The MessageService class provides methods used to unsend messages and corresponding listeners. The SDK allows you to unsend multiple types of messages, such as unsending a message for yourself and unsending messages for all members in group chats using revokeType.

Features

Prerequisites

• The SDK is initialized.
• IM accounts are created.

Workflow

The process for recalling messages of different types is similar. This section describes how to unsend messages in a private chat between User A and User B.

API call sequence diagram

The sequence diagram below only takes the scenario where a message is unsent for all participants in a P2P chat.

Notes

1. User A and User B call onMessageRevoked to register the observer for unsending messages. The observer can track notifications NIMRevokeMessage for events of unsending messages.

   • NIMRevokeMessage representation

     Parameter	Type	Description
     attach	String	message attachment
     customInfo	String	When unsending a message using the server API ([nimservermsg/recall.action](https://doc.commsease.com/messaging/docs/zE1NjUyNDg?platform=server#request parameter)), set the value to the msg field. The field indicates the description for unsending a message. The default value is "The message was unsent".
     message	NIMMessage	The unsent message.
     notificationType	int	Type of message to be unsent. 0: online message, 1: offline message, 2: roaming message. The default value is 0.
     revokeType	RevokeMessageType	The types of unsend operations, unsending or deleting a message for all participants of P2P chats, group chats, supergroup chats, and unsending or deleting a message for the current user only, and undefined type. The types are not supported on Windows and macOS.
     	fromAccount	String	the account that unsends a message.
     callbackExt	String	custom extension field.

   • Example

     dartNimCore.instance.messageService.onMessageRevoked.listen((event) {
     print('Test_Observer onMessageRevoked ${event.toString()}');
     });
     
     
     

2. User A sends a message and unsends the sent message by calling revokeMessage. After the call is successful, the SDK will trigger a callback to notify the upper layer of your app that the message is recalled and automatically delete the local message. If you want to display an alert that you have recalled a message, you can construct an alert message and call the method of inserting local messages.

** Sample code**:

dartNimCore.instance.messageService.revokeMessage(message: message);

Notes

For the requirement of overwriting the body of an unsent message, use the following implementation. For example, User A sends a message to User B and triggers APNs push. The message body is "Hello". User A unsends the message, and the message body "Hello" displayed on the notification bar is replaced by the preset message "A message was recalled".

• Add a key-value pair of apns-collapse-id as key and uuid as value using the NIMMessage.pushPayload method. It is the unique identifier of the message.
• When the message is recalled, specify the message body in the customApnsText parameter in the revokeMessage method to override the text of the recalled message and add the same key-value pair of apns-collapse-id in pushPayload.

API reference