/**

 * 集合数据监听

 * @param {String} collection 集合名称

 * @param {Object} where 过滤条件

 */

function addObserver(collection, where) {

  return new Promise((resolve, reject) => {

    const db = wx.cloud.database();

    const _ = db.command;

    const watcher = db.collection(collection).where(where).watch({

      onChange: snapshot => {

        console.log('[监听器] [监听'+collection+'] 新事件 ', snapshot.docs);

        if (snapshot.docs.length > 0) {

          resolve(snapshot.docs);

        }

      }, onError: err => {

        console.error('[监听器] [监听'+collection+'] 失败');

        reject(err);

      }

    });

  });

}