From 91c3755533dd4728e8e24ef983bb81eb7e24b429 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Christoffer=20R=C3=B8dvik?= <hi@kodemon.net>
Date: Mon, 11 Aug 2025 13:52:29 +0200
Subject: [PATCH] feat: add reducer factory method

---
 libraries/aggregate.ts   | 43 +++++++++++++++++++++++++++-------------
 libraries/event-store.ts | 11 ++++++++++
 2 files changed, 40 insertions(+), 14 deletions(-)

diff --git a/libraries/aggregate.ts b/libraries/aggregate.ts
index a203a82..a37eb00 100644
--- a/libraries/aggregate.ts
+++ b/libraries/aggregate.ts
@@ -2,7 +2,6 @@ import type { AnyEventStore, EventsInsertSettings } from "../libraries/event-sto
 import type { Unknown } from "../types/common.ts";
 import { AggregateSnapshotViolation, AggregateStreamViolation } from "./errors.ts";
 import { EventFactory } from "./event-factory.ts";
-import { makeAggregateReducer } from "./reducer.ts";
 
 /**
  * Represents an aggregate root in an event-sourced system.
@@ -20,6 +19,9 @@ export abstract class AggregateRoot<TEventFactory extends EventFactory> {
    */
   static readonly name: string;
 
+  /**
+   * Instance used for internal interaction with the originating event store.
+   */
   readonly #store: AnyEventStore;
 
   /**
@@ -52,13 +54,17 @@ export abstract class AggregateRoot<TEventFactory extends EventFactory> {
     this.#stream = value;
   }
 
-  get id() {
+  get id(): string {
     if (this.#stream === undefined) {
       this.#stream = crypto.randomUUID();
     }
     return this.#stream;
   }
 
+  get #self(): typeof AggregateRoot<TEventFactory> {
+    return this.constructor as typeof AggregateRoot<TEventFactory>;
+  }
+
   /**
    * Does the aggregate have pending events to submit to the event store.
    */
@@ -133,6 +139,27 @@ export abstract class AggregateRoot<TEventFactory extends EventFactory> {
   // Mutators
   // -------------------------------------------------------------------------
 
+  /**
+   * Generates a new snapshot for the aggregate instance.
+   *
+   * If the instance has any pending events, they will be commited before
+   * the snapshot operation is executed. If there are special settings for
+   * any pending events, make sure to `.save()` before snapshotting.
+   */
+  async snapshot() {
+    const stream = this.#stream;
+    if (stream === undefined) {
+      throw new AggregateSnapshotViolation(this.#self.name);
+    }
+    await this.save();
+    const reducer = this.#store.aggregate.reducer(this.#self);
+    await this.#store.createSnapshot({
+      name: this.#self.name,
+      stream,
+      reducer,
+    });
+  }
+
   /**
    * Saves all pending events to the attached event store.
    *
@@ -150,18 +177,6 @@ export abstract class AggregateRoot<TEventFactory extends EventFactory> {
     return this;
   }
 
-  async snapshot() {
-    const stream = this.#stream;
-    if (stream === undefined) {
-      throw new AggregateSnapshotViolation((this.constructor as typeof AggregateRoot<TEventFactory>).name);
-    }
-    await this.#store.createSnapshot({
-      name: this.constructor.name,
-      stream,
-      reducer: makeAggregateReducer(this.#store, this.constructor as typeof AggregateRoot<TEventFactory>),
-    });
-  }
-
   /**
    * Removes all events from the aggregate #pending list.
    */
diff --git a/libraries/event-store.ts b/libraries/event-store.ts
index 63f884d..fc3f5a2 100644
--- a/libraries/event-store.ts
+++ b/libraries/event-store.ts
@@ -184,6 +184,17 @@ export class EventStore<TEventFactory extends EventFactory, TEventStoreAdapter e
     ): InstanceType<TAggregate> => {
       return aggregate.from(this, snapshot);
     },
+
+    /**
+     * Create a new reducer instance for the given aggregate.
+     *
+     * @param aggregate - Aggregate to create a reducer for.
+     */
+    reducer: <TAggregate extends AggregateRootClass<TEventFactory>>(
+      aggregate: TAggregate,
+    ): Reducer<TEventFactory, InstanceType<TAggregate>> => {
+      return makeAggregateReducer(this, aggregate);
+    },
   };
 
   /*