login: Show progress indicator during server-settings request

Fixes-partly: zulip#35

Author: chrisbobbe

lib/widgets/login.dart

@@ -107,6 +107,8 @@ class AddAccountPage extends StatefulWidget {
 }
 
 class _AddAccountPageState extends State<AddAccountPage> {
+  bool _inProgress = false;
+
   final ServerUrlTextEditingController _controller = ServerUrlTextEditingController();
   late ServerUrlParseResult _parseResult;
 
@@ -138,8 +140,18 @@ class _AddAccountPageState extends State<AddAccountPage> {
     }
     assert(url != null);
 
-    // TODO(#35): show feedback that we're working, while fetching server settings
-    final serverSettings = await getServerSettings(realmUrl: url!);
+    final GetServerSettingsResult serverSettings;
+    setState(() {
+      _inProgress = true;
+    });
+    try {
+      serverSettings = await getServerSettings(realmUrl: url!);
+      // TODO(#35) report errors
+    } finally {
+      setState(() {
+        _inProgress = false;
+      });
+    }
     if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
     else {
       return;
@@ -173,10 +185,12 @@ class _AddAccountPageState extends State<AddAccountPage> {
                   ? null
                   : error.message(),
                 suffixIcon: IconButton(
-                  onPressed: error == null || error.shouldDeferFeedback()
+                  onPressed: !_inProgress && (error == null || error.shouldDeferFeedback())
                     ? () => _onSubmitted(context)
                     : null, // disabled; errorText will be present
-                  icon: const Icon(Icons.arrow_forward))))))));
+                  icon: _inProgress
+                    ? const CircularProgressIndicator.adaptive()
+                    : const Icon(Icons.arrow_forward))))))));
   }
 }