Corda customising transaction validation - Base Flow

SendMessageFlow.kt

@InitiatingFlow
open class SendMessageFlow(private val message: MessageState) :
  FlowLogic<SignedTransaction>() {

  open fun extraTransactionValidation(transaction: TransactionBuilder) {
    // to be implemented by subtype flows - otherwise do nothing
  }

  @Suspendable
  final override fun call(): SignedTransaction {
    // build transaction
    val tx = verifyAndSign(transaction)
    // collect signatures
    // save transaction
  }

  private fun verifyAndSign(transaction: TransactionBuilder): SignedTransaction {
    extraTransactionValidation(transaction)
    transaction.verify(serviceHub)
    return serviceHub.signInitialTransaction(transaction)
  }
}