compose_box: Replace compose box with a banner when cannot post in a channel

Fixes: #674

Author: sm-sayedi

assets/l10n/app_en.arb

@@ -188,6 +188,10 @@
   "@errorBannerDeactivatedDmLabel": {
     "description": "Label text for error banner when sending a message to one or multiple deactivated users."
   },
+  "errorBannerCannotPostInChannelLabel": "You do not have permission to post in this channel.",
+  "@errorBannerCannotPostInChannelLabel": {
+    "description": "Label text for error banner when sending a message in a channel with no posting permission."
+  },
   "composeBoxAttachFilesTooltip": "Attach files",
   "@composeBoxAttachFilesTooltip": {
     "description": "Tooltip for compose box icon to attach a file to the message."

lib/widgets/compose_box.dart

@@ -999,8 +999,21 @@ class _StreamComposeBoxState extends State<_StreamComposeBox> implements Compose
     super.dispose();
   }
 
+  Widget? _errorBanner(BuildContext context) {
+    final store = PerAccountStoreWidget.of(context);
+    final selfUser = store.users[store.selfUserId]!;
+    final channel = store.streams[widget.narrow.streamId]!;
+    return channel.hasPostingPermission(selfUser, realmWaitingPeriodThreshold: store.realmWaitingPeriodThreshold)
+      ? null : _ErrorBanner(label: ZulipLocalizations.of(context).errorBannerCannotPostInChannelLabel);
+  }
+
   @override
   Widget build(BuildContext context) {
+    final errorBanner = _errorBanner(context);
+    if (errorBanner != null) {
+      return _ComposeBoxContainer(child: errorBanner);
+    }
+
     return _ComposeBoxLayout(
       contentController: _contentController,
       contentFocusNode: _contentFocusNode,
@@ -1072,16 +1085,19 @@ class _FixedDestinationComposeBoxState extends State<_FixedDestinationComposeBox
   }
 
   Widget? _errorBanner(BuildContext context) {
-    if (widget.narrow case DmNarrow(:final otherRecipientIds)) {
-      final store = PerAccountStoreWidget.of(context);
-      final hasDeactivatedUser = otherRecipientIds.any((id) =>
-        !(store.users[id]?.isActive ?? true));
-      if (hasDeactivatedUser) {
-        return _ErrorBanner(label: ZulipLocalizations.of(context)
-          .errorBannerDeactivatedDmLabel);
-      }
+    final store = PerAccountStoreWidget.of(context);
+    switch (widget.narrow) {
+      case TopicNarrow():
+        final selfUser = store.users[store.selfUserId]!;
+        final channel = store.streams[(widget.narrow as TopicNarrow).streamId]!;
+        return channel.hasPostingPermission(selfUser, realmWaitingPeriodThreshold: store.realmWaitingPeriodThreshold)
+          ? null : _ErrorBanner(label: ZulipLocalizations.of(context).errorBannerCannotPostInChannelLabel);
+      case DmNarrow(:final otherRecipientIds):
+        final hasDeactivatedUser = otherRecipientIds.any((id) =>
+          !(store.users[id]?.isActive ?? true));
+        return hasDeactivatedUser ? _ErrorBanner(label: ZulipLocalizations.of(context)
+          .errorBannerDeactivatedDmLabel) : null;
     }
-    return null;
   }
 
   @override

test/widgets/action_sheet_test.dart

@@ -43,7 +43,7 @@ Future<void> setupToMessageActionSheet(WidgetTester tester, {
 
   await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
   final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
-  await store.addUser(eg.user(userId: message.senderId));
+  await store.addUsers([eg.selfUser, eg.user(userId: message.senderId)]);
   if (message is StreamMessage) {
     final stream = eg.stream(streamId: message.streamId);
     await store.addStream(stream);

test/widgets/autocomplete_test.dart

@@ -76,10 +76,12 @@ Future<Finder> setupToTopicInput(WidgetTester tester, {
   addTearDown(testBinding.reset);
   await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
   final store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+  await store.addUser(eg.selfUser);
   final connection = store.connection as FakeApiConnection;
 
   // prepare message list data
   final stream = eg.stream();
+  await store.addStream(stream);
   final message = eg.streamMessage(stream: stream, sender: eg.selfUser);
   connection.prepare(json: GetMessagesResult(
     anchor: message.id,