Does anyone know how to fix this issue?

Declared closure result 'InstockProduct?' is incompatible with contextual type 'InstockProduct'

class Database {
    let db = Firestore.firestore()
    var products = [InstockProduct]()

    func getProductsInCollection(collection: String, handler: @escaping ([InstockProduct]) -> Void) {
        db.collection("/products/instock/\(collection)").addSnapshotListener { querySnapshot, err in
            guard let documents = querySnapshot?.documents else{
                print("No document")
                handler([])
                return
            }
            self.products = documents.map { (queryDocumentSnapshot) -> InstockProduct in
                return try? queryDocumentSnapshot.data(as: InstockProduct.self)
            }
        }
    }
}

Code image

CodePudding user response：

You have products declared as [InstockProduct] (with non-optionals).

Also, in your closure, you have a return type specified as InstockProduct (again, non-optional).

But, in your return statement, you return an optional when you do this:

return try? queryDocumentSnapshot.data(as: InstockProduct.self)

Given that the try could fail, I'd change your code to return an optional and then use compactMap to remove those optionals from the final array:

self.products = documents.compactMap { (queryDocumentSnapshot) -> InstockProduct? in
    return try? queryDocumentSnapshot.data(as: InstockProduct.self)
}