子集合的Firestore侦听器

我想是因为

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

你没有定义一个正确的 CollectionReference 因为您没有识别“通道”集合的文档。

你应该做:

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

正如Doug在正确答案中指出的那样,您不能让一个侦听器从未知(数量或未指定)子集合接收更新。

然而,如果你能确定这些子集合的名称,那么答案就相当简单了。

其思想是读取通道的子节点,即通道0、通道1等,并使用这些文档id来构建对您感兴趣的节点的引用。

鉴于这种结构(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"

下面的代码将侦听器添加到每个通道,并响应该通道中的事件消息在控制台中通知消息id、消息文本和事件发生的通道。

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,输出将按预期显示每个子节点。从那时起,它将输出任何添加、修改或删除。

 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

对于一些选项,代码还需要一些额外的错误检查,但它应该提供一个解决方案。

不能让一个侦听器从未知数量的子集合接收更新。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,并为其附加一个侦听器。

如果你想 订阅Firstore子集合更改 在后端,这里是来自 Firebase docs :

// Listen for changes in all documents in the 'users' collection and all subcollections
exports.useMultipleWildcards = functions.firestore
    .document('users/{userId}/{messageCollectionId}/{messageId}')
    .onWrite((change, context) => {
      // If we set `/users/marie/incoming_messages/134` to {body: "Hello"} then
      // context.params.userId == "marie";
      // context.params.messageCollectionId == "incoming_messages";
      // context.params.messageId == "134";
      // ... and ...
      // change.after.data() == {body: "Hello"}
    });

请注意,即使是子集合名称也可以与通配符一起使用 {messageCollectionId} 任何地下采集 用户集合的一部分。

根据问题 Channels [collection] ----> channelID ---> Messages [collection] ---> messageID 代码可以是:

exports.onMessagesSubcollectionChange = functions.firestore
    .document('Channels/{channelID}/Messages/{messageID}')
    .onWrite((change, context) => {
   // the rest code
   // context.params.channelID
   // and context.params.messageID will be fulfilled.
}

不确定客户端是否支持它。

使用 collection group query 为此,请收听所有同名集合

    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");
    }
  }
});