feat(mobile): browse and join open channels

mobile/lib/features/channels/channel.dart

@@ -82,6 +82,7 @@ class Channel {
   bool get isForum => channelType == 'forum';
   bool get isDm => channelType == 'dm';
   bool get isPrivate => visibility == 'private';
+  bool get canJoin => visibility == 'open' && !isArchived && !isMember && !isDm;
 
   /// Whether [selfRole] may add *another* identity here, mirroring the relay's
   /// kind:9000 authority (`validate_admin_event` + `add_member`): DMs never,

mobile/lib/features/channels/channel_directory.dart

@@ -0,0 +1,43 @@
+part of 'channels_provider.dart';
+
+const _channelDirectoryPageSize = 500;
+const _maxChannelDirectoryPages = 100;
+
+Future<List<NostrEvent>> _fetchChannelDirectoryMetas(
+  RelaySessionNotifier session,
+) async {
+  final directoryMetas = <NostrEvent>[];
+  final seenDirectoryChannelIds = <String>{};
+  int? directoryUntil;
+  String? directoryBeforeId;
+  for (var pageIndex = 0; pageIndex < _maxChannelDirectoryPages; pageIndex++) {
+    final page = await session.queryRelay([
+      NostrFilter(
+        kinds: const [39000],
+        limit: _channelDirectoryPageSize,
+        until: directoryUntil,
+        extensions: {'before_id': ?directoryBeforeId},
+      ),
+    ]);
+    directoryMetas.addAll(page);
+
+    var madeProgress = false;
+    for (final event in page) {
+      final channelId = event.getTagValue('d');
+      if (channelId != null && seenDirectoryChannelIds.add(channelId)) {
+        madeProgress = true;
+      }
+    }
+    if (!madeProgress || page.length < _channelDirectoryPageSize) break;
+
+    final last = page.last;
+    directoryUntil = last.createdAt;
+    directoryBeforeId = last.id;
+    if (pageIndex == _maxChannelDirectoryPages - 1) {
+      throw StateError(
+        'Channel directory exceeded $_maxChannelDirectoryPages pages',
+      );
+    }
+  }
+  return directoryMetas;
+}

mobile/lib/features/channels/channels_page.dart

@@ -53,6 +53,7 @@ import '../../shared/read_state/read_state_time.dart';
 import 'unread_badge/observed_unread_event.dart';
 
 part 'channels_page/body.dart';
+part 'channels_page/browse_channels_sheet.dart';
 part 'channels_page/sections.dart';
 part 'channels_page/channel_tile.dart';
 part 'channels_page/sheets.dart';
@@ -62,7 +63,7 @@ part 'channels_page/community.dart';
 part 'channels_page/quick_actions.dart';
 part 'channels_page/quick_actions_launcher.dart';
 
-enum _QuickAction { createChannel, newDm }
+enum _QuickAction { createChannel, newDm, browseChannels }
 
 const double _kChannelSectionInset = Grid.gutter;
 const double _kChannelLeadingWidth = 22.0;

