将异步函数传递给 onSnapshot

Question

我需要运行来自 onSnapshot 的异步函数，但 onSnapshot 本身不是异步函数。如何在收到我的集合已更改的通知后运行一个异步函数？有没有办法解决这个问题？

//Adds a listener to the user's offercandidate doc then sets the remote description to what appears in the document
let unsubscribeFromOffer = negDoc
  .collection("users")
  .doc(sessionStorage.getItem("userID"))
  .collection("offer-candidates")
  .onSnapshot(() => {
    postReturnAnswer();
  });

async function postReturnAnswer() {
  let doc = await negDoc
    .collection("users")
    .doc(sessionStorage.getItem("userID"))
    .collection("offer-candidates")
    .doc("offer")
    .get();

  let newPeerConnection = new UserConnection(
    servers,
    doc.data()["offer"]["senderID"]
  );

  newPeerConnection.userPeerConnection.setRemoteDescription(
    doc.data()["offer"]["offer"]
  );

  let connAnswerDescription =
    await peerConnection.userPeerConnection.createAnswer();

  await peerConnection.userPeerConnection.setLocalDescription(
    connAnswerDescription
  );

  await negDoc
    .collection("users")
    .doc(sessionStorage.getItem("userID"))
    .collection("answer-candidates")
    .add({
      answer: JSON.stringify(
        peerConnection.userPeerConnection.localDescription
      ),
    });

  peerConnections.push(peerConnection);
}

错误信息：

Answer 1

根据上面的代码，您将获得一个关于异步函数的文档。查看 Get a Document，它显示了如何使用 get():

正确检索单个文档的内容

我使用上面的代码设法得到了未定义的错误。使用文档中的代码可以解决您的错误。您可能需要查看以下代码：

let unsubscribeFromOffer = negDoc
  .collection("users")
  .doc(sessionStorage.getItem("userID"))
  .collection("offer-candidates")
  .onSnapshot(() => {
    postReturnAnswer();
  });
    
async function postReturnAnswer() {
  let doc = await negDoc
    .collection("users")
    .doc(sessionStorage.getItem("userID"))
    .collection("offer-candidates")
    .doc("offer")
    .get()
    // This would map the data from the document
    .then((doc) => {
      // Check if the document exists.
      if (doc.exists) {   
        let newPeerConnection = new UserConnection(
          servers,
          // On this line, you already accessed on document level so you wont need to call the document again, just call the fieldname.
          doc.data().Fieldname
        );

        newPeerConnection.userPeerConnection.setRemoteDescription(
          // Same with this one.
          doc.data().Fieldname
        );

        let connAnswerDescription =
          await peerConnection.userPeerConnection.createAnswer();

        await peerConnection.userPeerConnection.setLocalDescription(
          connAnswerDescription
        );

        await negDoc
          .collection("users")
          .doc(sessionStorage.getItem("userID"))
          .collection("answer-candidates")
          .add({
            answer: JSON.stringify(
              peerConnection.userPeerConnection.localDescription
            ),
          });

        peerConnections.push(peerConnection);
      } else {
          // doc.data() will be undefined in this case
          console.log("No such document!");
      }
    }).catch((error) => {
        console.log("Error getting document:", error);
    });
}

将异步函数传递给 onSnapshot

Passing an asynchronous function into onSnapshot

javascript

async-await

google-cloud-firestore