I was developing the chat app in flutter using firestore, the users successfully chat one-to-one. but I also want to send the notification to the user that someone sends him/her a message. For that I decided to use the cloud messaging service of firebase, but how can I check that the user read the message or not.

CodePudding user response：

There is nothing built into Cloud Firestore to track which user has read a specific document already.

The two most common approaches that I know of are:

• To track for each individual message and each individual user whether they have read it, so by keeping a list of the user IDs in each message document. While this potentially stores a lot of extra data, it is the most direct mapping of your use-case.

• In most chat apps though, tracking of what messages each user has read is done as an "up to here". So if the user has read all messages up until a certain one or a certain time, the assumption is that they've also read all the messages before that one/moment.

  For this model you can instead just keep a single document ID or timestamp for each user (i.e. readUpTo), instead of having to mark each message for each user that has read it. In your UI, you then determine whether to show each message as unread based on the message ID/timestamp compared to the timestamp of readUpTo.