key: string, ...args: any) => Promise<any>
+}) => {
+  const indexStore = withGetter(derived(store, $items => indexBy(getKey, $items)))
+  const getItem = (key: string) => indexStore.get().get(key)
+  const pending = new Map<string, Promise<Maybe<T>>>()
+
+  const loadItem = async (key: string, ...args: any[]) => {
+    if (getFreshness(name, key) > now() - 3600) {
+      return indexStore.get().get(key)
+    }
+
+    if (pending.has(key)) {
+      await pending.get(key)
+    } else {
+      setFreshness(name, key, now())
+
+      const promise = load(key, ...args)
+
+      pending.set(key, promise)
+
+      await promise
+
+      pending.delete(key)
+    }
+
+    return indexStore.get().get(key)
+  }
+
+  const deriveItem = (key: Maybe<string>, ...args: any[]) => {
+    if (!key) {
+      return readable(undefined)
+    }
+
+    // If we don't yet have the item, or it's stale, trigger a request for it. The derived
+    // store will update when it arrives
+    load(key, ...args)
+
+    return derived(indexStore, $index => $index.get(key))
+  }
+
+  return {indexStore, deriveItem, loadItem, getItem}
+}