Write tests

test/viewmodels/base/BaseViewModel-test.ts

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+/*
+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 { BaseViewModel } from "../../../src/viewmodels/base/BaseViewModel";
+
+interface Props {
+    initialTodos: string[];
+}
+
+/**
+ * BaseViewModel is an abstract class, so we'll have to test it with a dummy view model.
+ */
+class TodoViewModel extends BaseViewModel<Props> {
+    public todos: string[];
+
+    public constructor(props: Props) {
+        super(props);
+        this.todos = [...props.initialTodos];
+    }
+
+    public doEmit(): void {
+        this.emit();
+    }
+}
+
+describe("BaseViewModel", () => {
+    describe("Test integrity of getSnapshot()", () => {
+        it("Multiple calls to getSnapshot returns a cached value", () => {
+            // To be sure that we don't run into
+            // https://react.dev/reference/react/useSyncExternalStore#im-getting-an-error-the-result-of-getsnapshot-should-be-cached
+            const vm = new TodoViewModel({ initialTodos: ["todo1", "todo2"] });
+            const snapshot1 = vm.getSnapshot();
+            const snapshot2 = vm.getSnapshot();
+            expect(snapshot1).toBe(snapshot2);
+        });
+        it("emit() causes snapshot value to change", () => {
+            // When emit() is called, the snapshot should change in order for react to rerender any subscribed components.
+            const vm = new TodoViewModel({ initialTodos: ["todo1", "todo2"] });
+            const snapshot1 = vm.getSnapshot();
+            vm.doEmit();
+            const snapshot2 = vm.getSnapshot();
+            expect(snapshot1).not.toBe(snapshot2);
+        });
+    });
+
+    describe("Subscriptions", () => {
+        it("subscribe() returns unsubscribe callback", () => {
+            const vm = new TodoViewModel({ initialTodos: ["todo1", "todo2"] });
+            const result = vm.subscribe(jest.fn());
+            expect(typeof result).toBe("function");
+        });
+
+        it("emit() calls subscribe callbacks", () => {
+            const vm = new TodoViewModel({ initialTodos: ["todo1", "todo2"] });
+
+            const callbacks = [jest.fn(), jest.fn()];
+            callbacks.forEach((c) => vm.subscribe(c));
+            vm.doEmit();
+
+            callbacks.forEach((c) => expect(c).toHaveBeenCalledTimes(1));
+        });
+
+        it("Invoking unsubscribe callback returned from subscribe() removes subscription", () => {
+            const vm = new TodoViewModel({ initialTodos: ["todo1", "todo2"] });
+
+            const callback1 = jest.fn();
+            const callback2 = jest.fn();
+            const unsubscribe = vm.subscribe(callback1);
+            vm.subscribe(callback2);
+
+            unsubscribe();
+            vm.doEmit();
+
+            expect(callback1).not.toHaveBeenCalled();
+            expect(callback2).toHaveBeenCalledTimes(1);
+        });
+    });
+});