数据添加到firestore的时候，想用onSnapshot监听，这样就可以自动调用数据了，但是不知道怎么用

Question

I want to listen for the value of message in the data structure in the attached image

I get an error with this code

secondDb.collection("chats")
  .doc(dockey)
  .onSnapshot(function (doc) {
    doc.docChanges().forEach(function (change) {
      if (change.type === "added") {
        console.log(doc.data().messages[0]);
      } else if (change.type === "modified") {
        // 
      } else if (change.type === "removed") {
        // 
      }
    })
  });

error code doc.docChanges is not a function

The data structure is as follows:

Answer 1

You can access the data over the collection you want to retrieve

var unsubscribe = firestore().collection('chats').onSnapshot(snapshot =>{
  snapshot .docChanges().map((change) => {
   if (change.type === "added") {
    console.log(doc.data().messages[0]);
   } else if (change.type === "modified") {
     // 
   } else if (change.type === "removed") {
     // 
   }
  })
})

Answer 2

Looking at your data structure, I would say that you probably don't want to call .onSnapshot on your entire collection.

Your initial code was fine. But, since you called .onSnapshot on a DocumentReference, you got a DocumentSnapshot . Instead of .docChanges you would just do doc.data().messages .

You should also check out the documentation on Best Practices for realtime updates.

If you're trying to make a realtime messaging app, you might consider using Firebase Cloud Messaging instead of onSnapshot . You could load all the messages for a conversation from Firestore when a user opens the app, and receive any new ones using FCM as long as the user is connected to the internet.