Warn users when their browser is offline

ui/src/components/common/UpdateStatus.test.tsx

@@ -1,4 +1,4 @@
-import { render } from 'testUtils';
+import { render, act } from 'testUtils';
 import UpdateStatus from './UpdateStatus';
 
 it('renders success by default', () => {
@@ -20,3 +20,24 @@ it('supports "error" status', () => {
   render(<UpdateStatus status={UpdateStatus.ERROR} />);
   expect(document.body).toHaveTextContent('Error saving');
 });
+
+it('shows message when browser is offline', async () => {
+  render(<UpdateStatus />);
+  expect(document.body).toHaveTextContent('Saved');
+
+  jest.spyOn(navigator, 'onLine', 'get').mockReturnValue(false);
+
+  await act(async () => {
+    window.dispatchEvent(new Event('offline'));
+  });
+
+  expect(document.body).toHaveTextContent('Offline');
+
+  jest.spyOn(navigator, 'onLine', 'get').mockReturnValue(true);
+
+  await act(async () => {
+    window.dispatchEvent(new Event('online'));
+  });
+
+  expect(document.body).toHaveTextContent('Saved');
+});

ui/src/components/common/UpdateStatus.tsx

@@ -1,3 +1,4 @@
+import { useEffect, useState } from 'react';
 import { FormattedMessage } from 'react-intl';
 import { Intent, Spinner, Tag } from '@blueprintjs/core';
 
@@ -13,6 +14,15 @@ type UpdateStatusProps = {
 
 function UpdateStatus({ status }: UpdateStatusProps) {
   const commonProps = { className: 'UpdateStatus', large: true, minimal: true };
+  const isOnline = useOnlineStatus();
+
+  if (!isOnline) {
+    return (
+      <Tag intent={Intent.DANGER} icon="error" {...commonProps}>
+        <FormattedMessage id="entity.status.offline" defaultMessage="Offline" />
+      </Tag>
+    );
+  }
 
   if (status === Status.ERROR) {
     return (
@@ -54,4 +64,21 @@ UpdateStatus.SUCCESS = Status.SUCCESS;
 UpdateStatus.ERROR = Status.ERROR;
 UpdateStatus.IN_PROGRESS = Status.IN_PROGRESS;
 
+function useOnlineStatus() {
+  const [isOnline, setIsOnline] = useState<boolean>(navigator.onLine);
+  const onChange = () => setIsOnline(navigator.onLine);
+
+  useEffect(() => {
+    window.addEventListener('online', onChange);
+    window.addEventListener('offline', onChange);
+
+    return () => {
+      window.removeEventListener('online', onChange);
+      window.removeEventListener('offline', onChange);
+    };
+  }, []);
+
+  return isOnline;
+}
+
 export default UpdateStatus;