如何获取按时间戳排序的消息

Question

我想按创建日期（即时间戳）获取所有消息。

我的数据结构是这样的：

messages
  XRJONcahNUgeNrfkH3cmITJt3409 (auto uid of owner - user)
    -KoOSxA2Iajk3kv20U_p (automatic uid generated by Firebase)
      message: "Hi"
      senderId: "X8Oxg5J9Imb12C1iYBXJT9P48909"
      senderName: "Dave"
      timestamp: 1505813320526 (automatic timestamp)
    -GdOSxR2Iajk3kv20U_p --- I NEED TO SORT THESE UIDS
      message: "Hello"
      senderId: "G7Oxg5J9Imb12C1iYBXJT9P48909"
      senderName: "Frank"
      timestamp: 1505813321024 (automatic timestamp)

我收到所有这样的消息：

database.child("messages").child(ownersId).observeSingleEvent(of: .value, with: { (snapshot) in
            print(snapshot)
            if let messagesDict = snapshot.value as? [String:AnyObject] {
                for (_,message) in messagesDict {
                    self.messages.append((message["message"] as? String)!)
                    self.users.append((message["senderName"] as? String)!)
                    self.timestamps.append((message["timestamp"] as? IntMax)!)
                }
            }
            completionHandler(true)

        }) { (error) in
            print(error.localizedDescription)
            completionHandler(false)
        }

所以问题是，我怎样才能让所有消息从最旧到最新排序？我不知道在这种特定情况下如何使用排序方法（queryOrderedByKey()、queryOrderedByValue() 或 queryOrdered(byChild: "timestamp")。Firebase 以随机顺序向我发送数据（我不知道它是否是随机的，但它是不按日期排序）。

Answer 1

正如 Rashwan 所说，您需要致电 queryOrderedByChild() 才能以正确的顺序退货。然后你循环 snapshot.children 以确保你也以正确的顺序处理它们。

所以：

database.child("messages").queryOrdered(byChild: "timestamp").observe(.value, with: { snapshot in
    for child in snapshot.children.allObjects as! [DataSnapshot] {
        print("\(child.key)")
    }

    completionHandler(true)
})

回调中的重要一点是不要将整个快照转换为字典，因为字典不会维护项目的顺序。

Answer 2

您使用database.queryOrdered 而不是database.child，如下所示：

database.queryOrdered(byChild: "messages"). observeSingleEvent(.value, with: { (snapshot) in
    print("\(snapshot.value)")
}

但您也可以像今天一样获取数据，然后对列表进行排序self.timestamps：

self.timestamps.sorted(by: { $0 > $1 })

更新：
创建一个模型并将您的属性附加到该模型，然后附加到一个列表并在此之后对该列表进行排序。像这样：

struct Model {
    let message: String?
    let senderId: String?
    let senderName: String?
    let timestamp: IntMax

    init(message: String?, senderId: String?, senderName: String?, timestamp: IntMax?) {
        self.message = message
        self.senderId = senderId
        self.senderName = senderName
        self.timestamp = timestamp
    }
}

var models = [Model]()
models.sorted(by: { $0.0.timestamp > $0.1.timestamp })

更新 2：
要添加模型，请查看以下代码示例：

database.child("messages").child(ownersId).observeSingleEvent(of: .value, with: { (snapshot) in
    print(snapshot)
    var models = [Model]()
    if let messagesDict = snapshot.value as? [String:AnyObject] {
        for (_,message) in messagesDict {
            if let message = message["message"] as? String, let senderName = message["senderName"] as? String, let timestamp = message["timestamp"] as? IntMax, let senderId = message["senderId"] as? String {
                let model = Model(message: message, senderId: senderId, senderName: senderName, timestamp: timestamp)
                models.append(model)
            }
        }
        models.sorted(by: { $0.0.timestamp > $0.1.timestamp })
    }
    completionHandler(true)

}) { (error) in
    print(error.localizedDescription)
    completionHandler(false)
}