Firestore-使用缓存直到在线内容更新 [英] Firestore - Using cache until online content updates

问题描述

我从Firestore开始.我已经阅读了有关脱机数据持久性的文档和教程，但是即使存储内容未经修改，Firestore是否还会再次下载数据，我也还不清楚. 例如，如果我有一个查询，查询结果每周更新一次，而在进行更改之前，我不需要应用再次下载内容，那么就效率而言，编写代码的最佳方法是什么? 谢谢！

I am starting with Firestore. I've read docs and tutorials about the offline data persistence but I have not really clear if Firestore downloads data again even if the content hasn't been modified. For example, if I have a query where the results will be updated once a week and I don't need that the app download the content again until the changes were made, what is the best way in terms of efficiency to write the code? Thanks!

推荐答案

您要使用快照监听器" API来监听查询: https://firebase.google.com/docs/firestore/query-data /listen#listen_to_multiple_documents_in_a_collection

You want to use the "snapshot listener" API to listen to your query: https://firebase.google.com/docs/firestore/query-data/listen#listen_to_multiple_documents_in_a_collection

以下是一些JavaScript示例:

Here's some JavaScript as an example:

db.collection("cities").where("state", "==", "CA")
    .onSnapshot(function(querySnapshot) {
        var cities = [];
        querySnapshot.forEach(function(doc) {
            cities.push(doc.data().name);
        });
        console.log("Current cities in CA: ", cities.join(", "));
    });

您第一次附加此侦听器时，Firestore将访问网络以将所有结果下载到查询中，并按您的期望为您提供查询快照.

The first time you attach this listener Firestore will access the network to download all of the results to your query and provide you with a query snapshot, as you'd expect.

如果您第二次附加相同的侦听器，并且正在使用脱机持久性，则将立即使用缓存中的结果触发该侦听器.这是检测结果是来自缓存还是本地的方法:

If you attach the same listener a second time and you're using offline persistence, the listener will be fired immediately with the results from the cache. Here's how you can detect if your result is from cache or local:

db.collection("cities").where("state", "==", "CA")
  .onSnapshot({ includeQueryMetadataChanges: true }, function(snapshot) {
      snapshot.docChanges.forEach(function(change) {
          if (change.type === "added") {
              console.log("New city: ", change.doc.data());
          }

          var source = snapshot.metadata.fromCache ? "local cache" : "server";
          console.log("Data came from " + source);
      });
  });

获得缓存结果后，Firestore将与服务器进行核对，以查看查询结果是否有任何更改.如果是，您将获得包含更改的另一个快照.

After you get the cached result, Firestore will check with the server to see if there are any changes to your query result. If yes you will get another snapshot with the changes.

如果希望仅包含元数据的更改得到通知(例如，如果没有文档更改但snapshot.metadata.fromCache更改)，则可以在发出查询时使用QueryListenOptions: https://firebase.google.com/docs/reference /android/com/google/firebase/firestore/QueryListenOptions

If you want to be notified of changes that only involve metadata (for example if no documents change but snapshot.metadata.fromCache changes) you can use QueryListenOptions when issuing your query: https://firebase.google.com/docs/reference/android/com/google/firebase/firestore/QueryListenOptions

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