actions [nfc]: Move subscribe-to-channel-with-feedback to ZulipActions

Author: chrisbobbe

lib/widgets/action_sheet.dart

@@ -490,27 +490,7 @@ class SubscribeButton extends ActionSheetMenuItemButton {
 
   @override
   void onPressed() async {
-    final store = PerAccountStoreWidget.of(pageContext);
-    final channel = store.streams[channelId];
-    if (channel == null || channel is Subscription) return; // TODO could give feedback
-
-    try {
-      await subscribeToChannel(store.connection, subscriptions: [channel.name]);
-    } catch (e) {
-      if (!pageContext.mounted) return;
-
-      String? errorMessage;
-      switch (e) {
-        case ZulipApiException():
-          errorMessage = e.message;
-          // TODO(#741) specific messages for common errors, like network errors
-          //   (support with reusable code)
-        default:
-      }
-
-      final title = ZulipLocalizations.of(pageContext).subscribeFailedTitle;
-      showErrorDialog(context: pageContext, title: title, message: errorMessage);
-    }
+    await ZulipAction.subscribeToChannel(pageContext, channelId: channelId);
   }
 }
 

lib/widgets/actions.dart

@@ -241,6 +241,32 @@ abstract final class ZulipAction {
     return fetchedMessage?.content;
   }
 
+  static Future<void> subscribeToChannel(BuildContext context, {
+    required int channelId,
+  }) async {
+    final store = PerAccountStoreWidget.of(context);
+    final channel = store.streams[channelId];
+    if (channel == null || channel is Subscription) return; // TODO could give feedback
+
+    try {
+      await channels_api.subscribeToChannel(store.connection, subscriptions: [channel.name]);
+    } catch (e) {
+      if (!context.mounted) return;
+
+      String? errorMessage;
+      switch (e) {
+        case ZulipApiException():
+          errorMessage = e.message;
+          // TODO(#741) specific messages for common errors, like network errors
+          //   (support with reusable code)
+        default:
+      }
+
+      final title = ZulipLocalizations.of(context).subscribeFailedTitle;
+      showErrorDialog(context: context, title: title, message: errorMessage);
+    }
+  }
+
   /// Unsubscribe from a channel, possibly after a confirmation dialog,
   /// showing an error dialog on failure.
   ///