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": "Error-banner text replacing the compose box when you do not have permission to send a message to the channel."
+  },
   "composeBoxAttachFilesTooltip": "Attach files",
   "@composeBoxAttachFilesTooltip": {
     "description": "Tooltip for compose box icon to attach a file to the message."

lib/widgets/compose_box.dart

@@ -1071,26 +1071,8 @@ class _FixedDestinationComposeBoxState extends State<_FixedDestinationComposeBox
     super.dispose();
   }
 
-  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);
-      }
-    }
-    return null;
-  }
-
   @override
   Widget build(BuildContext context) {
-    final errorBanner = _errorBanner(context);
-    if (errorBanner != null) {
-      return _ComposeBoxContainer(child: errorBanner);
-    }
-
     return _ComposeBoxLayout(
       contentController: _contentController,
       contentFocusNode: _contentFocusNode,
@@ -1127,8 +1109,36 @@ class ComposeBox extends StatelessWidget {
     }
   }
 
+  Widget? _errorBanner(BuildContext context) {
+    final store = PerAccountStoreWidget.of(context);
+    final selfUser = store.users[store.selfUserId]!;
+    switch (narrow) {
+      case ChannelNarrow narrow:
+        final channel = store.streams[narrow.streamId]!;
+        return channel.hasPostingPermission(selfUser, byDate: DateTime.now(), realmWaitingPeriodThreshold: store.realmWaitingPeriodThreshold)
+          ? null : _ErrorBanner(label: ZulipLocalizations.of(context).errorBannerCannotPostInChannelLabel);
+      case TopicNarrow narrow:
+        final channel = store.streams[narrow.streamId]!;
+        return channel.hasPostingPermission(selfUser, byDate: DateTime.now(), 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;
+      case CombinedFeedNarrow():
+      case MentionsNarrow():
+        return null;
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
+    final errorBanner = _errorBanner(context);
+    if (errorBanner != null) {
+      return _ComposeBoxContainer(child: errorBanner);
+    }
+
     final narrow = this.narrow;
     switch (narrow) {
       case ChannelNarrow():