@objc final class Database : NSObject
{
    var spamRef: FIRDatabaseReference!  = nil
    var snapshot:FIRDataSnapshot?       = nil

    override init()
    {
        FIRDatabase.database().persistenceEnabled = true
        FIRApp.configure()
        spamRef = FIRDatabase.database().reference(withPath: "spam_list")
        _ = spamRef.observe(FIRDataEventType.value, with: { (theSnapshot) in
            self.snapshot = theSnapshot
            ...