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Building a Transaction Manually

Creating a transaction manually allows you to have full control over every step, from selecting UTXOs to crafting the calldata and signing the transaction. This guide covers the process of manually building a transaction for interacting with a contract on OP_NET.

DEPRECATED

Manually building transactions can be error-prone due to gas miscalculations or incorrect UTXO usage. Prefer using the Simulation Method, which automates gas estimation and UTXO management, ensuring reliability and efficiency.

Steps to Build a Transaction​

1. Obtaining Proper UTXOs​

Before building a transaction, you need to gather the required UTXOs for funding. Use the utxoManager from the JSONRpcProvider to fetch UTXOs associated with your address.

const utxos = await provider.utxoManager.getUTXOs({
  address: wallet.p2tr,
});
UTXO Management

Ensure you have enough UTXOs to cover the transaction inputs, gas fees, and any additional outputs.

Learn more about UTXO Manager.

2. Estimating Gas​

Gas estimation is crucial for ensuring your transaction executes successfully. While the provider or simulation result often helps with gas estimation, adding a buffer (e.g., 15%) is recommended to account for fluctuations.

const gasParameters = await provider.gasParameters();
const gasEstimate = gasParameters.baseGas * 2n; // Double the base gas

3. Encoding Calldata​

Use your contract instance to encode the calldata for the specific function you want to call. The encodeCalldata method helps prepare the necessary data for contract interaction.

const contractInstance = getContract<IContract>(
  "contract-address",
  "contract-abi",
  provider,
  networks.regtest,
  wallet.address
);

// Call the function to get the calldata
const call = await contractInstance.foo("arg1", 123);
const calldata = call.calldata;
// Optionally, you can also get the calldata without calling the function
// const calldata = contractInstance.encodeCalldata(
//   "foo", // Function name
//   ["arg1", 123] // Function arguments
// );

4. Setting Interaction Parameters​

The IInteractionParameters object defines the details of the interaction, including the sender, contract address, UTXOs, gas settings, and calldata.

const interactionParameters: IInteractionParameters = {
  from: wallet.p2tr, // Sender's address
  to: "contract-address", // Contract address
  utxos, // UTXOs for funding
  signer: wallet.keypair, // Wallet keypair
  network: networks.regtest, // Target network
  feeRate: Number(gasEstimate), // Gas fee rate (satoshis per byte)
  priorityFee: 5000n, // Priority fee for faster processing
  calldata: calldata as Buffer, // Encoded calldata
};

5. Signing the Transaction​

The TransactionFactory allows you to sign the transaction with your wallet’s private key, ensuring the transaction is ready for broadcasting.

const transactionFactory = new TransactionFactory();
const signedTransaction = await transactionFactory.signInteraction(
  interactionParameters
);

console.log("Signed Transaction:", signedTransaction);

Full Example​

import { networks } from "@btc-vision/bitcoin";
import {
  IInteractionParameters,
  TransactionFactory,
  Wallet,
} from "@btc-vision/transaction";
import { getContract, JSONRpcProvider } from "opnet";

// Initialize wallet and provider
const wallet = Wallet.fromWif("your-private-key-wif", networks.regtest);
const provider = new JSONRpcProvider(
  "https://regtest.opnet.org",
  networks.regtest
);

// Fetch UTXOs for funding the transaction
const utxos = await provider.utxoManager.getUTXOs({
  address: wallet.p2tr,
});

// Define contract instance
const contractInstance = getContract<IContract>(
  "contract-address",
  "contract-abi",
  provider,
  networks.regtest,
  wallet.address
);

// Call the function to get the calldata
const call = await contractInstance.foo("arg1", 123);
const calldata = call.calldata;
// Optionally, you can also get the calldata without calling the function
// const calldata = contractInstance.encodeCalldata(
//   "foo", // Function name
//   ["arg1", 123] // Function arguments
// );

// Define interaction parameters
const interactionParameters: IInteractionParameters = {
  from: wallet.p2tr, // Sender's address
  to: "contract-address", // Contract address
  utxos, // UTXOs for funding
  signer: wallet.keypair, // Wallet keypair
  network: networks.regtest, // Target network
  feeRate: Number((await provider.gasParameters()).baseGas * 2n), // Gas fee rate (satoshis per byte)
  priorityFee: 5000n, // Priority fee for faster processing
  calldata: calldata as Buffer, // Encoded calldata
};

// Create and sign the transaction
const transactionFactory = new TransactionFactory();
const signedTransaction = await transactionFactory.signInteraction(
  interactionParameters
);

console.log("Signed Transaction:", signedTransaction);

Transaction Outputs​

The signInteraction method returns an array with the following structure:

  • [0]: The first transaction to send.
  • [1]: The second transaction to send.
  • [2]: An array of newly created UTXOs.
important

You must broadcast the transactions in the order they appear in the output array. (i.e., [0] first, then [1]).

Learn how to send a manually built transaction on OP_NET.

Best Practices​

  • Always fetch UTXOs that meet the requirements of your transaction and validate their sufficiency.
  • Ensure your gas estimate includes a buffer to prevent transaction failures due to fluctuating network conditions.
  • Verify the calldata matches the intended contract interaction to avoid unexpected behavior.
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