如何运行多个firebase承诺，然后一旦完成，执行功能答案

【问题标题】：how to run multiple firebase promises and then once completed, execute function如何运行多个firebase承诺，然后一旦完成，执行功能
【发布时间】：2018-05-26 07:11:12
【问题描述】：

我需要执行2 firebase calls 以从数据库中检索特定数据。 Once these promises resolve，我想用检索到的数据调用另一个函数。我怎样才能做到这一点？ ..Promise.All?的东西

代码如下：

app.post('/testtwilio', function(req, res) {
  //save request variables
  var to_UID = req.body.to;
  var from_UID = req.body.from;
  var experience_id = req.body.exp_id;

  //Query firebase and save 'zone_id' which we need later
  firebase.database().ref('experiences').child(experience_id).once('value').then((snap) => {
    zone_id = snap.val().ZoneID;
  });

  //Query firebase and save 'from_name' which we need later
  firebase.database().ref('users').child(from_UID).once('value').then((snap) => {
    from_name = snap.val().Name;
  });

  //Once we have the two variables returned and saved
  //Call a final firebase query and a twilio function with all the recieved data
  firebase.database().ref('users').child(to_UID).once('value').then((snap) => {
    //Do something with this aggregated data now
    client.messages.create({
      //blah blah do something with the saved data that we retrieved
      var phone = snap.val().Phone;
      var msg = from_name + zone_id + from_UID + experience_id
    });
  });
});

【问题讨论】：

标签： node.js firebase firebase-realtime-database promise

【解决方案1】：

是的，您可以使用Promise.all，因为once('value') 返回一个。

快速和肮脏的例子：

var promises = [];
promises.push(firebase.database().ref('experiences').child(experience_id).once('value'));
promises.push(firebase.database().ref('users').child(from_UID).once('value'));

// Wait for all promises to resolve
Promise.all(promises).then(function(res) {        
    // res[0] is your experience_id snapshot
    // res[1] is your from_UID snapshot
    // Do something...
});

【讨论】：

【解决方案2】：

如果您使用的是 NodeJS 7.6 及更高版本，您还可以使用 async 函数编写此代码，这样更易于阅读和维护

// ...

const wrap = require('express-async-wrap')

// ...
                        // need to wrap async function 
                        // to make it compatible with express
app.post('/testtwilio', wrap(async (req, res) => {
  const to_UID = req.body.to
  const from_UID = req.body.from
  const experience_id = req.body.exp_id

  const [
    snap1,
    snap2,
    snap3

  // waiting for all 3 promises
  ] = await Promise.all([
    firebase.database().ref('experiences').child(experience_id).once('value'),
    firebase.database().ref('users').child(from_UID).once('value'),
    firebase.database().ref('users').child(to_UID).once('value')
  ])

  const zone_id = snap1.val().ZoneID
  const from_name = snap2.val().Name
  const phone = snap3.val().Phone
  const msg = from_name + zone_id + from_UID + experience_id

  // ...
  client.messages.create(...)
}))

【讨论】：