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Leave a Group

In order to stop receiving updates and messages for any particular joined group, you will have to leave the group using the leaveGroup() method. 
private String GUID = "GUID";

CometChat.leaveGroup(GUID, new CometChat.CallbackListener<String>() {
  @Override
  public void onSuccess(String successMessage) {
    Log.d(TAG, successMessage);        
  }

  @Override
  public void onError(CometChatException e) {
    Log.d(TAG, "Group leaving failed with exception: " + e.getMessage());
  }
});
ParameterDescription
GUIDThe GUID of the group you would like to leave
Once a group is left, the user will no longer receive any updates or messages pertaining to the group.

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Real-time Group Member Left Events

In other words, as a member of a group, how do I know if someone has left it when my app is running? If a user leaves a group, the members of the group receive a real-time event in the onGroupMemberLeft() method of the GroupListener class. 
CometChat.addGroupListener(UNIQUE_LISTENER_ID, new CometChat.GroupListener() {
  @Override
  public void onGroupMemberLeft(Action action, User leftUser, Group leftGroup) {
    Log.d(TAG, "User left");
  }
});

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Missed Group Member Left Events

In other words, as a member of a group, how do I know if someone has left it when my app is not running? When you retrieve the list of previous messages if a member has left any group that the logged-in user is a member of, the list of messages will contain an Action message. An Action message is a sub-class of BaseMessage class. For the group member left event, in the Action object received, the following fields can help you get the relevant information-
  1. action - left
  2. actionBy - User object containing the details of the user who left the group
  3. actionFor - Group object containing the details of the group the user has left