CometChat provides 4 listeners (Delegates) viz.
  1. User Delegate (Listener)
  2. Group Delegate (Listener)
  3. Message Delegate (Listener)
  4. Call Delegate (Listener)

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User Delegate Methods

The CometChat provides you with live events related to users. Below are the callback methods provided by the CometChatUserDelegate.
Delegate MethodInformation
onUserOnline:This method is triggered when a user comes online and is available to chat. The details of the user can be obtained from the user object received as the method parameter.
onUserOffline:This method is triggered when a user goes offline. The details of the user can be obtained from the User object received as the parameter.
In order to use the Delegate methods you must add protocol conformance CometChatUserDelegate as Shown Below: 
extension ViewController : CometChatUserDelegate {
    
    func onUserOnline(user: User) {
        
        print(user.stringValue() + " status becomes online.")
    }
    
    func onUserOffline(user: User) {
        
        print(user.stringValue() + " status becomes offline.")
    }
}
Do not forget to set your view controller as a CometChat delegate probably in viewDidLoad() as CometChat.userdelegate = self

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Group Delegate Methods

The CometChatGroupDelegate provides you with all the real-time events related to groups. Below are the callback methods provided by the CometChatGroupDelegate.
MethodInformation
onGroupMemberJoined(Action action, User joinedUser, Group joinedGroup)This method is triggered when a user joins any group. All the members present in the group will receive this event.
onGroupMemberLeft(Action action, User leftUser, Group leftGroup)This method is triggered when a user who was a member of any group leaves the group. All the members of the group receive this event.
onGroupMemberKicked(Action action, User kickedUser, User kickedBy, Group kickedFrom)This method is triggered when a user is kicked from a group. All the members including the user receive this event
onGroupMemberBanned(Action action, User bannedUser, User bannedBy, Group bannedFrom)This method is triggered when a user is banned from a group. All the members including the user receive this event
onGroupMemberUnbanned(Action action, User unbannedUser, User unbannedBy, Group unbannedFrom)This method is triggered when a user is banned from a group. All the members of the group receive this event.
onGroupMemberScopeChanged(Action action, User updatedBy, User updatedUser, String scopeChangedTo, String scopeChangedFrom, Group group)This method is triggered when the scope of any Group Member has been changed. All the members that are a part of that group receive this event
onMemberAddedToGroup(Action action, User addedby, User userAdded, Group addedTo)This method is triggered when a user is added to any group. All the members including the user himself receive this event.
You can receive live events related to groups, In order to receive user Events, you must add protocol conformance CometChatGroupDelegate as shown below: 
extension AppDelegate: CometChatGroupDelegate {

  func onGroupMemberJoined(action: ActionMessage, joinedUser: User, joinedGroup: Group) {

    print("\(joinedUser.name) joined the group \(joinedGroup.name).")
  }

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

    print("\(leftUser.name) left the group \(leftGroup.name).")
  }

  func onGroupMemberKicked(action: ActionMessage, kickedUser: User, kickedBy: User, kickedFrom: Group) {

    print("\(kickedUser.name) kicked from the group \(kickedFrom.name) by \(kickedBy.name).")
  }

  func onGroupMemberBanned(action: ActionMessage, bannedUser: User, bannedBy: User, bannedFrom: Group) {

    print("\(bannedUser.name) banned from the group \(bannedFrom.name) by \(bannedBy.name).")
  }

  func onGroupMemberUnbanned(action: ActionMessage, unbannedUser: User, unbannedBy: User, unbannedFrom: Group) {

    print("\(unbannedUser.name) unbanned from the group \(unbannedFrom.name) by \(unbannedBy.name).")
  }

  func onGroupMemberScopeChanged(action: ActionMessage, scopeChangeduser: User, scopeChangedBy: User, scopeChangedTo: String, scopeChangedFrom: String, group: Group) {
    
    print("\(scopeChangedUser.name) scope changed from \(scopeChangedFrom) to \(scopeChangedTo) by \(scopeChangedBy.name) in the group \(group.name).")
  }
}
Do not forget to set your view controller as a CometChat delegate probably in viewDidLoad() as CometChat.groupdelegate = self

