Step 4: Initiate transfer transaction

Initiate transaction given the prior quote.

The full API reference for our /send endpoint can be found in the API reference section.

Similar to our quoting system, to initiate a transaction it requires all the following required information:

Bridge
Source chain
Destination chain
Source Token on the Source chain
Destination Token on the Destination chain
The amount you want to send
Source Wallet Address to pull funds from
Contract Call Information

Making a Request

POST: https://swap.prod.swing.xyz/v0/transfer/send

Body Parameters:

Key	Example	Description
`fromChain`	ethereum	The blockchain where the transaction originates.
`fromTokenAddress`	0x0000000000000000000000000000000000000000	Source Token Address
`fromUserAddress`	0x018c15DA1239B84b08283799B89045CD476BBbBb	Sender's wallet address
`tokenSymbol`	ETH	Source token slug
`toTokenAddress`	0x0000000000000000000000000000000000000000	Destination token address.
`toChain`	polygon	Destination chain
`toTokenAmount`	4376081	Amount of the destination token to be received.
`toTokenSymbol`	MATIC	Destination Chain slug
`toUserAddress`	0x018c15DA1239B84b08283799B89045CD476BBbBb	Receiver's wallet address
`tokenAmount`	1000000000000000000	Amount of the source token being sent (in wei for ETH).
`projectId`	`replug`	Your project's ID
`route`	see Get Qoute	Selected route
`contractCallInfo`	see Generating Contract Calldata section	Target contract info for deposits

Sample Request

const result = await axios.post(
  'https://swap.prod.swing.xyz/v0/transfer/send',
  {
    fromChain: 'polygon',
    tokenSymbol: 'USDC',
    fromTokenAddress: '0xcbe56b00d173a26a5978ce90db2e33622fd95a28',
    fromUserAddress: '0x018c15DA1239B84b08283799B89045CD476BBbBb',
    toChain: 'ethereum',
    toTokenSymbol: 'ETH',
    toTokenAddress: '0x0000000000000000000000000000000000000000',
    toUserAddress: '0x018c15DA1239B84b08283799B89045CD476BBbBb',
    tokenAmount: '1000000000',
    projectId: 'replug',
    route: [
      {
        bridge: 'stargate',
        bridgeTokenAddress: '0xc2132d05d31c914a87c6611c10748aeb04b58e8f',
        steps: ['allowance', 'approve', 'send'],
        name: 'USDT',
        part: 100,
      },
    ],
    contractCallInfo: {
      toContractAddress: '0x9ee91F9f426fA633d227f7a9b000E28b9dfd8599',
      toContractCallData:
        '0xa9059cbb0000000000000000000000001234567890abcdef1234567890abcdef12345678000000000000000000000000',
      outputTokenAddress: '0x018c15DA1239B84b08283799B89045CD476BBbBb',
    },
  },
);

Generating Contract Calldata

For Contract Calls, Swing's /send endpoint gives us the option to instruct Swing's API on what to do with our funds post swap.

For example, we can instruct Swing to deposit our funds into a smart contract of our choosing. A more practical instruction would be to stake our funds into a staking contract, and then send us that staking contract's LP token. We can achieve this using the contractCallInfo object parameter in the /send endpoint.

To execute a contract call via Swing, we'll need three (3) parameters to be passed to our contractCallInfo object:

Property	Example		Description
`toContractAddress`	0x9ee91F9f426fA633d227f7a9b000E28b9dfd8599	required	Smart Contract address we'll deposit your funds into
`toContractCallData`	0xa9059cbb0000000000000000000000abcdef........	required	Calldata to be passed to Swing
`outputTokenAddress`	0x018c15DA1239B84b08283799B89045CD476BBbBb	optional	Address of Liquidity Provider Token to be received by sender

When performing a contract call transaction, you are required to provide the contract calldata for the contract to which you wish to send your funds.

