Building and Sending Transactions

Kaptos boasts an expressive and type-safe DSL-style API for building and sending transactions on-chain. This guide will walk you through the process of building and sending transactions using Kaptos.

The typical flow for sending a transaction is as follows:

Create an account (if you don’t already have one).
Build the transaction.
Sign the transaction.
Submit the transaction.

Create an Account

To create a new account, you first generate new credentials then fund the account. On test networks, you can fund an account programmatically by asking a “faucet”.

val aliceAccount = Account.generate()
val bobAccount = Account.generate()

If you have a private key, you can use it to create an Account object to manage those credentials.

val privateKey = Ed25519PrivateKey("myEd25519privatekeystring")
val account = Account.fromPrivateKey(privateKey)

Build the Transaction

Kaptos provides a buildTransaction.simple method to build a transaction. You can specify the sender, entry function data like the function name, type arguments, and function arguments. You can also configure the transaction with the gas price and maximum gas amount. However, reasonable defaults are provided for these values in case you don’t specify them.

val txn = aptos.buildTransaction.simple(
    sender = aliceAccount.accountAddress,
    data = entryFunctionData {
        function = "0x1::coin::transfer"
        typeArguments = typeArguments {
            +TypeTagStruct("0x1::aptos_coin::AptosCoin")
        }
        functionArguments = functionArguments {
            +bobAccount.accountAddress
            +U64(SEND_AMOUNT)
        }
    },
)

Sign the Transaction

Once you have built a transaction, you can sign it using the sign method.

  val aliceAuthenticator = aptos.sign(
      sender = aliceAccount,
      transaction = txn,
  )

Submit the Transaction

Finally, you can submit the transaction to the network using the submit method.

val commitedTransaction = aptos.submitTransaction.simple(
      transaction = signedTransaction,
      senderAuthenticator = aliceAuthenticator,
)

ℹ️

You can collapse the signing and submitting steps into one by using the signAndSubmitTransaction method.

val executedTransaction = aptos.signAndSubmitTransaction(
      signer = aliceAccount,
      transaction = commitedTransaction,
)

Wait for the Transaction to Execute

Then you can wait for the transaction to be executed by using the waitForTransaction method.

val executedTransaction = aptos.waitForTransaction(HexInput.fromString(commitedTransaction.expect("Transaction failed").hash))

Full Kotlin Example

The following is a complete example of how to build and send a transaction to transfer APT:

transaction.kt

 
const val FUNDING_AMOUNT = 100_000_000L
const val SEND_AMOUNT_APT = 0.5f
const val UNIT_CONVERSION = 100_000_000
const val SEND_AMOUNT_UNITS = (SEND_AMOUNT_APT * UNIT_CONVERSION)
const val SEND_AMOUNT = 1_000_000UL
 
/**
 * This example demonstrates how to transfer APT from one account to another.
 *
 * Each run generates and creates new accounts on-chain using faucet funding. After funding, the APT
 * balance of each account is printed; if funding fails, an error is thrown.
 *
 * Next, a transaction is constructed to send 0.5 APT from Alice to Bob. The transaction is then
 * signed and submitted using the one-step `signAndSubmitTransaction` method. We wait for the
 * transaction to complete and print the updated balances of Alice and Bob. If the transaction
 * fails, an error is thrown.
 */
fun main() = runBlocking {
  val aptos = Aptos(AptosConfig(AptosSettings(network = Network.TESTNET)))
 
  println("Generating Alice and Bob's accounts")
 
  val alice = Account.generate()
  val bob = Account.generate()
 
  aptos.fundAccount(alice.accountAddress, FUNDING_AMOUNT).expect("Failed to fund Alice's account")
  aptos.fundAccount(bob.accountAddress, FUNDING_AMOUNT).expect("Failed to fund Bob's account")
 
  println("Created accounts on chain")
  println("Alice's balance: ${aptos.getAccountAPTAmount(alice.accountAddress)}")
  println("Bob's balance: ${aptos.getAccountAPTAmount(bob.accountAddress)}")
  println("=============================================")
  println(
    "Building transaction to send ${SEND_AMOUNT / 100_000_000u} APT to Bob: ${bob.accountAddress}"
  )
 
  val txn =
    aptos.buildTransaction.simple(
      sender = alice.accountAddress,
      data =
        entryFunctionData {
          function = "0x1::coin::transfer"
          typeArguments = typeArguments { +TypeTagStruct("0x1::aptos_coin::AptosCoin") }
          functionArguments = functionArguments {
            +bob.accountAddress
            +U64(SEND_AMOUNT_UNITS.toULong())
          }
        },
    )
 
  // Sign and submit the transaction
  val commitedTransaction = aptos.signAndSubmitTransaction(alice, txn)
 
  val executedTransaction =
    aptos.waitForTransaction(
      HexInput.fromString(commitedTransaction.expect("Transaction failed").hash)
    )
 
  println(
    "Transaction wait response: $executedTransaction\n============================================="
  )
 
  val aliceNewBalance =
    aptos.getAccountAPTAmount(alice.accountAddress).expect("Alice's account does not exist")
  val bobNewBalance =
    aptos.getAccountAPTAmount(bob.accountAddress).expect("Bob's account does not exist")
 
  println("Alice's new balance: $aliceNewBalance")
  println("Bob's new balance: $bobNewBalance")
}

Fetch Data via the Kotlin SDK AptosKit