code: public function build()
      {
          if (empty($this->msgType)) {
              throw new RuntimeException('message type not exist.');
          }
  
          if (empty($this->message)) {
              throw new RuntimeException('No message content to send.');
          }
  
          // 群发视频消息给用户列表时，视频消息格式需要另外处理，具体见文档
          if ($this->msgType === Broadcast::MSG_TYPE_VIDEO) {
              if (is_array($this->message)) {
                  $this->message = array_shift($this->message);
              }
              $this->msgType = 'mpvideo';
          }
  
          $content = (new Transformer($this->msgType, $this->message))->transform();
  
          $group = isset($this->to) ? $this->to : null;
  
          $message = array_merge($this->buildGroup($group), $content);
  
          return $message;
      }
docs: Build message.
  
  @return bool
  
  @throws RuntimeException