Let's say we've got some USDC on the Polygon chain and want to stake some ETH into LIDO's Ethereum staking contracts via Swing. The following will take place to make this transaction to be possible:

Swing will start the transaction by deducting the necessary funds from the fromUserAddress.
Swing will then bridge our USDC (tokenSymbol) from the Polygon chain (fromChain) to the Ethereum Chain (toChain).
Swing will then swap that USDC (tokenSymbol) to ETH (toTokenSymbol) on the destination chain (toChain) and send it to wallet address supplied to the toUserAddress.
Using the contractCallInfo parameter, Swing will then execute a function on a target Smart Contract and deposit your funds.
Optionally, If the target smart contract emits any token to the sender's wallet like an LP token, Swing can then send those token to the address supplied to the toUserAddress.

If you omit the toUserAddress parameter, Swing will default it's value to whatever value you supply to the fromUserAddress.

To get started, we'll first need to generate the necessary callData by using LIDO's staking contract ABI:

// Using partial ABI for simplicity
const LIDO_ABI = [
  {
    constant: false,
    inputs: [
      { name: '_amount', type: 'uint256' },
      { name: '_referral', type: 'address' },
    ],
    name: 'submit',
    outputs: [{ name: '', type: 'uint256' }],
    payable: true,
    stateMutability: 'payable',
    type: 'function',
  },
];

It's very important to remember what we're trying to achieve here. We want Swing to interact with LIDO's contract immediately after swapping our funds. When Swing is done swapping our assets, we want Swing to call the submit function on LIDO's contract.

For this example, we'll be using ethers.js to generate the callData required for our contract call example.

import { ethers } from 'ethers';

Using ethers to generate our callData is fairly straight forward. First we'll load up LIDO's staking smart contract using it's ABI and contract address:

const LIDO_ADDRESS = '0x9ee91F9f426fA633d227f7a9b000E28b9dfd8599';
const MY_REWARDS_ADDRESS = '0x184AbEfBdCa24Ce0Dd964a74f6d5E69CE44D9579';
const LIDO_OUTPUT_TOKEN_ADDRESS = '0xae7ab96520DE3A18E5e111B5EaAb095312D7fE84'; // sETH token address
const toAmount = '2578306944516966';
 
const contract = new ethers.Contract(
  LIDO_ADDRESS,
  LIDO_ABI,
  ethers.getDefaultProvider(),
);

Next, we'll encode the function data by using the encodeFunctionData() method to generate the final callData for the submit() function in LIDO's staking contract.

LIDO's submit() function takes two parameters, namely:

_amount — the amount of tokens to be staked
_referral — an address for incentivisation reward

const interface = new ethers.utils.Interface(LIDO_ABI);
const callData = interface.encodeFunctionData('submit', [
  toAmount,
  MY_REWARDS_ADDRESS,
]);

Putting it all together into a function called generateCallData():

const LIDO_ADDRESS = '0x9ee91F9f426fA633d227f7a9b000E28b9dfd8599';
const MY_REWARDS_ADDRESS = '0x184AbEfBdCa24Ce0Dd964a74f6d5E69CE44D9579';
const LIDO_OUTPUT_TOKEN_ADDRESS = '0xae7ab96520DE3A18E5e111B5EaAb095312D7fE84'; // sETH token address
const toAmount = '1000000000';
 
const LIDO_ABI = [
  {
    constant: false,
    inputs: [
      { name: '_amount', type: 'uint256' },
      { name: '_referral', type: 'address' },
    ],
    name: 'submit',
    outputs: [{ name: '', type: 'uint256' }],
    payable: true,
    stateMutability: 'payable',
    type: 'function',
  },
];
 
async function generateCallData() {
  const contract = new ethers.Contract(
    LIDO_ADDRESS,
    LIDO_ABI,
    ethers.getDefaultProvider(),
  );
 
  const interface = new ethers.utils.Interface(LIDO_ABI);
  const callData = await interface.encodeFunctionData('submit', [
    toAmount,
    MY_REWARDS_ADDRESS,
  ]);
 
  return callData;
}

Next, let's generate our callData and make a request to Swing to get our transaction ready to be sent over to the network:

const sendTransaction = async () => {
  const callData = await generateCallData(); //sample output: 0xa9059cbb0000000000000000000000001234567890abcdef1234567890abcdef12345678000000000000000000000000...
 
