• (NSFetchedResultsController *)fetchedResultsController {

  // 推荐写法，减少嵌套的层次
  if (_fetchedResultsController != nil) {

     return _fetchedResultsController;

  }

  // 先确定需要用到哪个实体
  NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"XMPPMessageArchiving_Message_CoreDataObject"];

  // 排序
  NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"timestamp" ascending:YES];

  request.sortDescriptors = @[sort];

  // 只读取当前用户的消息
  NSLog(@"streamBareJidStr============%@",[SXXMPPTools sharedXMPPTools].xmppStream.myJID);
  *

request.predicate = [NSPredicate predicateWithFormat:@"streamBareJidStr=%@",[SXXMPPTools sharedXMPPTools].xmppStream.myJID];

---

// 从自己写的工具类里的属性中得到上下文
NSManagedObjectContext *ctx = [SXXMPPTools sharedXMPPTools].xmppMessageArchivingCoreDataStorage.mainThreadManagedObjectContext;

// 实例化，里面要填上上面的各种参数
_fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:ctx sectionNameKeyPath:nil cacheName:nil];

_fetchedResultsController.delegate = self;

return _fetchedResultsController;

}

pragma mark fetcedresults 代理方法 上下文改变

• (void)controllerDidChangeContent:(NSFetchedResultsController *)controller{

  
   [self.tableview reloadData];
  

  }

加了这句查询条件 request.predicate = [NSPredicate predicateWithFormat:@"streamBareJidStr=%@",[SXXMPPTools sharedXMPPTools].xmppStream.myJID];我加了这句查询条件数据库内容改变就不会回调了
这个就是读取XMPP聊天消息的数据 因为要切换账号登录 我不想把之前账号的历史消息也查出来所有要加这个predicate