Path Payment

A path payment is where the asset sent can be different from the asset received. There are two possible path payment operations: 1) path_payment_strict_send, which allows the user to specify the amount of the asset to send, and 2) path_payment_strict_receive, which allows the user to specify the amount of the asset received. Read more in the Path Payments Encyclopedia Entry.

User experience

With BasicPay, the user sends a path payment by navigating to the Payments page, where they can either select a user from their contacts or input the public key of a destination address. They then select the Send and Receive Different Assets toggle and determine whether they want to specify the asset sent or received. Finally they select the asset sent and the asset received and the amounts and select the Preview Transaction button.

The user will then preview the transaction, input their pincode, and select the Confirm button to sign and submit the transaction to the network.

Code implementation

The /dashboard/send page

Most of this page has been discussed in the Payment section. Below, we're highlighting the unique pieces that are added to BasicPay to allow for the path payment feature.

/src/routes/dashboard/send/+page.svelte

<script>
  // We import some of our `$lib` functions
  import {
    fetchAccount,
    submit,
    fetchAccountBalances,
    findStrictSendPaths,
    findStrictReceivePaths,
  } from "$lib/stellar/horizonQueries";
  import {
    createCreateAccountTransaction,
    createPathPaymentStrictReceiveTransaction,
    createPathPaymentStrictSendTransaction,
    createPaymentTransaction,
  } from "$lib/stellar/transactions";

  /* ... */

  // Define some component variables that will be used throughout the page
  let destination = "";
  $: otherDestination = destination === "other";
  let otherPublicKey = "";
  let sendAsset = "native";
  let sendAmount = "";
  let receiveAsset = "";
  let receiveAmount = "";
  let memo = "";
  let createAccount = null;
  let pathPayment = false;
  let availablePaths = [];
  let strictReceive = false;
  let paymentXDR = "";
  let paymentNetwork = "";

  /* ... */

  // Query Horizon for available paths between a combination of source and destination assets and accounts.
  const findPaths = async () => {
    // Query the paths from Horizon
    let paths = strictReceive
      ? await findStrictReceivePaths({
          sourcePublicKey: data.publicKey,
          destinationAsset: receiveAsset,
          destinationAmount: receiveAmount,
        })
      : await findStrictSendPaths({
          sourceAsset: sendAsset,
          sourceAmount: sendAmount,
          destinationPublicKey: otherDestination ? otherPublicKey : destination,
        });
    // Fill the component variable `availablPaths` with our returned paths
    availablePaths = paths;
    // If both send and receive assets have been selected re-select the path
    // to update the relevant amount
    if (receiveAsset && sendAsset) {
      selectPath();
    }
  };

  // Select a path for use in the path payment operation, and set the component variables accordingly.
  const selectPath = () => {
    if (strictReceive) {
      // Set the `sendAmount` variable to the chosen path amount. The
      // filtering we do checks if the asset_type matches because that
      // will give us our 'native' XLM asset, otherwise we match on the
      // asset_code.
      sendAmount = availablePaths.filter(
        (path) =>
          path.source_asset_type === sendAsset ||
          sendAsset.startsWith(path.source_asset_code),
      )[0].source_amount;
    } else {
      // Set the `receiveAmount` variable to the chosen path amount. The
      // filtering we do checks if the asset_type matches because that
      // will give us our 'native' XLM asset, otherwise we match on the
      // asset_code.
      receiveAmount = availablePaths.filter(
        (path) =>
          path.destination_asset_type === receiveAsset ||
          receiveAsset.startsWith(path.destination_asset_code),
      )[0].destination_amount;
    }
  };

  /* ... */

  // Create a payment transaction depending on user selections, and present it to the user for approval or rejection.
  const previewPaymentTransaction = async () => {
    let { transaction, network_passphrase } = createAccount
      ? await createCreateAccountTransaction({
          /* ... */
        })
      : // highlight-start
        pathPayment && strictReceive
        ? await createPathPaymentStrictReceiveTransaction({
            source: data.publicKey,
            sourceAsset: sendAsset,
            sourceAmount: sendAmount,
            destination: otherDestination ? otherPublicKey : destination,
            destinationAsset: receiveAsset,
            destinationAmount: receiveAmount,
            memo: memo,
          })
        : pathPayment && !strictReceive
          ? await createPathPaymentStrictSendTransaction({
              source: data.publicKey,
              sourceAsset: sendAsset,
              sourceAmount: sendAmount,
              destination: otherDestination ? otherPublicKey : destination,
              destinationAsset: receiveAsset,
              destinationAmount: receiveAmount,
              memo: memo,
            })
          : // highlight-end
            await createPaymentTransaction({
              /* ... */
            });

    /* ... */
  };
</script>

<!-- HTML has been omitted from this tutorial. Please check the source file -->

Source: https://github.com/stellar/basic-payment-app/blob/main/src/routes/dashboard/send/+page.svelte

The transaction functions

In the above section, we used the createPathPaymentStrictReceiveTransaction and createPathPaymentStrictSendTransaction functions. These are used to create transactions that contain the actual path payment operation.

