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Grouped notifications in Xamarin.iOS

  • Article

By default, iOS 12 places all of an app's notifications in a group. The lock screen and Notification Center display this group as a stack with the most recent notification on top. Users can expand the group to see all the notifications it contains and dismiss the group as a whole.

Apps can also group notifications by thread, making it easier for users to find and interact with the specific information they're interested in.

Request authorization and allow foreground notifications

Before an app can send local notifications, it must request permission to do so. In the sample app's AppDelegate, the FinishedLaunching method requests this permission:

public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
{
    UNUserNotificationCenter center = UNUserNotificationCenter.Current;
    center.RequestAuthorization(UNAuthorizationOptions.Alert, (bool success, NSError error) =>
    {
        // Set the Delegate regardless of success; users can modify their notification
        // preferences at any time in the Settings app.
        center.Delegate = this;
    });
    return true;
}

The Delegate (set above) for a UNUserNotificationCenter decides whether or not a foreground app should display an incoming notification by calling the completion handler passed to WillPresentNotification:

[Export("userNotificationCenter:willPresentNotification:withCompletionHandler:")]
public void WillPresentNotification(UNUserNotificationCenter center, UNNotification notification, System.Action<UNNotificationPresentationOptions> completionHandler)
{
    completionHandler(UNNotificationPresentationOptions.Alert);
}

The UNNotificationPresentationOptions.Alert parameter indicates that the app should show the alert but not play a sound or update a badge.

Threaded notifications

Tap the sample app's Message with Alice button repeatedly to have it send notifications for a conversation with a friend named Alice. Since this conversation's notifications are part of the same thread, the lock screen and Notification Center group them together.

To start a conversation with a different friend, tap the Choose a new friend button. Notifications for this conversation appear in a separate group.

ThreadIdentifier

Any time the sample app starts a new thread, it creates a unique thread identifier:

void StartNewThread()
{
    threadId = $"message-{friend}";
    // ...
}

To send a threaded notification, the sample app:

  • Checks whether the app has authorization to send a notification.
  • Creates a UNMutableNotificationContent object for the notification's content and sets its ThreadIdentifier to the thread identifier created above.
  • Creates a request and schedules the notification:
async partial void ScheduleThreadedNotification(UIButton sender)
{
    var center = UNUserNotificationCenter.Current;

    UNNotificationSettings settings = await center.GetNotificationSettingsAsync();
    if (settings.AuthorizationStatus != UNAuthorizationStatus.Authorized)
    {
        return;
    }

    string author =  // ...
    string message = // ...

    var content = new UNMutableNotificationContent()
    {
        ThreadIdentifier = threadId,
        Title = author,
        Body = message,
        SummaryArgument = author
    };

    var request = UNNotificationRequest.FromIdentifier(
        Guid.NewGuid().ToString(),
        content,
        UNTimeIntervalNotificationTrigger.CreateTrigger(1, false)
    );

    center.AddNotificationRequest(request, null);

    // ...
}

All notifications from the same app with the same thread identifier will appear in the same notification group.

Note

To set a thread identifier on a remote notification, add the thread-id key to the notification's JSON payload. See Apple's Generating a Remote Notification document for more details.

SummaryArgument

SummaryArgument specifies how a notification will impact the summary text that appears on the lower-left corner of a notification group to which the notification belongs. iOS aggregates summary text from notifications in the same group to create an overall summary description.

The sample app uses the message's author as the summary argument. Using this approach, the summary text for a group of six notifications with Alice might be 5 more notifications from Alice and Me.

Unthreaded notifications

Each tap of the sample app's Appointment reminder button sends one of various appointment reminder notifications. Since these reminders are not threaded, they appear in the application-level notification group on the lock screen and in Notification Center.

To send an unthreaded notification, the sample app's ScheduleUnthreadedNotification method uses similar code as above. However, it does not set the ThreadIdentifier on the UNMutableNotificationContent object.