Firestore 和缓存

Question

我试图了解 Firestore 何时缓存某些内容。为此，我运行以下代码：我希望第一个 get 具有 isFromCache false 并且我希望第二个 get 来自缓存。

FirebaseFirestore.instance.collection('questions').doc("xyz").get().then((DocumentSnapshot documentSnapshot) {
  print("1st: " + documentSnapshot.metadata.isFromCache.toString());

  FirebaseFirestore.instance.collection('questions').doc("xyz").get(GetOptions(source: Source.cache)).then((DocumentSnapshot documentSnapshot) {
    print("2nd: " + documentSnapshot.metadata.isFromCache.toString());
  });
});

但是，当运行这个（飞镖，在 Chrome 中运行）时，我注意到以下日志很明显，文档没有被缓存。这是预期的行为吗？我错过了什么吗？

1st: false
zone.dart:1339 Uncaught [cloud_firestore/unavailable] Failed to get document from cache. (However, this document may exist on the server. Run again without setting 'source' in the GetOptions to attempt to retrieve the document from the server.)
    at Object.wrapException (http://localhost:41665/main.dart.js:2047:17)
    at Object.throwExpression (http://localhost:41665/main.dart.js:2061:15)
    at guard_closure0.call$1 (http://localhost:41665/main.dart.js:22924:16)
    at _RootZone.runUnary$2$2 (http://localhost:41665/main.dart.js:16321:18)
    at _RootZone.runUnary$2 (http://localhost:41665/main.dart.js:16325:19)
    at _FutureListener.handleError$1 (http://localhost:41665/main.dart.js:15447:19)
    at _Future__propagateToListeners_handleError.call$0 (http://localhost:41665/main.dart.js:15732:49)
    at Object._Future__propagateToListeners (http://localhost:41665/main.dart.js:5177:77)
    at _Future._completeError$2 (http://localhost:41665/main.dart.js:15577:9)
    at _Future__asyncCompleteError_closure.call$0 (http://localhost:41665/main.dart.js:15663:18)

编辑：当添加一个像下面这样的监听器时，我确实得到了“1st: false, 2nd: true”的结果。

    FirebaseFirestore.instance.collection('questions').doc("xyx").snapshots().listen((event)
    {
      print("update received");
    });

    FirebaseFirestore.instance.collection('questions').doc("xyz").get().then((DocumentSnapshot documentSnapshot) {
      print("1st: " + documentSnapshot.metadata.isFromCache.toString());

      FirebaseFirestore.instance.collection('questions').doc("xyz").get(GetOptions(source: Source.cache)).then((DocumentSnapshot documentSnapshot) {
        print("2nd: " + documentSnapshot.metadata.isFromCache.toString());
      });
    });

Answer 1

您正在根据您的代码发出标准请求并期望从缓存中提供它，这是对缓存如何工作的误解。

缓存需要启用离线访问，在documentation 中指出，仅当设备处于离线模式且文档在缓存中时才会返回 fromCache。