requestCredentials.swift

//1
let keychain = Keychain(server: domain, protocolType: .https).synchronizable(true)

let email = UserDefaults.standard.object(forKey: "account") as? String

//2
if let account = email, let password = try? keychain.get(account) {
    return password
}else {
  //3
    keychain.getSharedPassword({ [weak self] (account, password, error) in 
        guard let account = account, let password = password else { return }
        //4
        keychain[account] = password
        return password
    })
}