1 files changed, 51 insertions, 0 deletions
diff --git a/app/assets/javascripts/code_review/signals.js b/app/assets/javascripts/code_review/signals.js
new file mode 100644
index 00000000000..101b7996bb5
--- /dev/null
+++ b/app/assets/javascripts/code_review/signals.js
@@ -0,0 +1,51 @@
+import createApolloClient from '../lib/graphql';
+
+import { getDerivedMergeRequestInformation } from '../diffs/utils/merge_request';
+import { EVT_MR_PREPARED } from '../diffs/constants';
+
+import getMr from '../graphql_shared/queries/merge_request.query.graphql';
+import mrPreparation from '../graphql_shared/subscriptions/merge_request_prepared.subscription.graphql';
+
+function required(name) {
+  throw new Error(`${name} is a required argument`);
+}
+
+async function observeMergeRequestFinishingPreparation({ apollo, signaler }) {
+  const { namespace, project, id: iid } = getDerivedMergeRequestInformation({
+    endpoint: document.location.pathname,
+  });
+  const projectPath = `${namespace}/${project}`;
+
+  if (projectPath && iid) {
+    const currentStatus = await apollo.query({
+      query: getMr,
+      variables: { projectPath, iid },
+    });
+    const { id: gqlMrId, preparedAt } = currentStatus.data.project.mergeRequest;
+    let preparationObservable;
+    let preparationSubscriber;
+
+    if (!preparedAt) {
+      preparationObservable = apollo.subscribe({
+        query: mrPreparation,
+        variables: {
+          issuableId: gqlMrId,
+        },
+      });
+
+      preparationSubscriber = preparationObservable.subscribe((preparationUpdate) => {
+        if (preparationUpdate.data.mergeRequestMergeStatusUpdated?.preparedAt) {
+          signaler.$emit(EVT_MR_PREPARED);
+          preparationSubscriber.unsubscribe();
+        }
+      });
+    }
+  }
+}
+
+export async function start({
+  signalBus = required('signalBus'),
+  apolloClient = createApolloClient(),
+} = {}) {
+  await observeMergeRequestFinishingPreparation({ signaler: signalBus, apollo: apolloClient });
+}