React to Changes - .NET SDK

Register a Collection Change Listener

You can register a notification handler on a specific collection within a realm. The collection can be of realm objects (like realm.All<Person>()) or a collection property on a realm object (like house.Owners, where "Owners" is of type IList).

The handler receives a description of changes made to the collection since the last notification. Unlike realm-wide notifications, collection notifications contain detailed information about the change and provide the information you need to manage a list or other view that represents the collection in the UI.

Realm emits an initial notification when a subscription is added. After the initial notification, Realm delivers notifications asynchronously whenever a write transaction adds, modifies, or removes objects in the collection.

Specifically, the notification contains a ChangeSet with 6 properties:

• DeletedIndices is an int[] that contains the indices of the objects that were deleted.

• InsertedIndices is an int[] that contains the indices of the objects that were inserted.

• ModifiedIndices is an int[] that contains the old indices of the objects that were modified. These indices indicate the position of the modified objects in the original collection before any deletions or insertions ocurred.

• NewModifiedIndices is an int[] that represents the same entries as the ModifiedIndices property, but the indices represent the new locations in the collection after all changes have been accounted for.

• IsCleared is a boolean set to true when a collection has been cleared by calling the Clear() method.

• Moved is an array of ChangeSet.Move structs that contain the previous and new index of an object moved within the collection.

To subscribe to collection notifications, call the SubscribeForNotifications method. SubscribeForNotifications returns a subscription token which can be disposed at any time to stop receiving notifications on the collection.

The following code shows how to observe a collection for changes.

// Watch for collection notifications.
var subscriptionToken = realm.All<Dog>()
    .SubscribeForNotifications((sender, changes) =>
{
    if (changes == null)
    {
        // This is the case when the notification is called
        // for the first time.
        // Populate tableview/listview with all the items
        // from `collection`
        return;
    }
    // Handle individual changes
    foreach (var i in changes.DeletedIndices)
    {
        // ... handle deletions ...
    }
    foreach (var i in changes.InsertedIndices)
    {
        // ... handle insertions ...
    }
    foreach (var i in changes.NewModifiedIndices)
    {
        // ... handle modifications ...
    }
    if (changes.IsCleared)
    {
        // A special case if the collection has been cleared:
        // i.e., all items have been deleted by calling
        // the Clear() method.
    }
});

Handle the CollectionChanged Event

Every Realm collection implements INotifyCollectionChanged, which allows you to use a collection directly in data-binding scenarios. Because collections implement INotifyCollectionChanged, another approach to monitoring collection changes is to handle the CollectionChanged event and check for the type of NotifyCollectionChangedAction.

The following code shows you how to implement the CollectionChanged event handler:

{
    // Subscribe to a query
    realm.All<Dog>().AsRealmCollection().CollectionChanged +=
        HandleCollectionChanged;
    // Subscribe to a property collection
    gracie.Owners.AsRealmCollection().CollectionChanged +=
        HandleCollectionChanged;
   ...
}
private void HandleCollectionChanged(object? sender,
    NotifyCollectionChangedEventArgs e)
{
    // Use e.Action to get the
    // NotifyCollectionChangedAction type.
    if (e.Action == NotifyCollectionChangedAction.Add)
    {
        // etc.
    }
}