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Contract interactions

In this section, we'll write the logic in our app that will interact with the contract we have previously deployed. We'll be using the accounts already seeded in the Sandbox.

Showing user balance

Let's start by showing our user's private balance for the token across their accounts. To do this, we can leverage the balance_of_private unconstrained view function of the token contract:

balance_of_private
unconstrained fn balance_of_private(owner: AztecAddress) -> pub Field {
storage.balances.balance_of(owner).to_integer()
}
Source code: noir-projects/noir-contracts/contracts/token_contract/src/main.nr#L361-L365
info

Note that this function will only return a valid response for accounts registered in the Private eXecution Environment (PXE), since it requires access to the user's private state. In other words, you cannot query the private balance of another user for the token contract.

To do this, let's first initialize a new Contract instance using aztec.js that represents our deployed token contracts. Create a new src/contracts.mjs file with the imports for our artifacts and other dependencies:

// src/contracts.mjs
import { Contract } from "@aztec/aztec.js";
import { readFileSync } from "fs";
import TokenContractJson from "../contracts/token/target/token_contract-Token.json" assert { type: "json" };

And then add the following code for initializing the Contract instances:

get-tokens
export async function getToken(client) {
const addresses = JSON.parse(readFileSync('addresses.json'));
return Contract.at(AztecAddress.fromString(addresses.token), loadContractArtifact(TokenContractJson), client);
}
Source code: yarn-project/end-to-end/src/sample-dapp/contracts.mjs#L6-L11
info

You can use the typescript autogenerated interface instead of the generic Contract class to get type-safe methods.

We can now get the token instance in our main code in src/index.mjs, and query the private balance for each of the user accounts. To query a function, without sending a transaction, use the view function of the method:

showPrivateBalances
const accounts = await pxe.getRegisteredAccounts();
const token = await getToken(pxe);

for (const account of accounts) {
const balance = await token.methods.balance_of_private(account.address).simulate();
console.log(`Balance of ${account.address}: ${balance}`);
}
Source code: yarn-project/end-to-end/src/sample-dapp/index.mjs#L17-L26

Run this as node src/index.mjs and you should now see the following output:

Balance of 0x0c8a6673d7676cc80aaebe7fa7504cf51daa90ba906861bfad70a58a98bf5a7d: 100
Balance of 0x226f8087792beff8d5009eb94e65d2a4a505b70baf4a9f28d33c8d620b0ba972: 0
Balance of 0x0e1f60e8566e2c6d32378bdcadb7c63696e853281be798c107266b8c3a88ea9b: 0

Transferring private tokens

Now that we can see the balance for each user, let's transfer tokens from one account to another. To do this, we will first need access to a Wallet object. This wraps access to an PXE and also provides an interface to craft and sign transactions on behalf of one of the user accounts.

We can initialize a wallet using one of the getAccount methods from the accounts package, along with the corresponding signing and encryption keys:

import { getSchnorrAccount } from "@aztec/accounts/schnorr";
const wallet = await getSchnorrAccount(
client,
ENCRYPTION_PRIVATE_KEY,
SIGNING_PRIVATE_KEY
).getWallet();

For ease of use, accounts also ships with a helper getInitialTestAccountsWallets method that returns a wallet for each of the pre-initialized accounts in the Sandbox, so you can send transactions as any of them.

import { getInitialTestAccountsWallets } from "@aztec/accounts/testing";

We'll use one of these wallets to initialize the Contract instance that represents our private token contract, so every transaction sent through it will be sent through that wallet.

transferPrivateFunds
const [owner, recipient] = await getInitialTestAccountsWallets(pxe);
const token = await getToken(owner);

const tx = token.methods.transfer(owner.getAddress(), recipient.getAddress(), 1n, 0).send();
console.log(`Sent transfer transaction ${await tx.getTxHash()}`);
await showPrivateBalances(pxe);

console.log(`Awaiting transaction to be mined`);
const receipt = await tx.wait();
console.log(`Transaction has been mined on block ${receipt.blockNumber}`);
await showPrivateBalances(pxe);
Source code: yarn-project/end-to-end/src/sample-dapp/index.mjs#L60-L72

Let's go step-by-step on this snippet. We first get wallets for two of the Sandbox accounts, and name them owner and recipient. Then, we initialize the private token Contract instance using the owner wallet, meaning that any transactions sent through it will have the owner as sender.

