ios xmpp研究探索：获取消息列表

- (void)fetchMessageListWithCompletion:(HYBMessageListBlock)completion {
  self.messageListBlock = completion;
  
  NSManagedObjectContext *context = [self messageContext];
  NSEntityDescription *entity = [NSEntityDescription entityForName:@"XMPPMessageArchiving_Message_CoreDataObject"
                                            inManagedObjectContext:context];
  NSFetchRequest *request = [[NSFetchRequest alloc] init];
  [request setEntity:entity];
  
  if (self.currentChatingBuddyJidString != nil) {
    // 过滤内容，只找我与正要聊天的好友的聊天记录
    NSString *jidString = [NSString stringWithFormat:@"%@@%@", _jid, kServer];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(bareJidStr like %@) && (streamBareJidStr like %@)", self.currentChatingBuddyJidString , jidString];
    request.predicate = predicate;
  }
  __block NSError *error = nil;
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSArray *results =[context executeFetchRequest:request error:&error];
    NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:results.count];
    for (XMPPMessageArchiving_Message_CoreDataObject *object in results) {
      XHMessage *model = [[XHMessage alloc] initWithText:object.body
                                                  sender:object.bareJidStr
                                               timestamp:object.timestamp];
      [array addObject:model];
    }
    dispatch_async(dispatch_get_main_queue(), ^{
      if (completion) {
        completion(array, [error description]);
      }
    });
  });
}

获取消息列表，是需要过滤的，条件是：是我的好友且是我要查询的好友 的聊天记录：

 if (self.currentChatingBuddyJidString != nil) {
    // 过滤内容，只找我与正要聊天的好友的聊天记录
    NSString *jidString = [NSString stringWithFormat:@"%@@%@", _jid, kServer];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(bareJidStr like %@) && (streamBareJidStr like %@)", self.currentChatingBuddyJidString , jidString];
    request.predicate = predicate;
  }

先判断是否为空，如果当前没有正要聊天的好友，则不添加过滤条件，否则会崩溃。