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Message Delegate Methods

The CometChatMessageDelegate provides you with live events related to messages. Below are the callback methods provided by the CometChatMessageDelegate.
MethodInformation
onTextMessageReceived(TextMessage message)This event is triggered when a Text Message is received.
onMediaMessageReceived(MediaMessage message)This event is triggered when a Media Message is received.
onCustomMessageReceived(CustomMessage message)This event is triggered when a Custom Message is received.
onTypingStarted(TypingIndicator typingIndicator)This event is triggered when a user starts typing in a user/group conversation
onTypingEnded(TypingIndicator typingIndicator)This event is triggered when a user stops typing in a user/group conversation.
onMessagesDelivered(MessageReceipt messageReceipt)This event is triggered when a set of messages are marked as delivered for any particular conversation.
onMessagesRead(MessageReceipt messageReceipt)This event is triggered when a set of messages are marked as read for any particular conversation.
onMessageEdited(BaseMessage message)This method is triggered when a particular message has been edited in a user/group conversation.
onMessageDeleted(BaseMessage message)This event is triggered when a particular message is deleted in a user/group conversation.
In order to receive incoming messages, you must add protocol conformance CometChatMessageDelegate as Shown Below : 
extension ViewController: CometChatMessageDelegate {

  func onTextMessageReceived(textMessage: TextMessage) {

    print("TextMessage received successfully: " + textMessage.stringValue())
  }

  func onMediaMessageReceived(mediaMessage: MediaMessage) {

    print("MediaMessage received successfully: " + mediaMessage.stringValue())
  }
  
  func onCustomMessageReceived(customMessage: CustomMessage) {

    print("CustomMessage received successfully: " + customMessage.stringValue())
  }
  
  func onMessageEdited(message: BaseMessage) {
        
        print("received edited message successfully.")
    }
  func onMessageDeleted(message: BaseMessage) {
        
        print("received deleted message successfully.")
    }
  func onTypingStarted(_ typingDetails : TypingIndicator) {

    print("Typing started received successfully")
  }

  func onTypingEnded(_ typingDetails : TypingIndicator) {

    print("Typing ended received successfully")
  }
  func onMessageDelivered(receipt : MessageReceipt) {
       
        print("Message delivered receipt received.")
    }
    
    func onMessageRead(receipt : MessageReceipt) {
        
        print("Message read receipt received.")
    }
}
Do not forget to set your view controller as a CometChat delegate probably in viewDidLoad() as CometChat.messagedelegate = self

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Call Delegate Methods

The CometChatCallDelegate provides you with live events related to calls. Below are the callback methods provided by the CometChatCallDelegate.
MethodInformation
onIncomingCallReceived(Call call)This event is triggered when the logged-in user receives an incoming call. The details of the call can be obtained from the Call object received as the method parameter.
onOutgoingCallAccepted(Call call)This event is triggered when the call initiated by the logged-in user is accepted by the recipient. The details of the call can be obtained from the Call object received as the method parameter.
onOutgoingCallRejected(Call call)This event is triggered when the call initiated by the logged-in user is rejected by the recipient. The details of the call can be obtained from the Call object received as the method parameter
onIncomingCallCancelled(Call call)This event is triggered when an incoming call is canceled by the initiator of the call. The details of the call can be obtained from the Call object received as the method parameter
In order to receive all call events, you must add protocol conformance CometChatCallDelegate as Shown Below : 
extension ViewController: CometChatCallDelegate {

  func onIncomingCallReceived(incomingCall: Call ? , error : CometChatException ? ) {
    print(" Incoming call " + incomingCall!.stringValue());
  }

  func onOutgoingCallAccepted(acceptedCall: Call ? , error : CometChatException ? ) {
    print("Outgoing call " + acceptedCall!.stringValue());
  }

  func onOutgoingCallRejected(rejectedCall: Call ? , error : CometChatException ? ) {
    print("Rejected call " + rejectedCall!.stringValue());
  }
  func onIncomingCallCanceled(canceledCall: Call ? , error : CometChatException ? ) {
    print("Cancelled call " + canceledCall!.stringValue());
  }
}
Do not forget to set your view controller as a CometChat delegate probably in viewDidLoad() as CometChat.calldelegate = self