Using CometChat, you can send three types of messages:
  1. A Text Message, the most common and standard message type.
  2. A Media Message, for sending photos, videos and files.
  3. A Custom Message, for sending completely custom data using Map structures.
  4. A Interactive Messages , for sending end-user interactive messages of type form, card and custom Interactive
You can also send metadata along with a text or media message. Think, for example, if you’d want to share the user’s location with every message, you can use the metadata field.

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Text Message

In other words, as a sender, how do I send a text message? To send a text message to a single user or group, you need to use the sendMessage() method and pass a TextMessage object to it.

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Add Metadata

To send custom data along with a text message, you can use the setMetadata method and pass a Map to it. 
Map<String,String> metadata = {};
metadata["lattitude"] =  "50.6192171633316" ;
metadata["longitude"]  = "-72.68182268750002" ;
textMessage.metadata = metadata;

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Add Tags

To add a tag to a message you can assign value in .tags variable of the TextMessage Class. tags accepts a list of tags. 
List<String> tags = [];
tags.add("pinned");
textMessage.tags = tags;
Once the text message object is ready, you need to use the sendMessage() method to send the text message to the recipient. 
String receiverID = "cometchat-uid-1";
String messageText = "messageText";
String receiverType = CometChatConversationType.user;
String type = CometChatMessageType.text;

TextMessage textMessage = TextMessage(text: messageText,
                                    receiverUid: receiverID,
          													receiverType: receiverType,
           													type: type);
CometChat.sendMessage(textMessage, 
  onSuccess: (TextMessage message) {
	debugPrint("Message sent successfully:  $message");
  }, onError: (CometChatException e) {
	debugPrint("Message sending failed with exception:  ${e.message}");
  }
);
The TextMessage class constructor takes the following parameters:
ParameterDescription
receiverIDUID of the user or GUID of the group receiving the messageRequired
messageTextThe text messageRequired
receiverTypeThe type of the receiver- CometChatReceiverType.user (user) or CometChatReceiverType.group (group)Required
typeThe type of the message that needs to be sent which in this case can be: CometChatMessageType.text__(text)
When a text message is sent successfully, the response will include a TextMessage object which includes all information related to the sent message.

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Media Message

In other words, as a sender, how do I send a media message like photos, videos & files? To send a media message to any user or group, you need to use the sendMediaMessage() method and pass a MediaMessage object to it.

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Add Metadata

To send custom data along with a media message, you can use the setMetadata method and pass a Map to it. 
Map<String,dynamic> metadata = {};
metadata["lattitude"] =  "50.6192171633316" ;
metadata["longitude"]  = "-72.68182268750002" ;
mediaMessage.metadata = metadata;

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Add Caption(Text along with Media Message)

mediaMessage.caption = "Message Caption";

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Add Tags

To add a tag to a message you can use the setTags() method of the MediaMessage Class. The setTags() method accepts a list of tags. 
List<String> tags = [];
tags.add("pinned");
mediaMessage.tags = tags;
There are 2 ways you can send Media Messages using the CometChat SDK:
  1. By providing the File : You can directly share the file object while creating an object of the MediaMessage class. When the media message is sent using the sendMediaMessage() method, this file is then uploaded to CometChat servers and the URL of the file is sent in the success response of the sendMediaMessage() function.
String receiverID;
String messageType = CometChatMessageType.image;
String receiverType = CometChatConversationType.user ;
String filePath = "storage/emulated/0/Download/46.jpg";
MediaMessage mediaMessage = MediaMessage(
receiverType: receiverType,
type: messageType,
receiverUid: receiverID,
file: filePath);

await CometChat.sendMediaMessage(mediaMessage, 
  onSuccess: (MediaMessage message) {
  	debugPrint("Media message sent successfully:${mediaMessage.metadata}");
  }, onError: (e) {
  	debugPrint("Media message sending failed with exception: ${e.message}");
  }
);
The MediaMessage class constructor takes the following parameters:
ParameterDescription
receiverIdThe UID or GUID of the recipientRequired
fileThe file object to be sentRequired
messageTypeThe type of the message that needs to be sent which, in this case, can be:
1. CometChatMessageType.image (image)
2. CometChatMessageType.video (video)
3. CometChatMessageType.audio (audio)
4. CometChatMessageType.file (file)		Required
receiverTypeThe type of the receiver to whom the message is to be sent, i.e., CometChatReceiverType.user (user) or CometChatReceiverType.group (group)Required
  1. By providing the URL of the File: The second way to send media messages using the CometChat SDK is to provide the SDK with the URL of any file that is hosted on your servers or any cloud storage. To achieve this you will have to make use of the Attachment class that is available in the MediaMessage class. For more information, you can refer to the below code snippet:
MediaMessage mediaMessage = MediaMessage(
                          receiverType: receiverType,
                          type: messageType,
                          receiverUid: receiverID,
                          file: null);

