test(preview): add a test for observePreview memo

packages/sanity/src/core/preview/__test__/observeFields.test.ts

@@ -0,0 +1,47 @@
+import {describe, expect, it} from '@jest/globals'
+import {firstValueFrom, of, Subject} from 'rxjs'
+import {take, tap} from 'rxjs/operators'
+
+import {type ClientLike, createObserveFields} from '../observeFields'
+import {type InvalidationChannelEvent} from '../types'
+
+describe('observeFields', () => {
+  it('should cache the last known value and emit sync', async () => {
+    const client: ClientLike = {
+      observable: {
+        fetch: (query) => {
+          expect(query).toEqual('[*[_id in ["foo"]][0...1]{_id,_rev,_type,bar}][0...1]')
+          return of([
+            [
+              // no result
+            ],
+          ])
+        },
+      },
+      withConfig: () => client,
+    }
+
+    const invalidationChannel = new Subject<InvalidationChannelEvent>()
+    const observeFields = createObserveFields({
+      invalidationChannel,
+      client,
+    })
+    const first = firstValueFrom(observeFields('foo', ['bar']).pipe(take(1)))
+    invalidationChannel.next({type: 'connected'})
+
+    expect(await first).toMatchInlineSnapshot(`null`)
+
+    // After we got first value from server and it turned out to be `null`, we should have `null` as the memoized sync value
+    let syncValue = undefined
+    observeFields('foo', ['bar'])
+      .pipe(
+        tap((value) => {
+          syncValue = value
+        }),
+        take(1),
+      )
+      .subscribe()
+      .unsubscribe()
+    expect(syncValue).toBe(null)
+  })
+})