Firebase snapshot listener filter-CodePudding

I have a Firestore DB. Is it possible to filter snapshots coming from it based on some field if add add a listener? What I need is: “send me an updated document only if this field equals this value”

What I surely can do is just check manually each new snapshot and return/propagate document if it passes the filter but I was thinking about sparing some transferred data and hit less limits

CodePudding user response：

You can make a query by filtering something then adding a listener for this like the below code:

// get all document in collection "cities" that has attribute "state" equal to "CA"
db.collection("cities").whereField("state", isEqualTo: "CA")
    .addSnapshotListener { querySnapshot, error in
        guard let documents = querySnapshot?.documents else {
            print("Error fetching documents: \(error!)")
            return
        }
        let cities = documents.map { $0["name"]! }
        print("Current cities in CA: \(cities)")
    }

Ref: https://firebase.google.com/docs/firestore/query-data/listen#listen_to_multiple_documents_in_a_collection

CodePudding user response：

Cloud Firestore listeners fire on the document level. So if you have multiple fields in a document and the value of the fields in the document changes, your listener will fire. So you'll have to pay a read operation, each time something in the document changes.

It's true that you can attach a listener and get only the documents that have set a field to a particular value, but that doesn't mean you can restrict the SDK to read the value only when that field is changed to a value of your choice.

There is no way you can read a document, only when a field gets a particular value. You're always charged with a read operation, each time a value changes, no matter what the value is. So if the new value is the value that passes your filters, then you can go ahead with your logic.

I was thinking about sparing some transferred data and hitting less limits.

Everything in Firestore it's about the number of reads, writes, and deletes you perform. And the amount of bandwidth you are using. But unfortunately, you cannot reduce the costs that way.