subscription_list [nfc]: Allow onChannelSelect callback, notifying the selected channel

This will be used soon to provide specific behaviour when selecting
a channel, where if specified it will replace the default behaviour
of routing to the message list page of the selected channel narrow.

Author: rajveermalviya

lib/widgets/subscription_list.dart

@@ -13,16 +13,20 @@ import 'text.dart';
 import 'theme.dart';
 import 'unread_count_badge.dart';
 
+typedef OnChannelSelectCallback = void Function(BuildContext context, ChannelNarrow narrow);
+
 /// Scrollable listing of subscribed streams.
 class SubscriptionListPageBody extends StatefulWidget {
   const SubscriptionListPageBody({
     super.key,
     this.disableChannelActionSheet = false,
     this.hideChannelsIfUserCantPost = false,
+    this.onChannelSelect,
   });
 
   final bool disableChannelActionSheet;
   final bool hideChannelsIfUserCantPost;
+  final OnChannelSelectCallback? onChannelSelect;
 
   @override
   State<SubscriptionListPageBody> createState() => _SubscriptionListPageBodyState();
@@ -122,14 +126,16 @@ class _SubscriptionListPageBodyState extends State<SubscriptionListPageBody> wit
             _SubscriptionList(
               unreadsModel: unreadsModel,
               subscriptions: pinned,
-              disableChannelActionSheet: widget.disableChannelActionSheet),
+              disableChannelActionSheet: widget.disableChannelActionSheet,
+              onChannelSelect: widget.onChannelSelect),
           ],
           if (unpinned.isNotEmpty) ...[
             _SubscriptionListHeader(label: zulipLocalizations.unpinnedSubscriptionsLabel),
             _SubscriptionList(
               unreadsModel: unreadsModel,
               subscriptions: unpinned,
-              disableChannelActionSheet: widget.disableChannelActionSheet),
+              disableChannelActionSheet: widget.disableChannelActionSheet,
+              onChannelSelect: widget.onChannelSelect),
           ],
 
           // TODO(#188): add button leading to "All Streams" page with ability to subscribe
@@ -180,11 +186,13 @@ class _SubscriptionList extends StatelessWidget {
     required this.unreadsModel,
     required this.subscriptions,
     required this.disableChannelActionSheet,
+    this.onChannelSelect,
   });
 
   final Unreads? unreadsModel;
   final List<Subscription> subscriptions;
   final bool disableChannelActionSheet;
+  final OnChannelSelectCallback? onChannelSelect;
 
   @override
   Widget build(BuildContext context) {
@@ -198,7 +206,13 @@ class _SubscriptionList extends StatelessWidget {
         return SubscriptionItem(subscription: subscription,
           unreadCount: unreadCount,
           showMutedUnreadBadge: showMutedUnreadBadge,
-          disableChannelActionSheet: disableChannelActionSheet);
+          disableChannelActionSheet: disableChannelActionSheet,
+          onChannelSelect: onChannelSelect
+            ?? (context, narrow) {
+                Navigator.push(context,
+                  MessageListPage.buildRoute(context: context,
+                    narrow: narrow));
+              });
     });
   }
 }
@@ -211,12 +225,14 @@ class SubscriptionItem extends StatelessWidget {
     required this.unreadCount,
     required this.showMutedUnreadBadge,
     required this.disableChannelActionSheet,
+    required this.onChannelSelect,
   });
 
   final Subscription subscription;
   final int unreadCount;
   final bool showMutedUnreadBadge;
   final bool disableChannelActionSheet;
+  final OnChannelSelectCallback onChannelSelect;
 
   @override
   Widget build(BuildContext context) {
@@ -229,11 +245,7 @@ class SubscriptionItem extends StatelessWidget {
       // TODO(design) check if this is the right variable
       color: designVariables.background,
       child: InkWell(
-        onTap: () {
-          Navigator.push(context,
-            MessageListPage.buildRoute(context: context,
-              narrow: ChannelNarrow(subscription.streamId)));
-        },
+        onTap: () => onChannelSelect(context, ChannelNarrow(subscription.streamId)),
         onLongPress: !disableChannelActionSheet
           ? () => showChannelActionSheet(context, channelId: subscription.streamId)
           : null,