mobile/lib/features/channels/channels_page/browse_channels_sheet.dart

@@ -0,0 +1,154 @@
+part of '../channels_page.dart';
+
+class _BrowseChannelsSheet extends ConsumerWidget {
+  const _BrowseChannelsSheet();
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final channelsAsync = ref.watch(channelsProvider);
+    final channels = channelsAsync.asData?.value
+        .where((channel) => channel.canJoin)
+        .toList();
+    channels?.sort(
+      (left, right) =>
+          left.name.toLowerCase().compareTo(right.name.toLowerCase()),
+    );
+
+    return SafeArea(
+      top: false,
+      child: Padding(
+        padding: const EdgeInsets.fromLTRB(
+          Grid.gutter,
+          0,
+          Grid.gutter,
+          Grid.xs,
+        ),
+        child: CustomScrollView(
+          shrinkWrap: true,
+          slivers: [
+            SliverToBoxAdapter(
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  Text(
+                    'Join an open channel to add it to your conversations.',
+                    style: context.textTheme.bodyMedium?.copyWith(
+                      color: context.colors.onSurfaceVariant,
+                    ),
+                  ),
+                  const SizedBox(height: Grid.xs),
+                ],
+              ),
+            ),
+            if (channelsAsync.isLoading && channels == null)
+              const SliverToBoxAdapter(
+                child: Padding(
+                  padding: EdgeInsets.all(Grid.sm),
+                  child: Center(child: BuzzLoadingIndicator()),
+                ),
+              )
+            else if (channelsAsync.hasError && channels == null)
+              SliverToBoxAdapter(
+                child: Padding(
+                  padding: const EdgeInsets.symmetric(vertical: Grid.sm),
+                  child: Text(
+                    'Could not load open channels.',
+                    textAlign: TextAlign.center,
+                    style: context.textTheme.bodyMedium?.copyWith(
+                      color: context.colors.onSurfaceVariant,
+                    ),
+                  ),
+                ),
+              )
+            else if (channels == null || channels.isEmpty)
+              SliverToBoxAdapter(
+                child: Padding(
+                  padding: const EdgeInsets.symmetric(vertical: Grid.sm),
+                  child: Text(
+                    'No open channels available to join.',
+                    textAlign: TextAlign.center,
+                    style: context.textTheme.bodyMedium?.copyWith(
+                      color: context.colors.onSurfaceVariant,
+                    ),
+                  ),
+                ),
+              )
+            else
+              SliverList.builder(
+                itemCount: channels.length,
+                itemBuilder: (context, index) => _JoinableChannelTile(
+                  channel: channels[index],
+                  closeAfterJoin: true,
+                ),
+              ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class _JoinableChannelTile extends HookConsumerWidget {
+  final Channel channel;
+  final bool closeAfterJoin;
+
+  const _JoinableChannelTile({
+    required this.channel,
+    required this.closeAfterJoin,
+  });
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final isJoining = useState(false);
+    final actionError = useState<String?>(null);
+
+    Future<void> join() async {
+      if (isJoining.value) return;
+      isJoining.value = true;
+      actionError.value = null;
+      try {
+        await ref.read(channelActionsProvider).joinChannel(channel.id);
+        if (closeAfterJoin && context.mounted) Navigator.of(context).pop();
+      } catch (error) {
+        actionError.value = error.toString();
+      } finally {
+        isJoining.value = false;
+      }
+    }
+
+    return Column(
+      mainAxisSize: MainAxisSize.min,
+      children: [
+        ListTile(
+          key: Key('browse-channel-${channel.id}'),
+          contentPadding: EdgeInsets.zero,
+          leading: Icon(channelIcon(channel)),
+          title: Text(channel.name),
+          subtitle: channel.description.trim().isEmpty
+              ? null
+              : Text(
+                  channel.description,
+                  maxLines: 2,
+                  overflow: TextOverflow.ellipsis,
+                ),
+          trailing: FilledButton.tonal(
+            key: Key('browse-channel-join-${channel.id}'),
+            onPressed: isJoining.value ? null : () => unawaited(join()),
+            child: Text(isJoining.value ? 'Joining…' : 'Join'),
+          ),
+        ),
+        if (actionError.value case final error?)
+          Align(
+            alignment: Alignment.centerLeft,
+            child: Text(
+              error,
+              key: Key('browse-channel-error-${channel.id}'),
+              style: context.textTheme.bodySmall?.copyWith(
+                color: context.colors.error,
+              ),
+            ),
+          ),
+      ],
+    );
+  }
+}

mobile/lib/features/channels/channels_page/quick_actions.dart

@@ -7,7 +7,7 @@ const _kMorphCloseCurve = Cubic(0.22, 1, 0.36, 1);
 const double _kMorphOpenBounce = 0.14;
 const double _kMorphCloseBounce = 0.06;
 const double _kMorphClosedSize = 56;
-const double _kMorphOpenHeight = 160;
+const double _kMorphOpenHeight = 216;
 const double _kMorphOpenRadius = 20;
 const double _kMorphSlide = 40;
 const double _kMorphScale = 0.97;
@@ -274,6 +274,13 @@ class _QuickActionsMenu extends StatelessWidget {
             key: const Key('quick-action-new-dm-card'),
             onTap: () => onSelected(_QuickAction.newDm),
           ),
+          const SizedBox(height: Grid.xxs),
+          _QuickActionItem(
+            icon: LucideIcons.compass,
+            title: 'Browse channels',
+            key: const Key('quick-action-browse-channels-card'),
+            onTap: () => onSelected(_QuickAction.browseChannels),
+          ),
         ],
       ),
     );

mobile/lib/features/channels/channels_page/quick_actions_launcher.dart

@@ -109,6 +109,15 @@ class ChannelQuickActionsLauncher extends HookConsumerWidget {
           if (opened != null && context.mounted) {
             await openChannel(opened);
           }
+        case _QuickAction.browseChannels:
+          await showBuzzModalBottomSheet<void>(
+            context: context,
+            title: 'Browse channels',
+            constraints: _quickActionSheetConstraints(context),
+            isScrollControlled: true,
+            showDragHandle: true,
+            builder: (_) => const _BrowseChannelsSheet(),
+          );
       }
     }