A propertyWrapper for supporting declarative data fetching on SwiftUI with Firebase Cloud Firestore

Firestore+DeclarativeDataFetching.swift

@propertyWrapper
struct CollectionListener<Document: Codable>: DynamicProperty {
    final class FirestoreDocumentsListener {
        typealias OnUpdate = ([Document]) -> Void
        private var onUpdate: OnUpdate?
        private var listenerRegistration: ListenerRegistration?

        func listen(path: String, firestore: Firestore, onUpdate: OnUpdate? = nil) {
            self.onUpdate = onUpdate
            guard listenerRegistration == nil else { return }
            listenerRegistration = firestore.collection(path)
                .addSnapshotListener({ [weak self] snapshot, error in
                    guard let self = self else { return }
                    if let _ = error {
                        self.onUpdate?([])
                    } else if let snapshot = snapshot {
                        do {
                            let data = try snapshot.documents.compactMap {
                                try $0.data(as: Document.self)
                            }
                            self.onUpdate?(data)
                        } catch {
                            self.onUpdate?([])
                        }
                    } else {
                        self.onUpdate?([])
                    }
                })
        }
    }

    private let listener: FirestoreDocumentsListener = .init()
    @State var wrappedValue: [Document] = []
    @Environment(\.firestore) var firestore

    private let path: String

    func update() {
        listener.listen(
            path: path,
            firestore: firestore
        ) { documents in
            self.wrappedValue = documents
        }
    }

    init(path: String) {
        self.path = path
    }
}