Corda customising transaction validation - extra validation Flow

ExtraValidationSendMessageFlow.kt

class ExtraValidationSendMessageFlow(message: MessageState) :
  SendMessageFlow(message) {

  override fun extraTransactionValidation(transaction: TransactionBuilder) {
    requireThat {
      val messages = transaction.toLedgerTransaction(serviceHub).outputsOfType<MessageState>()
      "There must be only one output message" using (messages.size == 1)
      val message = messages.single()
      "Message must contain the secret passphrase" using (message.contents.contains("I love Corda"))
    }
  }
}