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Leave a Group

In order to stop receiving updates and messages for any particular joined group, you will have to leave the group using the leaveGroup() method. 
let guid = "cometchat-guid-11";

CometChat.leaveGroup(GUID: guid, onSuccess: { (response) in

	print("Left group successfully.")

}) { (error) in

  print("Group leaving failed with error:" + error!.errorDescription);
}
ParameterDescription
GUIDThe UID of the group you would like to leave
Once a group is left, the user will not receive any updates or messages pertaining to the group.

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Real-time Leave Group Event

In other words, as a member of a group, how do I know if someone has left it? If a user leaves any group, the members of the group receive a real-time event in the onGroupMemberLeft() method of the CometChatGroupDelegate. In order to receive user Events, you must add protocol conformance CometChatGroupDelegate as shown below : 
extension AppDelegate: CometChatGroupDelegate {

  func onGroupMemberLeft(action: ActionMessage, leftUser: User, leftGroup: Group) {

    print("\(leftUser.name) left the group \(leftGroup.name).")
  }
}
Do not forget to set your view controller as a CometChat delegate probably in viewDidLoad() as CometChat.groupdelegate = self

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Missed Group Member Left Events

In other words, as a member of a group, how do I know if someone has left it when my app is not running? When you retrieve the list of previous messages if a member has left any group that the logged-in user is a member of, the list of messages will contain an Action message. An Action message is a sub-class of BaseMessage class. For the group member left event, in the Action object received, the following fields can help you get the relevant information-
  1. action - left
  2. actionBy - User object containing the details of the user who left the group
  3. actionFor - Group object containing the details of the group the user has left