Move view model code to shared components package (#31024)

* Remove vm related code from element-web/src

* Add and export view model code from package

* Update imports

* Rewrite vm tests using vitest

* Add github action to run vm tests

* Fix lint errors

* Mvoe tests over to jest

* Try fixing code coverage

* Second attempt at fixing code coverage

packages/shared-components/src/viewmodel/BaseViewModel.ts

@@ -0,0 +1,51 @@
+/*
+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 ViewModel } from "./ViewModel";
+import { Disposables } from "./Disposables";
+import { Snapshot } from "./Snapshot";
+import { ViewModelSubscriptions } from "./ViewModelSubscriptions";
+
+export abstract class BaseViewModel<T, P> implements ViewModel<T> {
+    protected subs: ViewModelSubscriptions;
+    protected snapshot: Snapshot<T>;
+    protected props: P;
+    protected disposables = new Disposables();
+
+    protected constructor(props: P, initialSnapshot: T) {
+        this.props = props;
+        this.subs = new ViewModelSubscriptions();
+        this.snapshot = new Snapshot(initialSnapshot, () => {
+            this.subs.emit();
+        });
+    }
+
+    public subscribe = (listener: () => void): (() => void) => {
+        return this.subs.add(listener);
+    };
+
+    /**
+     * Returns the current snapshot of the view model.
+     */
+    public getSnapshot = (): T => {
+        return this.snapshot.current;
+    };
+
+    /**
+     * Relinquish any resources held by this view-model.
+     */
+    public dispose(): void {
+        this.disposables.dispose();
+    }
+
+    /**
+     * Whether this view-model has been disposed.
+     */
+    public get isDisposed(): boolean {
+        return this.disposables.isDisposed;
+    }
+}