Xamarin：Notification Service Extension创建UNNotificationCategory无效问题

我的应用不需要固定的操作按钮集合，而是要通过通知负载动态传递按钮信息，所以需要在每次收到带按钮的通知时，动态创建UNNotificationCategory。

为实现这个需求，我创建了Notification Service Extension来拦截通知、解析负载、创建类别并更新推送通知的类别ID。理论上这个方案可行，但实际测试发现，在Notification Service Extension中创建UNNotificationCategory后，推送通知并没有显示对应的操作按钮。

测试情况 #

• 在主项目的FinishedLaunching方法中添加了打印所有UNNotificationCategory的代码，结果始终为空。
• 不确定FinishedLaunching是否在Notification Extension之前执行。
• 曾怀疑UNUserNotificationCenter.Current实例在主项目和扩展中不一致，但测试发现库项目中创建的类别可以在主项目中访问。

其他说明 #

如果在FinishedLaunching中预先创建测试用的UNNotificationCategory，仅在Notification Extension中修改推送通知的categoryID，功能可以正常工作，但这无法实现动态创建的需求。

请问：为什么在Notification Service Extension中创建的类别无法被推送通知识别？

AppDelegate.cs（主项目） #

public override bool FinishedLaunching(UIApplication app, NSDictionary options)
{
    Rg.Plugins.Popup.Popup.Init();
    global::Xamarin.Forms.Forms.Init();
    IOSMigrationHelper migrationHelper = new IOSMigrationHelper();
    // must be done before Firebase.Core.App.Configure() to migrate the saved fcm token
    // and before LoadApplication() so we don't run the initial setup process
    // and after Xamarin.Forms.Forms.Init() so we can use DependencyService
    migrationHelper.Migrate();
    Forms9Patch.iOS.Settings.Initialize(this);
    LoadApplication(new App());
    bool success = base.FinishedLaunching(app, options);

    if (success)
    {
        AllGesturesRecognizer allGesturesRecognizer = new AllGesturesRecognizer(delegate
        {
            SessionManager.Instance.ExtendSession();
        });

        Window.AddGestureRecognizer(allGesturesRecognizer);
    }

    if (string.IsNullOrEmpty(AppPreferences.DeviceIdentifier))
    {
        // this will change if the user removes and reinstalls the app but there isn't anything else to use 
        AppPreferences.DeviceIdentifier = UIDevice.CurrentDevice.IdentifierForVendor.ToString();
    }

    // Test Categories. Creating them here and then just updating the categoryID in the notification extension works
    var actions1 = new List<UNNotificationAction>();
    var action11 = UNNotificationAction.FromIdentifier($"C1 B1", "C1 B1", UNNotificationActionOptions.None); // TODO: change to foregound if urlaction != post
    var action12 = UNNotificationAction.FromIdentifier($"C1 B2", "C1 B2", UNNotificationActionOptions.None);

    var actions2 = new List<UNNotificationAction>();
    var action21 = UNNotificationAction.FromIdentifier($"C2 B1", "C2 B1", UNNotificationActionOptions.None);
    var action22 = UNNotificationAction.FromIdentifier($"C2 B2", "C2 B2", UNNotificationActionOptions.None);

    actions1.Add(action11);
    actions1.Add(action12);

    actions2.Add(action21);
    actions2.Add(action22);

    var intentIDs = new string[] { };
    var category1 = UNNotificationCategory.FromIdentifier("cat1", actions1.ToArray(), intentIDs, UNNotificationCategoryOptions.AllowInCarPlay | UNNotificationCategoryOptions.AllowAnnouncement);
    var category2 = UNNotificationCategory.FromIdentifier("cat2", actions2.ToArray(), intentIDs, UNNotificationCategoryOptions.AllowInCarPlay | UNNotificationCategoryOptions.AllowAnnouncement);

    // Register category
    var categories = new UNNotificationCategory[] { category1, category2 };
    UserNotificationCenter.Current.SetNotificationCategories(new NSSet<UNNotificationCategory>(categories));
    PrintCategories();