String fileUrl  = "https://pngimg.com/uploads/mario/mario_PNG125.png";
String fileName   = "test";
String fileExtension = "png";
String fileMimeType = "image/png";

Attachment attach =  Attachment(fileUrl,fileName,fileExtension,fileMimeType,null);
mediaMessage.attachment= attach;

await CometChat.sendMediaMessage(mediaMessage,
  onSuccess: (MediaMessage message) {
  	debugPrint( "Media message sent successfully: ${mediaMessage}");                            
  }, onError: (CometChatException e) {
  	debugPrint("Media message sending failed with exception: ${e.message}");
  }
);
When a media message is sent successfully, the response will include a MediaMessage object which includes all information related to the sent message. If you wish to send a caption or some text along with the Media Message, you can use the caption field provided by the MediaMessage class. To get and set the caption you can use the .caption variable . As with text messages, the metadata field can be used with media messages as well. Any additional information can be passed along with the media message as a Map.

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Custom Message

In other words, as a sender, how do I send a custom message like location co-ordinates? CometChat allows you to send custom messages which are neither text nor media messages. In order to send a custom message, you need to use the sendCustomMessage() method. The sendCustomMessage() methods takes an object of the CustomMessage which can be obtained using the below constructor. 
CustomMessage customMessage = CustomMessage( receiverUid: UID,
    type: type,
    customData: customData,
    receiverType: receiverType,
    subType: subType,
  );
The above constructor, helps you create a custom message with the message type set to whatever is passed to the constructor and the category set to custom. The parameters involved are:
  1. receiverUid - Unique id of the user or group to which the message is to be sent.
  2. receiverType - Type of the receiver i.e user or group
  3. customType - custom message type that you need to set
  4. customData - The data to be passed as the message in the form of a Map.
You can also use the subType field of the CustomMessage class to set a specific type for the custom message. This can be achieved using the subtype field.

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Add Tags

To add a tag to a message you can assign value in .tags variable of the CustomMessage Class. tags accepts a list of tags. 
List<String> tags = [];
tags.add("pinned");
textMessage.tags = tags;
Once the object of CustomMessage class is ready you can send the custom message using the sendCustomMessage() method. 
String UID = "UID";
String subType = "LOCATION";
String receiverType = CometChatConversationType.user;
String type = CometChatMessageType.custom;

Map<String, String> customData = {};
customData["latitude"] = "19.0760";
customData["longitude"] = "72.8777";

CustomMessage customMessage = CustomMessage(
  receiverUid: UID,
  type: type,
  customData: customData,
  receiverType: receiverType,
  subType: subType,
);

CometChat.sendCustomMessage(customMessage, onSuccess: (CustomMessage message) {
  debugPrint("Custom Message Sent Successfully : $message");
}, onError: (CometChatException e) {
  debugPrint("Custom message sending failed with exception: ${e.message}");
});
The above sample explains how custom messages can be used to share the location with a user. Similarly, you can send custom messages to groups. On success, you will receive an object of CustomMessage class.

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Update Conversation

How can I decide whether the custom message should update the last message of a conversation? By default, a custom message will update the last message of a conversation. If you wish to not update the last message of the conversation when a custom message is sent, please use updateConversation (boolean value) method of the Custom Message. 
String UID = "UID";
String subType = "LOCATION";
String receiverType = CometChatConversationType.user;
String type = CometChatMessageType.custom;

Map<String, String> customData = {};
customData["latitude"] = "19.0760";
customData["longitude"] = "72.8777";

CustomMessage customMessage = CustomMessage(
  receiverUid: UID,
  type: type,
  customData: customData,
  receiverType: receiverType,
  subType: subType,
);
customMessage.updateConversation = false;

CometChat.sendCustomMessage(customMessage, onSuccess: (CustomMessage message) {
  debugPrint("Custom Message Sent Successfully : $message");
}, onError: (CometChatException e) {
  debugPrint("Custom message sending failed with exception: ${e.message}");
});

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Custom Notification Body

How can i customise the notification body of custom message? To add a custom notification body for Push, Email & SMS notification of a custom message you can use the conversationText method of Custom Message class. 
String UID = "UID";
String subType = "LOCATION";
String receiverType = CometChatConversationType.user;
String type = CometChatMessageType.custom;

Map<String, String> customData = {};
customData["latitude"] = "19.0760";
customData["longitude"] = "72.8777";

CustomMessage customMessage = CustomMessage(
  receiverUid: UID,
  type: type,
  customData: customData,
  receiverType: receiverType,
  subType: subType,
);
customMessage.conversationText = "Custom Notification Body";

CometChat.sendCustomMessage(customMessage, onSuccess: (CustomMessage message) {
  debugPrint("Custom Message Sent Successfully : $message");
}, onError: (CometChatException e) {
  debugPrint("Custom message sending failed with exception: ${e.message}");
});
It is also possible to send interactive messages from CometChat , to know more click here