Receiving messages with CometChat has two parts:
  1. Adding a listener to receive real-time messages when your app is running
  2. Calling a method to retrieve missed messages when your app was not running

​
Real-time Messages

In other words, as a recipient, how do I receive messages when my app is running? 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())
  }
}
Do not forget to set your view controller as a CometChat delegate probably in viewDidLoad() as CometChat.messagedelegate = self
As a sender, you will not receive your own message in a real-time message event. However, if a user is logged-in using multiple devices, they will receive an event for their own message in other devices.

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Missed Messages

In other words, as a recipient, how do I receive messages that I missed when my app was not running? For most cases, you will need to add functionality to load missed messages. If you’re building an on-demand or live streaming app, you may choose to skip this and fetch message history instead. Using the same MessagesRequest class and the filters provided by the MessagesRequestBuilder class, you can fetch the message that we sent to the logged-in user but were not delivered to him as he was offline. For this, you will require the Id of the last message received. You can either maintain it at your end or use the getLastDeliveredMessageId() method provided by the CometChat class. This Id needs to be passed to the setMessageId() method of the builder class. Now using the fetchNext() method, you can fetch all the messages that were sent to the user when he/she was offline. Calling the fetchNext() method on the same object repeatedly allows you to fetch all the offline messages for the logged in user in a paginated manner

​
Fetch Missed Messages of a particular one-on-one conversation

let limit = 50;
let latestId = CometChat.getLastDeliveredMessageId()
let UID = "cometchat-uid-2"

let messagesRequest = MessagesRequest.MessageRequestBuilder().set(messageID: latestId).set(uid:UID).set(limit: limit).build();

messagesRequest.fetchNext(onSuccess: { (messages) in

 for message in messages!{

    if let receivedMessage = (message as? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as? MediaMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
  	}
  }

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}

​
Fetch Missed Messages of a particular group conversation

let limit = 50;
let latestId = CometChat.getLastDeliveredMessageId()
let GUID = "cometchat-guid-1"

let messagesRequest = MessagesRequest.MessageRequestBuilder().set(messageID: latestId).set(guid:GUID).set(limit: limit).build();

messagesRequest.fetchNext(onSuccess: { (messages) in

 for message in messages!{

    if let receivedMessage = (message as? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as? MediaMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
  	}
  }

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}

​
Unread Messages

In other words, as a logged-in user, how do I fetch the messages I’ve not read? Using the MessagesRequest class described above, you can fetch the unread messages for the logged-in user. In order to achieve this, you need to set the unread variable in the builder to true using the setUnread() method so that only the unread messages are fetched.

​
Fetch Unread Messages of a particular one-on-one conversation

let limit = 50;
let UID = "cometchat-uid-2"
let messagesRequest = MessagesRequest.MessageRequestBuilder().set(unread: true).set(limit: limit).set(uid:UID).build();

messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as? MediaMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
  	}
  }

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}

​
Fetch Unread Messages of a particular group conversation

let limit = 50;
let GUID = "cometchat-uid-2"
let messagesRequest = MessagesRequest.MessageRequestBuilder().set(unread: true).set(limit: limit).set(guid:GUID).build();

messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as? MediaMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
  	}
  }

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}
BaseMessageThe list of messages received is in the form of objects of BaseMessage class. A BaseMessage can either be a TextMessage or a MediaMessage. You can use the instanceOf operator to check the type of object.

​
Message History

In other words, as a logged-in user, how do I fetch the complete message history?

​
Fetch Message History of a particular one-on-one conversation

Using the MessagesRequest class and the filters for the MessagesRequestBuilder class as shown in the below code snippet, you can fetch the entire conversation between the logged in user and any particular user. For this use case, it is mandatory to set the UID parameter using the setUID() method of the builder. This UID is the unique id of the user for which the conversation needs to be fetched. 
let limit = 50;
let UID = "cometchat-uid-2"

let messagesRequest = MessagesRequest.MessageRequestBuilder().set(limit: limit).set(uid:UID).build();

messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as ? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as ? MediaMessage) {
      print("Message received successfully. " + receivedMessage!.stringValue())
  }
}

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}
Calling the fetchPrevious() method on the same object repeatedly allows you to fetch all the previous messages in a paginated way.

​
Fetch Message History of a particular group conversation

Using the MessagesRequest class and the filters for the MessagesRequestBuilder class as shown in the below code snippet, you can fetch the entire conversation for any group provided you have joined the group. For this use case, it is mandatory to set the GUID parameter using the setGUID() method of the builder. This GUID is the unique identifier of the Group for which the messages are to be fetched. 
let limit = 50;
let GUID = "cometchat-guid-1"

let messagesRequest = MessagesRequest.MessageRequestBuilder().set(limit: limit).set(guid:GUID).build();

messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as ? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as ? MediaMessage) {
      print("Message received successfully. " + receivedMessage!.stringValue())
  }
}

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}
Calling the fetchPrevious() method on the same object repeatedly allows you to fetch the entire conversation between the logged in user and the specified user. This can be implemented with upward scrolling to display the entire conversation.