Next, we send a transfer transaction, moving 1 unit of balance to the recipient account address. This has no immediate effect, since the transaction first needs to be simulated locally and then submitted and mined. Only once this has finished we can query the balances again and see the effect of our transaction. We are using a showPrivateBalances helper function here which has the code we wrote in the section above.

Run this new snippet and you should see the following:

Sent transfer transaction 16025a7c4f6c44611d7ac884a5c27037d85d9756a4924df6d97fb25f6e83a0c8

Balance of 0x0c8a6673d7676cc80aaebe7fa7504cf51daa90ba906861bfad70a58a98bf5a7d: 100
Balance of 0x226f8087792beff8d5009eb94e65d2a4a505b70baf4a9f28d33c8d620b0ba972: 0
Balance of 0x0e1f60e8566e2c6d32378bdcadb7c63696e853281be798c107266b8c3a88ea9b: 0

Awaiting transaction to be mined
Transaction has been mined on block 4

Balance of 0x0c8a6673d7676cc80aaebe7fa7504cf51daa90ba906861bfad70a58a98bf5a7d: 99
Balance of 0x226f8087792beff8d5009eb94e65d2a4a505b70baf4a9f28d33c8d620b0ba972: 1
Balance of 0x0e1f60e8566e2c6d32378bdcadb7c63696e853281be798c107266b8c3a88ea9b: 0
info

At the time of this writing, there are no events emitted when new private notes are received, so the only way to detect of a change in a user's private balance is via polling on every new block processed. This will change in a future release.

Working with public state

While private and public state are fundamentally different, the API for working with private and public functions and state from aztec.js is equivalent. To query the balance in public tokens for our user accounts, we can just call the balance_of_public view function in the contract:

showPublicBalances
const accounts = await pxe.getRegisteredAccounts();
const token = await getToken(pxe);

for (const account of accounts) {
const balance = await token.methods.balance_of_public(account.address).simulate();
console.log(`Balance of ${account.address}: ${balance}`);
}
Source code: yarn-project/end-to-end/src/sample-dapp/index.mjs#L76-L85
info

Since this we are working with pubic balances, we can now query the balance for any address, not just those registered in our local PXE. We can also send funds to addresses for which we don't know their public encryption key.

Here, since the token contract does not mint any initial funds upon deployment, the balances for all of our user's accounts will be zero. But we can send a transaction to mint tokens, using very similar code to the one for sending private funds:

mintPublicFunds
const [owner] = await getInitialTestAccountsWallets(pxe);
const token = await getToken(owner);

const tx = token.methods.mint_public(owner.getAddress(), 100n).send();
console.log(`Sent mint transaction ${await tx.getTxHash()}`);
await showPublicBalances(pxe);

console.log(`Awaiting transaction to be mined`);
const receipt = await tx.wait();
console.log(`Transaction has been mined on block ${receipt.blockNumber}`);
await showPublicBalances(pxe);
Source code: yarn-project/end-to-end/src/sample-dapp/index.mjs#L89-L101

And get the expected results:

Sent mint transaction 041d5b4cc68bcb5c6cb45cd4c79f893d94f0df0792f66e6fddd7718c049fe925
Balance of 0x0c8a6673d7676cc80aaebe7fa7504cf51daa90ba906861bfad70a58a98bf5a7d: 0
Balance of 0x226f8087792beff8d5009eb94e65d2a4a505b70baf4a9f28d33c8d620b0ba972: 0
Balance of 0x0e1f60e8566e2c6d32378bdcadb7c63696e853281be798c107266b8c3a88ea9b: 0

Awaiting transaction to be mined
Transaction has been mined on block 5

Balance of 0x0c8a6673d7676cc80aaebe7fa7504cf51daa90ba906861bfad70a58a98bf5a7d: 100
Balance of 0x226f8087792beff8d5009eb94e65d2a4a505b70baf4a9f28d33c8d620b0ba972: 0
Balance of 0x0e1f60e8566e2c6d32378bdcadb7c63696e853281be798c107266b8c3a88ea9b: 0

Next steps

In the next and final section, we'll set up automated tests for our application.