# Events

> Understand the event system for tracking changes in the status and relationships of friends.

Most relationships don’t change often, but when there are changes, users want to know about them as soon as possible. The Friends SDK fetches all a user's relationships at startup, then keeps the list up-to-date using events.

The SDK sends events as soon as changes occur. User relationship changes that trigger an event include:

* A friend updates their [presence](./presence.md) status.
* The user adds a new relationship. A user creates a `FRIEND_REQUEST` relationship or a `FRIEND_REQUEST` is accepted creating a `FRIEND` relationship.
* The user removes a relationship like a `FRIEND_REQUEST`, a `FRIEND`, or a `BLOCK` relationship.

You can use callbacks that run in response to notification events to subscribe the current user to notifications.

You can subscribe (and unsubscribe) to the following event types:

| **Parameter**       | **Description**                                                                                                       |
| ------------------- | --------------------------------------------------------------------------------------------------------------------- |
| RelationshipAdded   | Event invoked when a [relationship](./relationships.md) involving the current user gets added.                        |
| RelationshipDeleted | Event invoked when a relationship involving the current user gets deleted.                                            |
| PresenceUpdated     | Event invoked when the [presence](./presence.md) of a member associated with the current user updates their presence. |
| MessageReceived     | Event invoked when a user sends a message to the current user.                                                        |