Messages that are started from a particular message are called Threaded messages or simply threads. Each Thread is attached to a message which is the Parent message for that thread.

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Send Message in a Thread

As mentioned in the Send a Message section. You can either send a message to a User or a Group based on the receiverType and the UID/GUID specified for the message. A message can belong to either of the below types:
  1. Text Message
  2. Media Message
  3. Custom Message.
Any of the above messages can be sent in a thread. As mentioned, a thread is identified based on the Parent message. So while sending a message the parentMessageId must be set for the message to indicate that the message to be sent needs to be a part of the thread with the specified parentMessageId. This can be achieved using the parentMessageId property provided by the object of the TextMessage, MediaMessage, and CustomMessage class. The id specified in the parentMessageId property maps the message sent to the particular thread. Example to Send a Text Message in a thread in a user conversation. 
let receiverID = "cometchat-uid-2"
let text = "Hello"

let textMessage = TextMessage(receiverUid: receiverID, text: text, receiverType: .user)
textMessage?.parentMessageId = 100
CometChat.sendTextMessage(message: textMessage, onSuccess: { (message) in

  print("TextMessage sent successfully. " + message.stringValue())

}) { (error) in

  print("TextMessage sending failed with error: " + error!.errorDescription);
}
The above snippet shows how a message with the text “Hello” can be sent in the thread with parentMessageId 100. Similarly, using the parentMessageId property, Media and Custom Messages can be sent in threads too.

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Receiving Real-Time Messages

The procedure to receive real-time messages is exactly the same as mentioned in the Receive Messages. This can be achieved using the CometChatMessageDelegate class provided by the SDK. In order to receive incoming messages, you must add protocol conformance CometChatMessageDelegate as Shown in the below code example. The only thing that needs to be checked is if the received message belongs to the active thread. This can be done using the parentMessageId field of the message object. 
var activeThreadId = 100;

extension ViewController: CometChatMessageDelegate {

  func onTextMessageReceived(textMessage: TextMessage) {
		if textMessage.id == activeThreadId {
    	print("TextMessage received successfully: " + textMessage.stringValue())
    }
  }

  func onMediaMessageReceived(mediaMessage: MediaMessage) {
	  if textMessage.id == activeThreadId {
    	print("MediaMessage received successfully: " + mediaMessage.stringValue())
    }
  }
  
  func onCustomMessageReceived(customMessage: CustomMessage) {
		if textMessage.id == activeThreadId {
    	print("CustomMessage received successfully: " + customMessage.stringValue())
    }
  }
}

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Fetch all the messages for any particular thread.

You can fetch all the messages belonging to a particular thread by using the MessagesRequest class. In order to get an object of the MessagesRequest class, you need to use the MessagesRequestBuilder class. and use the setParentMessageId() method of the MessagesRequestBuilder to inform the SDK that you only need the messages belonging to the thread with the specified parentMessageId. Once you have the object of the MessagesRequest class, you need to call the fetchPrevious() method to get the latest messages in the thread. In one integration, a maximum of 100 messages can be fetched. If you wish to fetch the next set of messages, you need to call the fetchPrevious() method again on the same object. 
let limit = 50;

let messagesRequest = MessageRequestBuilder().setParentMessageId(parentMessageId: 100).set(limit: limit).build()

messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as ? TextMessage) {
      print("Text Messages for thread fetched successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as ? MediaMessage) {
      print("Media Messages for thread fetched successfully. " + receivedMessage!.stringValue())
  }
}

}) { (error) in

  print("Messages fetching failed with error: " + error!.errorDescription);
}

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Avoid Threaded Messages in User/Group Conversations

While fetching messages for normal user/group conversations using the MessagesRequest, the threaded messages by default will be a part of the list of messages received. In order to exclude the threaded messages from the list of user/group messages, you need to use the hideReplies() method of the MessagesRequestBuilder class. This method takes a boolean argument which when set to true excludes the messages belonging to threads from the list of messages. 
let limit = 50;

let messageRequest = MessagesRequest.MessageRequestBuilder().set(uid: "cometchat-uid-1").set(limit: limit).hideReplies(hide: true).build()
       
messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as ? TextMessage) {
      print("Text Messages for thread fetched successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as ? MediaMessage) {
      print("Media Messages for thread fetched successfully. " + receivedMessage!.stringValue())
  }
}

}) { (error) in

  print("Messages fetching failed with error: " + error!.errorDescription);
}
The above snippet will return messages between the logged in user and cometchat-uid-1 excluding all the threaded messages belonging to the same conversation.