Firestore实时监听多个文档的实时监听器 [英] Firestore realtime listener to multiple documents async await

问题描述

我想知道Firestore的实时侦听器是否支持 async await 而不是promise?

I am wondering if the realtime listener for Firestore supports async await instead of promise?

文档建议:

var unsubscribe = db.collection("cities").where("state", "==", "CA").onSnapshot(function(querySnapshot) {
    var cities = [];
    querySnapshot.forEach(function(doc) {
       cities.push(doc.data().name);
    });
    console.log("Current cities in CA: ", cities.join(", "));
});
unsubscribe();

我可以使用 async await 编写上述实时侦听器吗?我尝试了以下操作，但侦听器不再起作用.另外，将无法再分离侦听器.

Could I write the above realtime listener using async await? I tried the following and the listener does not work anymore. Also, it won't be possible to detach the listener anymore.

var querySnapshot = db.collection("cities").where("state", "==", "CA").onSnapshot
var cities = [];
querySnapshot.forEach(function(doc) {
   cities.push(doc.data().name);
});
console.log("Current cities in CA: ", cities.join(", "));

我如何使用 async await 编写它，并且还可以使用分离器?

How could I write it using async await and would be able to use the detacher as well?

推荐答案

async/await(与promises一起使用)与侦听器一起使用是没有意义的.承诺表示以最终值或错误结尾的工作单元.侦听器是一个持续的过程，直到使用取消订阅功能将其删除后，该过程才会结束.它们是根本不同的东西.

async/await (used with promises) doesn't make sense to use with listeners. A promise represents a unit of work that finishes with a final value, or an error. A listener is an ongoing process that doesn't finish until the listener is removed using the unsubscribe function. They are fundamentally different things.

如果您要进行一次查询以保证可以等待，则应使用 get()代替如上所述的 onSnapshot()在文档中.当结果随时间变化时，如果要更新查询，请仅使用侦听器.

If you want to do a one-time query that gives you a promise that you can await, you should use get() instead of onSnapshot() as described in the documentation. Only use a listener if you want updates to a query as the results change over time.

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