I'm trying to create a singleton, but the Class 'IAPHelper' it's instantiating has a public init method that takes a parameter.

I can't see how to pass the required parameter, so I get the error message:

"Cannot use instance member 'productIdentifiers' within property initializer; property initializers run before 'self' is available"

open class IAPHelper: NSObject  {

    private let productIdentifiers: Set<ProductIdentifier>
    private var purchasedProductIdentifiers: Set<ProductIdentifier> = []
    private var productsRequest: SKProductsRequest?
    private var productsRequestCompletionHandler: ProductsRequestCompletionHandler?

    static let shared = IAPHelper(productIds: productIdentifiers)  

    public init(productIds: Set<ProductIdentifier>) {
        productIdentifiers = productIds
        for productIdentifier in productIds {
            let purchased = KeychainWrapper.standard.bool(forKey: productIdentifier)
            if purchased ?? false {
                purchasedProductIdentifiers.insert(productIdentifier)
                print("Previously purchased: \(productIdentifier)")
            } else {
                print("Not purchased: \(productIdentifier)")
            }
        }
        super.init()

        SKPaymentQueue.default().add(self)
    }
}