子集合的Firestore侦听器

Question

我通过以下方式设置Firestore：

通道采集-> channelID ->消息采集-> messageID

如何将snapshotListener添加到子集合'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)
        }
    }

这对我不起作用

Answer 1

您不能让单个侦听器接收来自未知数量的子集的更新。集合上的侦听器没有“通配符”运算符。您必须选择一个特定的集合或查询，并将侦听器附加到该集合或查询。

Answer 2

正如Doug在他的正确答案中指出的那样，您不能让单个侦听器接收来自未知(数量或未指定的)子集合的更新。

但是，如果您可以确定这些子集合的名称，那么答案就非常简单了。

其思想是读取Channels的子节点，即channel_0、channel_1等，并使用这些文档id构建对您感兴趣的侦听节点的引用。

因此，给定此结构(与问题中的结构相匹配)：

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"

下面的代码向每个通道添加侦听器，并响应该通道中的事件消息，在控制台中通知消息id、msg文本和发生事件的通道。

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)")
                    }
                }
            }
        }
    })
}

第一次运行时，如预期的那样，输出将显示每个子节点。从那时起，它将输出任何添加、修改或删除的内容。

 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

代码还需要对一些可选选项进行一些额外的错误检查，但它应该提供一个解决方案。

Answer 3

通过侦听具有相同名称的所有集合来使用collection group query

    db.collectionGroup("Messages"). docRef.addSnapshotListener(new EventListener<DocumentSnapshot>() {
  @Override
  public void onEvent(@Nullable DocumentSnapshot snapshot,
                      @Nullable FirestoreException e) {
    if (snapshot != null && snapshot.exists()) {
      System.out.println("Current data: " + snapshot.getData());
    } else {
      System.out.print("Current data: null");
    }
  }
});