Test MediaViewModel

This was the result of me playing around with RxJS marble testing to understand how to get things done with its TestScheduler. I discovered that it lacks a clear way to fire arbitrary actions during the test, so I built a small helper function called schedule which does this for us.

src/utils/test.ts

@@ -1,5 +1,5 @@
 /*
-Copyright 2023 New Vector Ltd
+Copyright 2023-2024 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -13,7 +13,9 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 */
-import { vi } from "vitest";
+import { map } from "rxjs";
+import { RunHelpers, TestScheduler } from "rxjs/testing";
+import { expect, vi } from "vitest";
 
 export function withFakeTimers(continuation: () => void): void {
   vi.useFakeTimers();
@@ -23,3 +25,36 @@ export function withFakeTimers(continuation: () => void): void {
     vi.useRealTimers();
   }
 }
+
+export interface OurRunHelpers extends RunHelpers {
+  /**
+   * Schedules a sequence of actions to happen, as described by a marble
+   * diagram.
+   */
+  schedule: (marbles: string, actions: Record<string, () => void>) => void;
+}
+
+/**
+ * Run Observables with a scheduler that virtualizes time, for testing purposes.
+ */
+export function withTestScheduler(
+  continuation: (helpers: OurRunHelpers) => void,
+): void {
+  new TestScheduler((actual, expected) => {
+    expect(actual).deep.equals(expected);
+  }).run((helpers) =>
+    continuation({
+      ...helpers,
+      schedule(marbles, actions) {
+        const actionsObservable = helpers
+          .cold(marbles)
+          .pipe(map((value) => actions[value]()));
+        const results = Object.fromEntries(
+          Object.keys(actions).map((value) => [value, undefined] as const),
+        );
+        // Run the actions and verify that none of them error
+        helpers.expectObservable(actionsObservable).toBe(marbles, results);
+      },
+    }),
+  );
+}