问题
I'm writing a chat application with the help of firebase.Here I have a problem , if the device is offline the data is stored in cache and when device is back online the cache will be synced , but how to know which data is synced and which not (User may have poor internet connection , so syncing may be delayed).How to notify user that message has been sent or sending , and that too we need to manage a huge list of messages Thank you!
回答1:
There are two ways:
Using a completion listener as shown here: Firebase Android - how to tell if node has been synced This approach works as long as the app stays active. Completion listener don't survive an app restart, so it won't work if the app is restarted.
Using a sentinel value. Writes to the database from a single client are guaranteed to be executed in order. Using this knowledge, you could write a sentinel (think: dummy) value when the app is restarted, or the connection is restored, and detect when that one is written. Once the sentinel value is confirmed, you can be certain that all older messages have also been handled by the server.
Also see:
- Firebase synchronisation of locally-modified data: handling errors & global status
来源：https://stackoverflow.com/questions/61320556/how-to-know-which-offline-data-has-been-synced-to-firebase-realtime-database