1 files changed, 145 insertions, 0 deletions
diff --git a/spec/frontend/code_review/signals_spec.js b/spec/frontend/code_review/signals_spec.js
new file mode 100644
index 00000000000..03c3580860e
--- /dev/null
+++ b/spec/frontend/code_review/signals_spec.js
@@ -0,0 +1,145 @@
+import { start } from '~/code_review/signals';
+
+import diffsEventHub from '~/diffs/event_hub';
+import { EVT_MR_PREPARED } from '~/diffs/constants';
+import { getDerivedMergeRequestInformation } from '~/diffs/utils/merge_request';
+
+jest.mock('~/diffs/utils/merge_request');
+
+describe('~/code_review', () => {
+  const io = diffsEventHub;
+
+  beforeAll(() => {
+    getDerivedMergeRequestInformation.mockImplementation(() => ({
+      namespace: 'x',
+      project: 'y',
+      id: '1',
+    }));
+  });
+
+  describe('start', () => {
+    it.each`
+      description                     | argument
+      ${'no event hub is provided'}   | ${{}}
+      ${'no parameters are provided'} | ${undefined}
+    `('throws an error if $description', async ({ argument }) => {
+      await expect(() => start(argument)).rejects.toThrow('signalBus is a required argument');
+    });
+
+    describe('observeMergeRequestFinishingPreparation', () => {
+      const callArgs = {};
+      const apollo = {};
+      let querySpy;
+      let apolloSubscribeSpy;
+      let subscribeSpy;
+      let nextSpy;
+      let unsubscribeSpy;
+      let observable;
+
+      beforeEach(() => {
+        querySpy = jest.fn();
+        apolloSubscribeSpy = jest.fn();
+        subscribeSpy = jest.fn();
+        unsubscribeSpy = jest.fn();
+        nextSpy = jest.fn();
+        observable = {
+          next: nextSpy,
+          subscribe: subscribeSpy.mockReturnValue({
+            unsubscribe: unsubscribeSpy,
+          }),
+        };
+
+        querySpy.mockResolvedValue({
+          data: { project: { mergeRequest: { id: 'gql:id:1', preparedAt: 'x' } } },
+        });
+        apolloSubscribeSpy.mockReturnValue(observable);
+
+        apollo.query = querySpy;
+        apollo.subscribe = apolloSubscribeSpy;
+
+        callArgs.signalBus = io;
+        callArgs.apolloClient = apollo;
+      });
+
+      it('does not query at all if the page does not seem like a merge request', async () => {
+        getDerivedMergeRequestInformation.mockImplementationOnce(() => ({}));
+
+        await start(callArgs);
+
+        expect(querySpy).not.toHaveBeenCalled();
+        expect(apolloSubscribeSpy).not.toHaveBeenCalled();
+      });
+
+      describe('on a merge request page', () => {
+        it('requests the preparedAt (and id) for the current merge request', async () => {
+          await start(callArgs);
+
+          expect(querySpy).toHaveBeenCalledWith(
+            expect.objectContaining({
+              variables: {
+                projectPath: 'x/y',
+                iid: '1',
+              },
+            }),
+          );
+        });
+
+        it('does not subscribe to any updates if the preparedAt value is already populated', async () => {
+          await start(callArgs);
+
+          expect(apolloSubscribeSpy).not.toHaveBeenCalled();
+        });
+
+        describe('if the merge request is still asynchronously preparing', () => {
+          beforeEach(() => {
+            querySpy.mockResolvedValue({
+              data: { project: { mergeRequest: { id: 'gql:id:1', preparedAt: null } } },
+            });
+          });
+
+          it('subscribes to updates', async () => {
+            await start(callArgs);
+
+            expect(apolloSubscribeSpy).toHaveBeenCalledWith(
+              expect.objectContaining({ variables: { issuableId: 'gql:id:1' } }),
+            );
+            expect(observable.subscribe).toHaveBeenCalled();
+          });
+
+          describe('when the MR has been updated', () => {
+            let emitSpy;
+            let behavior;
+
+            beforeEach(() => {
+              emitSpy = jest.spyOn(diffsEventHub, '$emit');
+              nextSpy.mockImplementation((data) => behavior?.(data));
+              subscribeSpy.mockImplementation((handler) => {
+                behavior = handler;
+
+                return { unsubscribe: unsubscribeSpy };
+              });
+            });
+
+            it('does nothing if the MR has not yet finished preparing', async () => {
+              await start(callArgs);
+
+              observable.next({ data: { mergeRequestMergeStatusUpdated: { preparedAt: null } } });
+
+              expect(unsubscribeSpy).not.toHaveBeenCalled();
+              expect(emitSpy).not.toHaveBeenCalled();
+            });
+
+            it('emits an event and unsubscribes when the MR is prepared', async () => {
+              await start(callArgs);
+
+              observable.next({ data: { mergeRequestMergeStatusUpdated: { preparedAt: 'x' } } });
+
+              expect(unsubscribeSpy).toHaveBeenCalled();
+              expect(emitSpy).toHaveBeenCalledWith(EVT_MR_PREPARED);
+            });
+          });
+        });
+      });
+    });
+  });
+});