问题
I'm creating a multi page application where i need to create and store transactions even when the users are offline. How do i achieve this using firestore ? Also i need some idea on how to persist the data received from the firestore locally.
回答1:
You can't run transactions when offline,but if you think that your data is not changed while you are offline you can get the data from cache and update it it there using dbRef.addSnapshotListener(MetadataChanges.INCLUDE) and dbRef.update()
回答2:
How to perform transactions in Firestore when the user is offline?
You cannot! Transactions are not supported for offline use, they can't be cached or saved for later. This is because a transaction absolutely requires round trip communications with the server in order to ensure that the code inside the transaction completes successfully. So you can use transaction only while online because the transactions are network dependent.
Also I need some idea on how to persist the data received from the Firestore locally.
According to the official documentation of Cloud Firestore regarding offline persistence:
For Android and iOS, offline persistence is enabled by default. To disable persistence, set the PersistenceEnabled option to false.
For the web, offline persistence is disabled by default. To enable persistence, call the enablePersistence method. Cloud Firestore's cache isn't automatically cleared between sessions. Consequently, if your web app handles sensitive information, make sure to ask the user if they're on a trusted device before enabling persistence.
Important: For the web, offline persistence is an experimental feature that is supported only by Chrome, Safari, and Firefox web browsers. Also, if a user opens multiple browser tabs that point to the same Cloud Firestore database, and offline persistence is enabled, Cloud Firestore will work correctly only in the first tab.
Edit:
The Firestore SDK for Android has a local cache that's enabled by default. So all read operations will come from the cache when there is no connectivity. So Firestore provides this feature to handle offline data. This means that if the user tries to add/delete documents while offline, every operation is added to a queue. Once the user regains the connection, every change that is made while offline will be updated on Firebase servers. In other words, all queries will be committed on the server.
Please also note that when you are offline, pending writes that have not yet been synced to the server are held in a queue. If you do too many write operations without going online to sync them, that queue will grow fast and it will not slow down only the write operations it will also slow down your read operations. So I suggest using this database for its online capabilities.
来源：https://stackoverflow.com/questions/54197073/how-to-perform-transactions-in-firestore-when-user-is-offline