login: Show progress

Fixes-partly: zulip#35

Author: chrisbobbe

lib/widgets/login.dart

@@ -106,6 +106,8 @@ class _EmailPasswordLoginPageState extends State<EmailPasswordLoginPage> {
     });
   }
 
+  bool _inProgress = false;
+
   Future<int> _getUserId(FetchApiKeyResult fetchApiKeyResult) async {
     final FetchApiKeyResult(:email, :apiKey) = fetchApiKeyResult;
     final auth = Auth(
@@ -126,13 +128,20 @@ class _EmailPasswordLoginPageState extends State<EmailPasswordLoginPage> {
     // TODO(#35): validate email is in the shape of an email
 
     final FetchApiKeyResult result;
+    setState(() {
+      _inProgress = true;
+    });
     try {
       result = await fetchApiKey(
         realmUrl: realmUrl, username: email, password: password);
     } on Exception catch (e) { // TODO(#37): distinguish API exceptions
       // TODO(#35): give feedback to user on failed login
       debugPrint(e.toString());
       return;
+    } finally {
+      setState(() {
+        _inProgress = false;
+      });
     }
 
     // TODO(server-7): Rely on user_id from fetchApiKey.
@@ -168,7 +177,11 @@ class _EmailPasswordLoginPageState extends State<EmailPasswordLoginPage> {
   Widget build(BuildContext context) {
     assert(!PerAccountStoreWidget.debugExistsOf(context));
     return Scaffold(
-      appBar: AppBar(title: const Text('Log in')),
+      appBar: AppBar(title: const Text('Log in'),
+        bottom: _inProgress
+          ? const PreferredSize(preferredSize: Size.fromHeight(4),
+              child: LinearProgressIndicator(minHeight: 4))
+          : null),
       body: SafeArea(
         minimum: const EdgeInsets.all(8),
         child: Center(
@@ -197,7 +210,7 @@ class _EmailPasswordLoginPageState extends State<EmailPasswordLoginPage> {
                           icon: _obscurePassword ? const Icon(Icons.visibility_off) : const Icon(Icons.visibility))))),
                   const SizedBox(height: 8),
                   ElevatedButton(
-                    onPressed: _submit,
+                    onPressed: _inProgress ? null : _submit, // disabled if in progress
                     child: const Text('Log in')),
                 ])))))));
   }