States and statuses

/**
 * The StatusState interface should be implemented for all [ContractState]s that require a status field.
 *
 * [status] is nullable so that when there is no input or output state in a transaction, the status can be represented as [null]
 *
 */
interface StatusState: ContractState {
    val status: Status?
}

/**
 * Statuses are defined as enum classes in the StatusState which should implement this Status interface.
 */
@CordaSerializable
interface Status

@BelongsToContract(AgreementContract::class)
data class AgreementState(override val status: AgreementStatus?,
                          val buyer: Party,
                          val seller: Party,
                          val goods: String,
                          val price: Amount<Currency>,
                          val proposer: Party,
                          val consenter: Party,
                          val rejectionReason: String? = null,
                          val rejectedBy: Party?= null,
                          override val participants: List<AbstractParty> = listOf(buyer, seller),
                          override val linearId: UniqueIdentifier = UniqueIdentifier()) : LinearState, StatusState

enum class AgreementStatus: Status {
    PROPOSED,
    REJECTED,
    AGREED
}