Nrthugu

I have my Firestore setup in the following way:

Channels [collection] ----> channelID ---> Messages [collection] --->
messageID

How would I add snapshotListener to sub collection 'Messages' ?

  Firestore.firestore().collection("Channels").document().collection("Messages").addSnapshotListener { (querySnapshot, error) in

        guard let snapshot = querySnapshot else {

            print("Error listening for channel updates: (error?.localizedDescription ?? "No error")")

            return

        }



        snapshot.documentChanges.forEach { change in 

           print(change)

        }

    }

This didn't work for me

You can't have a single listener receive updates from an unknown number of subcollection. There are no "wildcard" operators for listeners on collections. You have to choose a specific collection or query and attach a listener to that.

I think it is because with

Firestore.firestore().collection("Channels").document().collection("Messages")

you are not defining a correct CollectionReference since you don't identify the document of the "Channels" collection.

You should do:

Firestore.firestore().collection("Channels").document(channelID).collection("Messages")

As Doug pointed out in his correct answer, you cannot have a single listener receive updates from an unknown (number of or unspecified) subcollection.

However, if you can determine those subcollection names, then the answer is pretty straightforward.

The idea is to read the child nodes of Channels, which will be channel_0, channel_1 etc and use those document id's to build references to the nodes you are interested in listening to.

So given this struture (which matches the structure in the question):

Channels

  channel_0

    Messages

      message_0

        msg: "chan 0 msg 0"

      message_1

        msg: "chan 0 msg 1"

      message_2

        msg: "chan 0 msg 2"

  channel_1

    Messages

      message_0

        msg: "chan 1 msg 0"

      message_1

        msg: "chan 1 msg 1"

Here's the code that adds listeners to each channel, and responds to events within that channels messages notifying in console the message id, msg text and the channel the event occurred in.

func addChannelCollectionObserver() {

    let channelsRef = self.db.collection("Channels")

    channelsRef.getDocuments(completion: { snapshot, error in



        guard let documents = snapshot?.documents else {

            print("Collection was empty")

            return

        }



        for doc in documents {

            let docID = doc.documentID

            let eachChannelRef = channelsRef.document(docID)

            let messagesRef = eachChannelRef.collection("Messages")



            messagesRef.addSnapshotListener { querySnapshot, error in



                querySnapshot?.documentChanges.forEach { diff in

                    if diff.type == .added {

                        let doc = diff.document

                        let msgId = doc.documentID

                        let channelId = messagesRef.parent!.documentID

                        let msg = doc.get("msg") as? String ?? "no message"

                        print(" added msgId: (msgId) with msg: (msg) in channel: (channelId)")

                    }



                    if diff.type == .modified {

                        let doc = diff.document

                        let msgId = doc.documentID

                        let msg = doc.get("msg") as? String ?? "no message"

                        print(" modified msgId: (msgId) with msg: (msg)")

                    }



                    if diff.type == .removed {

                        let doc = diff.document

                        let msgId = doc.documentID

                        let msg = doc.get("msg") as? String ?? "no message"

                        print(" removed msgId: (msgId) with msg: (msg)")

                    }

                }

            }

        }

    })

}

When first run the output will show, as expected, each child node. From then on it will output any addititions, modifications or deletions.

 added msgId: message_0 with msg: chan 0 msg 0 in channel: channel_0

 added msgId: message_1 with msg: chan 0 msg 1 in channel: channel_0

 added msgId: message_2 with msg: chan 0 msg 2 in channel: channel_0

 added msgId: message_0 with msg: chan 1 msg 0 in channel: channel_1

 added msgId: message_1 with msg: chan 1 msg 1 in channel: channel_1

the code needs some additional error checking as well for some of the optionals but it should provide a solution.