feat: add test

Author: MichaelVerdon

packages/firestore/e2e/DocumentReference/onSnapshotsInSync.e2e.js

@@ -27,10 +27,61 @@ describe('firestore().doc().onSnapshot()', function () {
   describe('modular', function () {
     it('onSnapshotsInSync() returns an unsubscribe function', function () {
         const firestore = firebase.firestore();
-        const unsubscribe = firestore.onSnapshotsInSync(function () {});
+        const unsubscribe = onSnapshotsInSync(firestore);
 
         expect(unsubscribe).to.be.a('function');
         expect(unsubscribe()).to.equal(undefined);
     });
   });
+
+  it('onSnapshotsInSync fires after listeners are in sync', () => {
+    const testDocs = {
+      a: { foo: 1 }
+    };
+    return withTestCollection(persistence, testDocs, async (coll, db) => {
+      let events = [];
+      const gotInitialSnapshot = (() => {
+        let resolve, reject;
+        const promise = new Promise((res, rej) => {
+          resolve = res;
+          reject = rej;
+        });
+        return { promise, resolve, reject };
+      })();
+      const docA = doc(coll, 'a');
+
+      onSnapshot(docA, snap => {
+        events.push('doc');
+        gotInitialSnapshot.resolve();
+      });
+      await gotInitialSnapshot.promise;
+      events = [];
+
+      const done = (() => {
+        let resolve, reject;
+        const promise = new Promise((res, rej) => {
+          resolve = res;
+          reject = rej;
+        });
+        return { promise, resolve, reject };
+      })();
+      onSnapshotsInSync(db, () => {
+        events.push('snapshots-in-sync');
+        if (events.length === 3) {
+          // We should have an initial snapshots-in-sync event, then a snapshot
+          // event for set(), then another event to indicate we're in sync
+          // again.
+          expect(events).to.deep.equal([
+            'snapshots-in-sync',
+            'doc',
+            'snapshots-in-sync'
+          ]);
+          done.resolve();
+        }
+      });
+
+      await setDoc(docA, { foo: 3 });
+      await done.promise;
+    });
+  });
 });