GmailObserver.gs

/**
 * Observe MailBox
 *
 * @param {string} condition メール検索条件(require)
 * @param {function} onSuccess 条件合致時のコールバック(optional)
 * @param {function} onFailure 例外発生時のコールバック(optional)
**/
function observe(condition, onSuccess, onFailure) {
  try {
    var observer = new GmailObserver(condition),
        response = observer.getMails();
    return onSuccess ? onSuccess(response) : response;
  } catch(ex) {
    var response = ex.toString();
    return onFailure ? onFailure(response) : response;
  }
}

/**
 * Constructor GmailObserver
 *
 * @param {string} condition メール検索条件(require)
**/
function GmailObserver(condition) {
  this.condition = condition;
  this.setProcessedIds();
}

/**
 * Set processed ids
 *
**/
GmailObserver.prototype.setProcessedIds = function () {
  var ids = PropertiesService.getUserProperties().getProperty(this.condition);
  this.processed_ids = ids ? ids.split(',') : [];
}

/**
 * Update processed ids
 *
**/
GmailObserver.prototype.updateProcessedIds = function () {
  PropertiesService.getUserProperties().setProperty(this.condition, this.processed_ids.join(','));
}

/**
 * Get mails that match the condition
 *
 * @return {object} array [GmailMessage, ...]
**/
GmailObserver.prototype.getMails = function () {
  var threads = GmailApp.search(this.condition),
      result  = [];
  for(var i in threads) {
    var messages = threads[i].getMessages();
    for(var j in messages) {
      var id = messages[j].getId();
      if (this.processed_ids.indexOf(id) == -1) {
        result.push(messages[j]);
        this.processed_ids.push(id);
      }
    }
  }
  this.updateProcessedIds();
  return result;
}