如何仅从 Firebase 获取没有现有数据的新数据?

问题描述

我在 Firebase 中有一个节点，不断更新日志文件中的信息.节点是 lines/ 并且 lines/ 的每个子节点都来自 post() 所以它有一个唯一的 ID.

I have a node in Firebase getting continually updated with information from a logfile. The node is lines/ and each child of lines/ is from a post() so it has a unique ID.

当客户端第一次加载时，我希望能够获取最后 X 个条目.我希望我会用 once() 做到这一点.但是，从那时起，我想将 on() 与 child_ added 一起使用，以便我获得所有新数据.但是，child_ added 获取存储在 Firebase 中的所有数据，并且在初始设置后，只需要新的东西.

When a client first loads, I want to be able to grab the last X number of entries. I expect I'll do this with once(). From then on, however, I want to use an on() with child_added so that I get all new data. However, child_added gets all data stored in the Firebase and, after the initial setup, only want the new stuff.

我看到我可以在 on() 上添加一个 limitToLast()，但是，如果我说 limitToLast(1) 并且大量条目涌入，我的应用程序还会获得所有新条目吗?有没有其他方法可以做到这一点?

I see that I can add a limitToLast() on the on(), but, if I say limitToLast(1) and a flood of entries come in, will my app still get all the new entries? Is there some other way to do this?

推荐答案

您需要包含 timestamp 属性并运行查询.

You need to include a timestamp property and run a query.

// Get the current timestamp
var now = new Date().getTime();
// Create a query that orders by the timestamp
var query = ref.orderByChild('timestamp').startAt(now);
// Listen for the new children added from that point in time
query.on('child_added', function (snap) {
  console.log(snap.val()
});

// When you add this new item it will fire off the query above
ref.push({
  title: "hello",
  timestamp: Firebase.ServerValue.TIMESTAMP
});

Firebase SDK 具有排序方法 orderByChild() 和创建范围的方法 startAt().当您将两者结合使用时，您可以限制从 Firebase 返回的内容.

The Firebase SDK has methods for ordering, orderByChild() and methods for creating a range startAt(). When you combine the two you can limit what comes back from Firebase.

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