​
Search Messages

In other words, as a logged-in user, how do I search a message? In order to do this, you can use the set(searchKeyword: String) method. This method accepts string as input. When set, the SDK will fetch messages which match the searchKeyword.
By default, the searchKey is searched only in message text. However, if you enable Conversation & Advanced Search, the searchKey will be searched in message text, file name, mentions & mime type of the file.The Conversation & Advanced Search is only available in Advanced & Custom plans. If you’re already on one of these plans, please enable the Conversation & Advanced Search from CometChat Dashboard (Open your app, navigate to Chats -> Settings -> General Configuration)
FeatureBasic SearchAdvance Search
Text search✅✅
File name search❌✅
Mentions search❌✅
Mime type search❌✅

​
Search Messages in a particular one-on-one conversation

let limit = 50;
let UID = "cometchat-uid-2"

let messagesRequest = MessagesRequest.MessageRequestBuilder().set(uid: UID).set(limit: limit).set(searchKeyword: "Hello").build();

messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as ? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as ? MediaMessage) {
      print("Message received successfully. " + receivedMessage!.stringValue())
  	}
  }

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}

​
Search Messages in a particular group conversation

let limit = 50;
let GUID = "cometchat-guid-1"

let messagesRequest = MessagesRequest.MessageRequestBuilder().set(guid: GUID).set(limit: limit).set(searchKeyword: "Hello").build();

messagesRequest.fetchPrevious(onSuccess: { (messages) in

  for message in messages!{

    if let receivedMessage = (message as ? TextMessage) {
      print("Message received successfully. " + receivedMessage.stringValue())
    }
    else if let receivedMessage = (message as ? MediaMessage) {
      print("Message received successfully. " + receivedMessage!.stringValue())
  	}
  }

}) { (error) in

  print("Message receiving failed with error: " + error!.errorDescription);
}

​
Unread Messages Count

In other words, as a logged-in user, how do I find out the number of unread messages that I have?

​
Fetch Unread Message Count of a particular one-on-one conversation

In other words, how do I find out the number of unread messages I have from a particular user? In order to get the unread message count for a particular user (with respect to the logged-in user), you can use the getUnreadMessageCountForUser(). This method has the two variants: 
CometChat.getUnreadMessageCountForUser(UID)
If you wish to ignore the messages from blocked users you can use the below syntax setting the boolean parameter to true: 
CometChat.getUnreadMessageCountForUser(UID, hideMessagesFromBlockedUsers:true)

let UID = "cometchat-uid-1"

CometChat.getUnreadMessageCountForUser(UID, onSuccess: { (response) in

    print("Unread count for users: \(response)")

}) { (error) in

    print("Error in unread count for users: \(error)")
}
In the onSuccess() callback, you will receive a dictionary that will contain the UID of the user as the key and the unread message count as the value.

​
Fetch Unread Message Count of a particular group conversation

In order to get the unread message count for a particular group, you can use the getUnreadMessageCountForGroup(). This method has the below two variants: 
CometChat.getUnreadMessageCountForGroup(GUID)
If you wish to ignore the messages from blocked users you can use the below syntax setting the boolean parameter to true: 
CometChat.getUnreadMessageCountForGroup(GUID, hideMessagesFromBlockedUsers:true)

let GUID = "cometchat-guid-1"

CometChat.getUnreadMessageCountForGroup(GUID, onSuccess: { (response) in

    print("Unread count for groups: \(response)")

}) { (error) in

    print("Error in unread count for group: \(error.errorDescription)")
}
In the onSuccess() callback, you will receive a dictionary that will contain the GUID of the group as the key and the unread message count as the value.

​
Fetch Unread Message Count of all one-on-one & group conversation

In other words, how do I find out the number of unread messages for every one-on-one and group conversation? In order to get all the unread message counts, you can use the getUnreadMessageCount() method. This method has two variants: 
CometChat.getUnreadMessageCount(callbacks)
If you wish to ignore the messages from blocked users you can use the below syntax setting the boolean parameter to true: 
CometChat.getUnreadMessageCount(hideMessagesFromBlockedUsers:true, callbacks)

CometChat.getUnreadMessageCount(onSuccess: { (response) in

    print("Unread message count: \(response)")

}) { (error) in

    print("Error in fetching unread count: \(error)")
}
In the onSuccess() callback, you will receive a dictionary having two keys:
  1. user - The value for this key holds another dictionary that holds the UIDs of the users and their corresponding unread message counts
  2. group - The value for this key holds another dictionary that holds the GUIDs of the groups and their corresponding unread message counts

​
Fetch Unread Message Count of all one-on-one conversation

In other words, how do I find out the number of unread messages I have for every user? In order to fetch the unread message counts for just the users, you can use the getUnreadMessageCountForAllUsers() method. This method, just like others, has two variants: 
CometChat.getUnreadMessageCountForAllUsers(callbacks)
If you wish to ignore the messages from blocked users you can use the below syntax setting the boolean parameter to true: 
CometChat.getUnreadMessageCountForAllUsers(hideMessagesFromBlockedUsers:true, callbacks)

CometChat.getUnreadMessageCountForAllUsers(onSuccess: { response in

    print("Unread count for all users: \(response)")

}) { (error) in

    print("Error in fetching unread count for all users: \(error?.errorDescription)")
}
In the onSuccess() callback, you will receive a dictionary that will contain the UIDs of the users as the key and the unread message counts as the values.

​
Fetch Unread Message Count of all group conversation

In other words, how do I find out the number of unread messages for every group? In order to fetch the unread message counts for all groups, you can use the getUnreadMessageCountForAllGroups() method. This method has two variants: 
CometChat.getUnreadMessageCountForAllGroups(callbacks)
If you wish to ignore the messages from blocked users you can use the below syntax setting the boolean parameter to true: 
CometChat.getUnreadMessageCountForAllGroups(hideMessagesFromBlockedUsers:true, callbacks)

CometChat.getUnreadMessageCountForAllGroups(onSuccess: { response in

    print("Unread count for all groups: \(response)")

}) { (error) in

    print("Error in fetching unread count for all groups: \(error.errorDescription)")
}
In the onSuccess() callback, you will receive a dictionary that will contain the GUIDs of the groups as the key and the unread message counts as the values.