Pfthb

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.