action_sheet: Always confirm before unsubscribing from a channel

Fixes zulip#1827.

(That is, always confirm when unsubscribing via the action sheet --
I've added a test that we *don't* always confirm in the "All
channels" page, since that could be annoying, as mentioned in the
issue.)

Author: chrisbobbe

lib/widgets/actions.dart

@@ -247,8 +247,11 @@ abstract final class ZulipAction {
   ///
   /// A confirmation dialog is shown if the user would not have permission
   /// to resubscribe.
+  /// If [alwaysAsk] is true (the default),
+  /// a confirmation dialog is shown unconditionally.
   static Future<void> unsubscribeFromChannel(BuildContext context, {
     required int channelId,
+    bool alwaysAsk = true,
   }) async {
     final store = PerAccountStoreWidget.of(context);
     final subscription = store.subscriptions[channelId];
@@ -258,9 +261,9 @@ abstract final class ZulipAction {
     final couldResubscribe = !subscription.inviteOnly
       || store.selfHasPermissionForGroupSetting(subscription.canSubscribeGroup,
            GroupSettingType.stream, 'can_subscribe_group');
+    final zulipLocalizations = ZulipLocalizations.of(context);
     if (!couldResubscribe) {
       // TODO(#1788) warn if org would lose content access (nobody can subscribe)
-      final zulipLocalizations = ZulipLocalizations.of(context);
 
       final dialog = showSuggestedActionDialog(context: context,
         title: zulipLocalizations.unsubscribeConfirmationDialogTitle('#${subscription.name}'),
@@ -269,6 +272,12 @@ abstract final class ZulipAction {
         actionButtonText: zulipLocalizations.unsubscribeConfirmationDialogConfirmButton);
       if (await dialog.result != true) return;
       if (!context.mounted) return;
+    } else if (alwaysAsk) {
+      final dialog = showSuggestedActionDialog(context: context,
+        title: zulipLocalizations.unsubscribeConfirmationDialogTitle('#${subscription.name}'),
+        actionButtonText: zulipLocalizations.unsubscribeConfirmationDialogConfirmButton);
+      if (await dialog.result != true) return;
+      if (!context.mounted) return;
     }
 
     try {

lib/widgets/all_channels.dart

@@ -139,7 +139,8 @@ class _SubscribeToggle extends StatelessWidget {
             subscriptions: [channel.name]);
         } else {
           await ZulipAction.unsubscribeFromChannel(context,
-            channelId: channel.streamId);
+            channelId: channel.streamId,
+            alwaysAsk: false);
         }
       },
       // TODO(#741) interpret API errors for user

test/widgets/action_sheet_test.dart

@@ -573,9 +573,15 @@ void main() {
 
         connection.prepare(json: {});
         await tapButton(tester);
-        await tester.pump(Duration.zero);
+        await tester.pump();
 
-        checkNoDialog(tester);
+        final (unsubscribeButton, cancelButton) = checkSuggestedActionDialog(tester,
+          expectedTitle: 'Unsubscribe from #${channel.name}?',
+          expectDestructiveActionButton: false,
+          expectedActionButtonText: 'Unsubscribe');
+        await tester.tap(find.byWidget(unsubscribeButton));
+        await tester.pump();
+        await tester.pump(Duration.zero);
 
         check(connection.lastRequest).isA<http.Request>()
           ..method.equals('DELETE')

test/widgets/all_channels_test.dart

@@ -241,6 +241,24 @@ void main() {
       });
   });
 
+  testWidgets('Toggle "off" to unsubscribe, public channel', (tester) async {
+    final channel = eg.stream(inviteOnly: false);
+    final subscription = eg.subscription(channel);
+
+    await setupAllChannelsPage(tester, channels: [subscription]);
+
+    connection.prepare(json: {});
+    await tester.tap(find.byType(Toggle));
+    await tester.pump(Duration.zero);
+    checkNoDialog(tester);
+    check(connection.lastRequest).isA<http.Request>()
+      ..method.equals('DELETE')
+      ..url.path.equals('/api/v1/users/me/subscriptions')
+      ..bodyFields.deepEquals({
+        'subscriptions': jsonEncode([channel.name]),
+      });
+  });
+
   testWidgets('Toggle "off" to unsubscribe, but without resubscribe permission', (tester) async {
     final channel = eg.stream(
       inviteOnly: true, canSubscribeGroup: eg.groupSetting(members: []));