Signing constraints

    fun verifySigningConstraints(tx: LedgerTransaction){

        // This implementation assumes there is a maximum of one Primary input state and a maximum one Primary output state.
        // For Contracts which assume multiple Primary inputs or Output a different approach will be required

        val command = tx.commands.requireSingleCommand<AgreementContract.Commands>()
        val inputStates = tx.inputsOfType<AgreementState>()
        val outputStates = tx.outputsOfType<AgreementState>()

        // This is a guard which shouldn't be triggered because the Path constraints should have already ensured there is
        // a maximum of one Primary input state and a maximum one Primary output state
        requireThat{
            "when checking signing constraints there should be a maximum of one Primary input state." using (inputStates.size <= 1)
            "When checking signing constraints there should be a maximum of one Primary output state." using (outputStates.size <= 1)
        }

        val inputState = inputStates.singleOrNull()
        val outputState = outputStates.singleOrNull()

        val commandName = command.value::class.java.simpleName

        fun checkSigner(signerDescription: String, signer: Party?){
            requireThat { "When the Command is $commandName the $signerDescription must sign." using (command.signers.contains(signer?.owningKey))}

        }

        when (command.value){
            is Commands.Propose -> {
                checkSigner("output.proposer", outputState?.proposer)   // Can add multiple signing check against each Command
            }
            is Commands.Reject -> {
                checkSigner("output.rejectedBy", outputState?.rejectedBy)
            }
            is Commands.Repropose -> {
                checkSigner("output.proposer", outputState?.proposer)
            }
            is Commands.Agree -> {
                checkSigner("input.consenter", inputState?.consenter )
            }
            is Commands.Complete -> {
                checkSigner("input.seller", inputState?.seller)
            }
        }
    }