How to get handlers on custom paywalls?

the below method has two handlers for the purchase

public func presentPaywallIfNeeded(
        fonts: PaywallFontProvider = DefaultPaywallFontProvider(),
        shouldDisplay: @escaping @Sendable (CustomerInfo) -> Bool,
        purchaseCompleted: PurchaseOrRestoreCompletedHandler? = nil,
        restoreCompleted: PurchaseOrRestoreCompletedHandler? = nil
)

How can our application detect when the purchase is completed or restored when using a custom paywall?