承诺后Firestore返回

Question

我正在使用 Firestore、Cloud Functions 和 Dialogflow 来创建 Google Home 应用。我的云函数应该根据集合中的值返回一个计算，但我只能返回一个 Promise。

// query firestore based on user
var transactions = db.collection('accounts')
               .doc(request.body.result.parameters.accountowner)
               .collection('transactions');
var accountbalance = await transactions.get()
    .then(snapshot => {
        var workingbalance = 0
        snapshot.forEach(doc => {
            workingbalance = workingbalance + doc.data().amount;
        });
        return workingbalance
    })
    .catch(err => {
        console.log('Error getting transactions', err);
    });

console.log(accountbalance)
balanceresponse = {
    "speech": request.body.result.parameters.accountowner + " has a balance of $" + accountbalance,
    "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + accountbalance,
    "data": {},
    "contextOut": [],
    "source": "system"
}

response.send(balanceresponse);    

如何在调用 response.send 之前等待 promise 返回实际值？我也尝试将 response.send 放在 then 中，但函数完成并在操作完成之前发送标头。

Answer 1

如果您使用 Promise，则需要将值作为 Promise.then 链的一部分返回。这通常是通过将一堆.then() 调用/块链接在一起来完成的。请记住，then() 部分只会在 Promise 解析时被调用。

在您的情况下，您似乎可以在同一代码块内计算workingbalance 后生成balanceresponse 并调用response.send()。对response.send() 的调用不需要在函数的末尾。因此，您可能会使该块看起来像：

.then(snapshot => {
    var workingbalance = 0
    snapshot.forEach(doc => {
        workingbalance = workingbalance + doc.data().amount;
    });

    balanceresponse = {
      "speech": request.body.result.parameters.accountowner + " has a balance of $" + workingbalance,
      "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + workingbalance,
      "data": {},
      "contextOut": [],
      "source": "system"
    }

    response.send(balanceresponse); 
})

如果出于某种原因，您想分开计算并在不同的块中计算 balanceresponse，您可以这样做。我不一定推荐这个，但它有一些优点（例如，您可以在每个部分之后放置多个 catch 块，我在这里展示过）：

// query firestore based on user
var transactions = db.collection('accounts')
               .doc(request.body.result.parameters.accountowner)
               .collection('transactions');
await transactions.get()

    .then(snapshot => {
        var workingbalance = 0
        snapshot.forEach(doc => {
            workingbalance = workingbalance + doc.data().amount;
        });
        return workingbalance
    })

    .catch(err => {
        console.log('Error getting transactions', err);
    });

    .then( accountbalance => {
        balanceresponse = {
            "speech": request.body.result.parameters.accountowner + " has a balance of $" + accountbalance,
            "displayText": request.body.result.parameters.accountowner + " has an account balance of $" + accountbalance,
            "data": {},
            "contextOut": [],
            "source": "system"
        }
        response.send(balanceresponse);    
    })

    .catch(err => {
        console.log('Error sending response', err);
    });