Using Chained APIs

Chained APIs 简体中文版

When deploying and calling a contract, it is often necessary to calculate the transaction fee and the change output. Without knowing the size of the unlocking script, it is difficult to calculate both. scryptlib extends the bsv library and provides a set of chained APIs to simplify building transactions in these cases.

Deploy Contract

The following is a function that deploys any type of contracts.

async function deployContract(contract, amount) {

  const address = privateKey.toAddress()
  const tx = new bsv.Transaction()
  tx.from(await fetchUtxos(address))  // Add UTXOs/bitcoins that are locked into the contract and pay for miner fees. In practice, wallets only add enough UTXOs, not all UTXOs as done here for ease of exposition.
  .addOutput(new bsv.Transaction.Output({  
    script: contract.lockingScript, // Deploy the contract to the 0-th output
    satoshis: amount,
  }))
  .change(address)   // Add change output
  .sign(privateKey) // Sign inputs. Only apply to P2PKH inputs.

  await sendTx(tx) // Broadcast transaction
  return tx
}

Call contract

There are two cases when calling a public function of a contract: 1. The function takes a parameter of type SigHashPreimage 2. No SigHashPreimage parameter.

No SigHashPreimage

If the public function of the contract being called does not contain parameter of type SigHashPreimage, it is relatively simple to build a transaction calling it. Generally, you can build a transaction according to the following steps:

1. Create an input from a previous transaction where the contract is located by createInputFromPrevTx(tx, outputIndex) and add it to the transaction.
2. Use change(address) to add a change output.
3. Use setInputScript(inputIndex, (tx, output) => bsv.Script) to set the unlocking script for the input added in step 1.
4. Call seal() to finalize the transaction, and automatically calculate the correct transaction fee and change balance.

const demo = new Demo(4, 7);

const deployTx = await deployContract(demo, 1000); // Deploy a contract

const unlockingTx = new bsv.Transaction();

unlockingTx.addInput(createInputFromPrevTx(deployTx))
.change(privateKey.toAddress())
.setInputScript(0, (_) => {  // Set the unlocking script for 0-th input
    return demo.add(11).toScript();
})
.feePerKb(250) // Set transaction fee rate. Optional, default is 500 satoshis per KB
.seal()  // Finalize the transaction and automatically calculate the correct transaction fee and change amount

// Broadcast transaction
await sendTx(unlockingTx)

SigHashPreimage

If the public function of the contract contains parameters of type SigHashPreimage, it is more complicated to build the transaction calling it. This is because unlocking script containing SigHashPreimage affects the calculation of transaction fee and thus the satoshis amount in the output. In turn, satoshis affects the calculation of SigHashPreimage. The chained APIs provided by scryptlib hides the details of processing these inter-dependent calculations. You only need to build the transaction in the following way:

1. Transaction fees paid by the bitcoins locked in the contract:

In this case, no separate change output is included. The balances of all outputs are unknown before the transaction is built. Transaction fees will affect the calculation of all output balances.

1. Create an input from the transaction where the contract is located by createInputFromPrevTx(tx, outputIndex) and add it to the transaction.

2. Use setOutput(outputIndex, (tx) => bsv.Transaction.Output) to add one or more outputs. The output balance usually needs to subtract transaction fee;

   javascript const newAmount = amount - tx.getEstimateFee();

3. Use setInputScript(inputIndex, (tx, output) => bsv.Script) to set the unlocking script for the input added in step 1. The unlocking parameter usually contains a parameter that specifies the new balance of the contract and its calculation is the same as the previous step.

4. Call seal() to finalize the transaction and automatically calculate the correct transaction fee and change amount.

const counter = new StateCounter(0)

let amount = 8000

const lockingTx =  await deployContract(counter, amount) // Deploy a contract

const unlockingTx = new bsv.Transaction();

unlockingTx.addInput(createInputFromPrevTx(prevTx))
.setOutput(0, (tx) => {
    const newLockingScript = counter.getNewStateScript({
        counter: i + 1
    })
    const newAmount = amount - tx.getEstimateFee(); // To calculate the new balance of the contract, you need to subtract the transaction fee
    return new bsv.Transaction.Output({
        script: newLockingScript,
        satoshis: newAmount,
        })
})
.setInputScript(0, (tx, output) => {  // output contains the locking script and satoshis balance of the UTXO
    const preimage = getPreimage(tx, output.script, output.satoshis)
    const newAmount = amount - tx.getEstimateFee(); // To calculate the new balance of the contract, you need to subtract the transaction fee
    return counter.unlock(new SigHashPreimage(toHex(preimage)), newAmount).toScript()
})
.seal() // Finalize the transaction

// Broadcast transaction
await sendTx(unlockingTx)

2. Transaction fees paid by adding other inputs:

In this case, a separate change output is included. The balance of other outputs can generally be calculated before the transaction is built. Transaction fees only affect the calculation of change output.

1. Create an input from the transaction where the contract is located by createInputFromPrevTx(tx, outputIndex) and add it to the transaction
2. Use from([utxo]) to add inputs used to pay transaction fee.
3. Use addOutput() to add one or more outputs (add according to the contract business logic).
4. Use change(address) to add a change output.
5. Use setInputScript(inputIndex, (tx, output) => bsv.Script) to set the unlocking script for the input added in step 1. The unlocking parameters usually contains a parameter that specifies the change amount, which can be obtained through tx.getChangeAmount().
6. Use sign(privateKey) to sign all inputs added to pay transaction fee.
7. Call seal() to finalize the transaction.

...

const unlockingTx = new bsv.Transaction();

const newAmount = amount + spendAmount； // The new balance of contract has nothing to do with transaction fee

unlockingTx.addInput(createInputFromPrevTx(lockingTx))
.from(await fetchUtxos(privateKey.toAddress())) // Add inputs to pay transaction fees
.addOutput(new bsv.Transaction.Output({ // Add one or more outputs, newAmount is the new balance of the contract
    script: newLockingScript,
    satoshis: newAmount,
}))
.change(privateKey.toAddress())  // Add change output
.setInputScript(0, (tx, output) => {
    let preimage = getPreimage(
    tx,
    output.script,
    output.satoshis,
    0,
    sighashType
    );

    return advTokenSale.buy(
    new SigHashPreimage(toHex(preimage)), 
    new Ripemd160(toHex(pkh)), // Change address
    tx.getChangeAmount(), // Calculate the change balance
    new Bytes(toHex(publicKeys[i])), 
    numBought 
    ).toScript();

})
.sign(privateKey) // Sign all inputs added to pay transaction fee
.seal() // Finalize the transaction

// Broadcast transaction
lockingTxid = await sendTx(unlockingTx)

Extended APIs list

Api	description	parameters
setInputScript	Set input unlocking script	1. inputIndex Input index 2. (tx, output) => bsv.Script A callback function returns the unlocking script of the input
setOutput	Add output to the specified index	1. outputIndex Output index 2. (tx) => bsv.Transaction.Output A callback function returns the output
setLockTime	Set the transaction's nLockTime	1. nLockTime
setInputSequence	Set the sequenceNumber	1. inputIndex Input index 2. sequenceNumber
seal	Seal transactions. Transactions after sealing can no longer be modified	-
getChangeAmount	Get the balance of the change output	-
getEstimateFee	Estimate transaction fee based on transaction size and fee rate	-
checkFeeRate	Check whether the transaction fee meets the fee rate	1. feePerKb fee rate, satoshis per KB
prevouts	returns the serialization of all input outpoints	-