    SetupPushNotifications(app);
    return success;
}

async void PrintCategories()
{
    try
    {
        NSSet<UNNotificationCategory> categories = await iOSNotificationCenter.Current.GetNotificationCategoriesAsync();

        // Categories is null when the categories created in the AppDelegate are commented out and moved to the Notification Extension
        if (categories == null)
        {
            App.Current.MainPage.DisplayAlert($"No categories", "", "Ok");
            return;
        }

        foreach (UNNotificationCategory c in categories)
        {
            App.Current.MainPage.DisplayAlert($"Category: {c.Identifier}", "", "Ok");
        }
    }
    catch (Exception e)
    {
        App.Current.MainPage.DisplayAlert(e.ToString(), "", "Ok");
    }
} 

NotificationService.cs（Notification Service Extension） #

public override void DidReceiveNotificationRequest(UNNotificationRequest request, Action<UNNotificationContent> contentHandler)
{
    ContentHandler = contentHandler;
    BestAttemptContent = (UNMutableNotificationContent)request.Content.MutableCopy();

    var categoryID = "but1";

    // creating the categories in the notification service extension doesn't work. The notification doesn't have any buttons 
    var actions = new List<UNNotificationAction>();
    var action1 = UNNotificationAction.FromIdentifier($"Button 1", "Button 1", UNNotificationActionOptions.None);
    var action2 = UNNotificationAction.FromIdentifier($"Button 2", "Button 2", UNNotificationActionOptions.None);

    actions.Add(action1);
    actions.Add(action2);

    var intentIDs = new string[] { };
    var category = UNNotificationCategory.FromIdentifier(categoryID, actions.ToArray(), intentIDs, UNNotificationCategoryOptions.AllowInCarPlay | UNNotificationCategoryOptions.AllowAnnouncement);

    //// Register category
    var categories = new UNNotificationCategory[] { category };
    UNUserNotificationCenter.Current.SetNotificationCategories(new NSSet<UNNotificationCategory>(categories));

    BestAttemptContent.CategoryIdentifier = categoryID;
    ContentHandler(BestAttemptContent);
}

核心原因 #

iOS系统在处理推送通知并展示操作按钮的流程中，会在触发Notification Service Extension之前就检查UNNotificationCategory的注册状态。也就是说，当你的Extension开始创建类别时，系统已经完成了对通知类别有效性的校验，此时新注册的类别无法被当前通知复用。

另外，Notification Service Extension和主App是两个独立的进程，虽然它们共享同一个UNUserNotificationCenter的底层存储，但系统在处理当前通知时，不会等待Extension中的类别注册完成再去加载按钮配置。

解决方案 #

1. 提前注册所有可能的类别模板：如果按钮的类型、数量有固定的几种组合，可以在主App启动时（FinishedLaunching）提前注册所有对应的UNNotificationCategory，通知负载中只传递类别ID即可。这也是你测试中能正常工作的方案。

2. 使用通知服务扩展配合本地通知转发：

   • 在Notification Service Extension中拦截远程通知，解析出按钮信息后，先取消原远程通知的展示。
   • 动态创建UNNotificationCategory并注册，然后创建一个本地通知，设置对应的类别ID，最后展示这个本地通知。
     注意：这种方式需要处理好通知的展示时机和权限，同时要避免重复通知。

3. 使用iOS 14+的交互式通知新API：如果你的应用只需要支持iOS 14及以上版本，可以考虑利用通知内容扩展（Notification Content Extension）来自定义通知界面，直接在扩展中渲染按钮并处理点击事件，无需依赖UNNotificationCategory。

额外说明 #

你在FinishedLaunching中打印不到Extension注册的类别，是因为主App进程和Extension进程是分开启动的。当Extension运行时，主App可能处于未启动状态，此时Extension注册的类别会被存储，但主App启动时如果没有重新注册这些类别，调用GetNotificationCategoriesAsync可能无法获取到（因为主App进程的UNUserNotificationCenter实例需要重新加载存储的类别）。

内容的提问来源于stack exchange，提问作者Tyler Wanta