Create a base vm class:

- Implements callback aggregation for subscribe method
- Provides `emit` method for re-rendering the UI

src/viewmodels/base/BaseViewModel.ts

@@ -0,0 +1,48 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import type { ViewModelNew } from "../../shared-components/ViewModel";
+
+export abstract class BaseViewModel<P> implements ViewModelNew {
+    /**
+     * We are relying on {@link https://react.dev/reference/react/useSyncExternalStore|useSyncExternalStore}
+     * to keep the react component in sync with view model.
+     * We only use this snapshot as way of convincing react that it should do a re-render when {@link emit} is called.
+     */
+    private snapshot: unknown = {};
+    private callbacks: Set<() => void> = new Set();
+
+    public constructor(protected props: P) {}
+
+    public getSnapshot(): unknown {
+        return this.snapshot;
+    }
+
+    public subscribe(callback: () => void): () => void {
+        this.callbacks.add(callback);
+        return () => {
+            this.callbacks.delete(callback);
+        };
+    }
+
+    /**
+     * Re-render any subscribed components
+     */
+    protected emit(): void {
+        /**
+         * When we invoke the callbacks, react will check if the result of getSnapshot()
+         * matches the previously known snapshot value via Object.is().
+         * Since the intention of calling this method is to make the UI re-render, we want
+         * that comparison to fail.
+         * We can do that by assigning a new empty object to snapshot.
+         */
+        this.snapshot = {};
+        for (const callback of this.callbacks) {
+            callback();
+        }
+    }
+}