How verify function is working in Cordapp - corda

Question

I have developed one sample cordapp where I am instantiating the state object inside the InitiatorFlow.
BookingState outputState = new BookingState(customerName, customerAge,checkInDate,checkOutDate,roomType,roomRate,bookingAmount,getOurIdentity(),lemonTree);
Will this statement automatically call the verify method which presents inside the Contract class?
or Do I need to call the verify method inside the Flow class?
Do TransactionBuilder need verify method?
Will the below code will collect the signature from counterparty? I am asking because in maximum cases I have seen this code to collect the counterparty signature. (SignedTransaction stx = subFlow(new CollectSignaturesFlow(ptx, session));)
private final ProgressTracker.Step GATHERING_SIGNATURE = new ProgressTracker.Step("Gathering the counterparty's signature"){
#Override
public ProgressTracker childProgressTracker() {
return CollectSignaturesFlow.Companion.tracker();
}
}

Answer 1

Answering your question in the same order:
No, the initiator of the flow must explicitly call the verify() function before signing it locally.
Same as above.
The code that you shared does not collect signatures; it only defines that step in the progress tracker so it prints it out on your node's shell when you run the flow. However calling CollectSignaturesFlow will send the transaction to all provided counter-parties FlowSession, and receive back their signatures when they call SignTransactionFlow in the responder flow. See an example of calling CollecSignaturesFlow here.
Looking at your code:
If you call CollectSignaturesFlow in initiator, you must call SignTransactionFlow in responder; otherwise your initiator flow will hang, because it has a receive (which is called inside CollectSignaturesFlow), and that receive will wait until a responder flow will send something, and in that case the responder must call SignTransactionFlow which internally has a send to send the signed transaction by the counter-party. See IOU example here.
The reason why your flow still verifies the transaction and throws an error when you supply a faulty one is because you call FinalityFlow inside your initiator which implicitly verifies the transaction before notarizing it (see here).
You might ask, why do I need to call transactionBuilder.verify() if the notary already does it? Well, why sign the transaction and send it to other parties without verifying it first? You're consuming resources and sending messages between nodes (to send and receive signed transactions); instead, first check whether the transaction is valid or not, then sign it locally and send it to other peers. Btw, counter-parties in responder flow also run the verify() method implicitly when they receive the transaction through calling SignTransactionFlow (see here).

Answer 2

Below is a sample code snippet to answer your questions.
// 1. This will only create BookingState instance
BookingState outputState = new BookingState(customerName, customerAge,checkInDate,checkOutDate,roomType,roomRate,bookingAmount,getOurIdentity(),lemonTree);
TransactionBuilder transactionBuilder = new TransactionBuilder(notary);
transactionBuilder.addOutputState(outputState);
transactionBuilder.addCommand(new BookingContract.Commands.Issue() ,
ImmutableList.of(issuer.getOwningKey(),owner.getOwningKey()));
//2. You should call the verify method from transactionbuilder class which will //run the verify method of contract class.
transactionBuilder.verify(getServiceHub());
//3. Below code is used to collect signatures from the counterparty
FlowSession session = initiateFlow(owner);
SignedTransaction fullySignedTransaction = subFlow(new CollectSignaturesFlow(signedTransaction, singletonList(session)));