  const result = await axios.post(
    'https://swap.prod.swing.xyz/v0/transfer/send',
    {
      fromChain: 'polygon',
      tokenSymbol: 'USDC',
      fromTokenAddress: '0x3c499c542cef5e3811e1192ce70d8cc03d5c3359',
      fromUserAddress: '0x018c15DA1239B84b08283799B89045CD476BBbBb',
      toChain: 'ethereum',
      toTokenSymbol: 'ETH',
      toTokenAddress: '0x0000000000000000000000000000000000000000',
      tokenAmount: '1000000000',
      projectId: 'replug', // create your project here: https://platform.swing.xyz/
      route: [
        {
          bridge: 'stargate',
          bridgeTokenAddress: '0xc2132d05d31c914a87c6611c10748aeb04b58e8f',
          steps: ['allowance', 'approve', 'send'],
          name: 'USDT',
          part: 100,
        },
      ],
      contractCallInfo: {
        toContractAddress: LIDO_ADDRESS,
        toContractCallData: callData,
        outputTokenAddress: LIDO_OUTPUT_TOKEN_ADDRESS, //Address of Liquidity Provider Token to be received by sender
      },
    },
  );
  return result.data;
};

Since performing a contract call will change the state of a user's wallet, the next step of this process requires invoking a contract-level function on the wallet address you supply to the fromUserAddress parameter. Swing only returns the necessary call-data that will need to be signed and executed by the local wallet (i.e. Metamask, WalletConnect, Coinbase Wallet).

Sending our transaction over the network

It's important to note that the /send endpoint only builds an unsigned transaction. To actually send the transaction, you'll have to give Swing the permission to do so by signing the transaction using a non-custodial wallet like MetaMask or Coinbase.

Now that we've got our txData, we next need to invoke a function in one of Swing's contracts to be able to take funds from a user's wallet and begin the transaction.

We'll be using ethers.js with MetaMask Wallet to demonstrate how you can execute our txData, but feel free to use whatever wallet provider you're comfortable using.

The txData from the /send will look something like this:

{
   ....
    "tx": {
        "from": "0x018c15DA1239B84b08283799B89045CD476BBbBb",
        "to": "0x39E3e49C99834C9573c9FC7Ff5A4B226cD7B0E63",
        "data": "0x301a3720000000000000000000000000eeeeeeeeeeee........",
        "value": "0x0e35fa931a0000",
        "gas": "0x06a02f"
    }
   ....
}

First, we'll call the /send endpoint via the sendTransaction() function and retrieve our txData from it's response:

const transaction = await sendTransaction();
 
let txData = {
  data: transaction.tx.data,
  from: transaction.tx.from,
  to: transaction.tx.to,
  value: transaction.tx.value,
  gasLimit: transaction.tx.gas,
};

Next, we'll create a function called getSigner() to retrieve our Signer Object using MetaMask's Injected Provider:

async function getSigner() {
  const provider = await new ethers.providers.Web3Provider(window.ethereum);
  return provider.getSigner();
}

Now that we've got our Signer Object, we can go ahead and execute the txData by calling the sendTransaction() function present in our signer object.

async function executeTxData(txData) {
  const signer = await getSigner();
 
  const txResponse = await signer.sendTransaction(txData);
 
  const receipt = await txResponse.wait();
  console.log('Transaction receipt:', receipt);
 
  return txResponse.txHash; // We'll need this later for checking the transaction's status
}

Finally, immediately after we get our txData from the /send Response, we'll prompt a user's MetaMask wallet to execute our txData by calling executeTxData():

const transaction = await sendTransaction();
 
let txData = {
  data: transaction.tx.data,
  from: transaction.tx.from,
  to: transaction.tx.to,
  value: transaction.tx.value,
  gasLimit: transaction.tx.gas,
};
 
const txHash = await executeTxData(txData); // This will open MetaMask and ask the user to confirm the transaction
 
console.log(txHash);