Example for custom collection dispatchers

examples/custom-collections/README.md

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+Custom Collections Example
+========================
+
+This example demonstrates how to implement custom collections in Fedify.
+Custom collections allow you to define your own ActivityPub collections with 
+custom logic for dispatching items and counting collection sizes.
+
+
+~~~~ sh
+deno task codegen  # At very first time only
+deno run -A ./main.ts 
+~~~~

examples/custom-collections/main.ts

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+import { createFederation, MemoryKvStore, Note } from "@fedify/fedify";
+
+// Mock data - in a real application, this would query your database
+const POSTS = [
+  new Note({
+    id: new URL("https://example.com/posts/post-1"),
+    content: "ActivityPub is a decentralized social networking protocol...",
+    tags: [
+      new URL("https://example.com/tags/ActivityPub"),
+      new URL("https://example.com/tags/Decentralization"),
+    ],
+  }),
+
+  new Note({
+    id: new URL("https://example.com/posts/post-2"),
+    content: "Fedify makes it easy to build federated applications...",
+  }),
+
+  new Note({
+    id: new URL("https://example.com/posts/post-3"),
+    content: "WebFinger is a protocol for discovering information...",
+    tags: [new URL("https://example.com/tags/ActivityPub")],
+  }),
+
+  new Note({
+    id: new URL("https://example.com/posts/post-4"),
+    content: "HTTP Signatures provide authentication for ActivityPub...",
+  }),
+
+  new Note({
+    id: new URL("https://example.com/posts/post-5"),
+    content: "Understanding ActivityPub's data model is crucial...",
+  }),
+];
+
+function getTagFromUrl(url: string): string {
+  const parts = url.split("/");
+  return parts[parts.length - 1];
+}
+
+function getTaggedPostsByTag(tag: string): Note[] {
+  return POSTS
+    .filter((post) => {
+      if (!post.tagIds) {
+        return false;
+      }
+      return post.tagIds.some((tagId) => {
+        return getTagFromUrl(tagId.toString()) === tag;
+      });
+    });
+}
+
+async function demonstrateCustomCollection(): Promise<Response> {
+  // Federation instance created for demonstration
+  const federation = createFederation<void>({ kv: new MemoryKvStore() });
+
+  federation.setCollectionDispatcher(
+    "TaggedPosts",
+    Note,
+    "/users/{userId}/tags/{tag}",
+    (
+      _ctx: { url: URL },
+      values: Record<string, string>,
+      cursor: string | null,
+    ) => {
+      if (!values.tag) {
+        throw new Error("Missing userId or tag in values");
+      }
+
+      // Normally here you would look up posts from a database by user ID and tag name:
+      const posts = getTaggedPostsByTag(values.tag);
+
+      if (cursor != null) {
+        const idx = Number.parseInt(cursor, 10);
+        if (Number.isNaN(idx) || idx > posts.length || idx < 0) {
+          return { items: [], nextCursor: null, prevCursor: null };
+        }
+        return {
+          items: idx < posts.length ? [posts[idx]] : [],
+          nextCursor: idx < posts.length - 1 ? (idx + 1).toString() : null,
+          prevCursor: idx > 0 ? (idx - 1).toString() : null,
+        };
+      }
+      return { items: posts, nextCursor: null, prevCursor: null };
+    },
+  ).setCounter((_ctx, values) => {
+    // Return the total count of tagged posts
+    const count = getTaggedPostsByTag(values.tag).length;
+    return count;
+  });
+
+  return await federation.fetch(
+    new Request(
+      "https://example.com/users/123/tags/ActivityPub",
+      {
+        headers: {
+          Accept: "application/activity+json",
+        },
+      },
+    ),
+    {
+      contextData: undefined,
+    },
+  );
+}
+
+if (import.meta.main) {
+  const response = await demonstrateCustomCollection();
+
+  if (response.ok) {
+    const jsonResponse = await response.json();
+    console.log("Custom collection data:", jsonResponse);
+  } else {
+    const errorText = await response.text();
+    console.log("Error response:", errorText);
+  }
+}