/src/lib/stellar/transactions.js

// Constructs and returns a Stellar transaction that will contain a path payment strict send operation to send/receive different assets.
export async function createPathPaymentStrictSendTransaction({
  source,
  sourceAsset,
  sourceAmount,
  destination,
  destinationAsset,
  destinationAmount,
  memo,
}) {
  // First, we setup our transaction by loading the source account from the
  // network, and initializing the TransactionBuilder. This is the first step
  // in constructing all Stellar transactions.
  let server = new Server(horizonUrl);
  let sourceAccount = await server.loadAccount(source);
  let transaction = new TransactionBuilder(sourceAccount, {
    networkPassphrase: networkPassphrase,
    fee: maxFeePerOperation,
  });

  // We work out the assets to be sent by the source account and received by
  // the destination account
  let sendAsset =
    sourceAsset === "native"
      ? Asset.native()
      : new Asset(sourceAsset.split(":")[0], sourceAsset.split(":")[1]);
  let destAsset =
    destinationAsset === "native"
      ? Asset.native()
      : new Asset(
          destinationAsset.split(":")[0],
          destinationAsset.split(":")[1],
        );

  // We will calculate an acceptable 2% slippage here for... reasons?
  let destMin = ((98 * parseFloat(destinationAmount)) / 100).toFixed(7);

  // If a memo was supplied, add it to the transaction
  if (memo) {
    transaction.addMemo(Memo.text(memo));
  }

  // Add a single `pathPaymentStrictSend` operation
  transaction.addOperation(
    Operation.pathPaymentStrictSend({
      sendAsset: sendAsset,
      sendAmount: sourceAmount.toString(),
      destination: destination,
      destAsset: destAsset,
      destMin: destMin,
    }),
  );

  // Before the transaction can be signed, it requires timebounds, and it must
  // be "built"
  let builtTransaction = transaction.setTimeout(standardTimebounds).build();
  return {
    transaction: builtTransaction.toXDR(),
    network_passphrase: networkPassphrase,
  };
}

// Constructs and returns a Stellar transaction that will contain a path payment strict receive operation to send/receive different assets.
export async function createPathPaymentStrictReceiveTransaction({
  source,
  sourceAsset,
  sourceAmount,
  destination,
  destinationAsset,
  destinationAmount,
  memo,
}) {
  // First, we setup our transaction by loading the source account from the
  // network, and initializing the TransactionBuilder. This is the first step
  // in constructing all Stellar transactions.
  let server = new Server(horizonUrl);
  let sourceAccount = await server.loadAccount(source);
  let transaction = new TransactionBuilder(sourceAccount, {
    networkPassphrase: networkPassphrase,
    fee: maxFeePerOperation,
  });

  // We work out the assets to be sent by the source account and received by
  // the destination account
  let sendAsset =
    sourceAsset === "native"
      ? Asset.native()
      : new Asset(sourceAsset.split(":")[0], sourceAsset.split(":")[1]);
  let destAsset =
    destinationAsset === "native"
      ? Asset.native()
      : new Asset(
          destinationAsset.split(":")[0],
          destinationAsset.split(":")[1],
        );

  /** @todo Figure out a good number to use for slippage. And why! And how to calculate it?? */
  // We will calculate an acceptable 2% slippage here for... reasons?
  let sendMax = ((100 * parseFloat(sourceAmount)) / 98).toFixed(7);

  // If a memo was supplied, add it to the transaction
  if (memo) {
    transaction.addMemo(Memo.text(memo));
  }

  // Add a single `pathPaymentStrictSend` operation
  transaction.addOperation(
    Operation.pathPaymentStrictReceive({
      sendAsset: sendAsset,
      sendMax: sendMax,
      destination: destination,
      destAsset: destAsset,
      destAmount: destinationAmount,
    }),
  );

  // Before the transaction can be signed, it requires timebounds, and it must
  // be "built"
  let builtTransaction = transaction.setTimeout(standardTimebounds).build();
  return {
    transaction: builtTransaction.toXDR(),
    network_passphrase: networkPassphrase,
  };
}

Source: https://github.com/stellar/basic-payment-app/blob/main/src/lib/stellar/transactions.js