# Front-end app: smart contract interaction

## Intro

In this tutorial, you will learn how to make the front-end of your App interact with smart contracts using [`@polkadot/api-contract`](https://polkadot.js.org/docs/api-contract), more specifically:

* how to read values stored in your smart contract
* how to send signed transactions to your smart contract

{% hint style="info" %}
Before we start, you may want to check out our [Bulletin Board Example](https://github.com/Cardinal-Cryptography/bulletin-board-example) repository. It is a simple yet comprehensive tutorial dApp that can be used to learn more about writing smart contracts in ink! and how to build your first dApp on the Aleph Zero ecosystem, or simply bootstrap your project. All the code snippets in this tutorial come from this repository.
{% endhint %}

{% hint style="info" %}
It is also worth checking out the [Aleph Zero Signer Integration](https://docs.alephzero.org/aleph-zero/build/aleph-zero-signer-integration) tutorial: it could be a good starting point for you.
{% endhint %}

{% hint style="info" %}
**For beginners**: blocks like this contain some explanation about basic concepts. If you're an experienced developer, please feel free to skip them.
{% endhint %}

## Connect to a deployed smart contract

{% hint style="info" %}
**For beginners**: when interacting with the blockchain, you actually connect to a single node running as part of the network. The states of the nodes are constantly being synchronized by the protocol, so it doesn't matter which node you connect to.

If you're using the public Aleph Zero endpoints described below, you connect to a single endpoint that serves as an umbrella for a few nodes run by the Aleph Zero Foundation: it will automatically choose the best endpoint.
{% endhint %}

The first step will be to create an API instance to connect to a running node. For that, we need a provider: the default instance of `WsProvider` connects to `"ws://127.0.0.1:9944"` which usually is your local node's endpoint. For Aleph Zero Testnet use `"wss://ws.test.azero.dev"` and for Aleph Zero Mainnet: `"wss://ws.azero.dev"`.

```typescript
import { ApiPromise, WsProvider } from '@polkadot/api';
...

const APP_PROVIDER_URL = "ws://127.0.0.1:9944";

const wsProvider = new WsProvider(APP_PROVIDER_URL);
const api = await ApiPromise.create({ provider: wsProvider });
```

The `@polkadot/api-contract` comes with 4 general helpers (see the [docs](https://polkadot.js.org/docs/api-contract/start/basics/)). Here, we are using `ContractPromise` which allows us to interact with already deployed contracts.

{% hint style="info" %}
**For beginners**: the contract's ABI (Application Binary Interface) is a JSON file describing, among other things, what methods are available, what selectors they have, what are the parameters and return types and so on. It also contains the docstrings you put in your code that will be displayed by the [Contracts UI](https://contracts-ui.substrate.io) if you choose to use it.

Importantly, the terms **ABI** and **metadata** are used interchangeably in this context.
{% endhint %}

To create the contract-api instance, we will need the contract's address and ABI. The contract ABI can be found in the artifacts generated when building the contract (see [Creating your first contract](https://docs.alephzero.org/aleph-zero/aleph-zero-smart-contracts-basics/creating-your-first-contract#compiling)).

```typescript
import { ContractPromise } from '@polkadot/api-contract';
...
import bulletinBoardMetadata from '../metadata/metadata_bulletin_board.json';
import addresses from '../metadata/addresses.json';

const bulletin_board_address = addresses.bulletin_board_address;

const contract = new ContractPromise(
    api,
    bulletinBoardMetadata,
    bulletin_board_address
  );
```

## Query a contract state

{% hint style="info" %}
**For beginners**: for the purpose of this tutorial, we can assume that an extrinsic is the same as a transaction that you send to the blockchain. If you want to read more, please refer to the [Substrate documentation on extrinsics](https://docs.substrate.io/learn/transaction-types/).

The concept of gas (fees for executing transactions) is approached slightly differently to what you may be used to from EVM-based chains: gas is a two-dimensional value and there's no concept of gas price (it is calculated based only on how much resources are used + an optional tip). To learn more, please refer to the [Substrate documentation of transaction fees](https://docs.substrate.io/build/tx-weights-fees/).
{% endhint %}

Under the hood, querying a contract is an extrinsic dry run. It is not submitted on the chain, however, it requires to specify the `gasLimit` which refers to the maximum resources used by a contract call. Because we are not submitting an extrinsic to the chain (we're only performing a dry-run), it is safe to use sufficiently high values to ensure that the call won’t be reverted. For contract calls, Substrate uses a 2-dimensional weight (gas) system which consists of:

* `refTime`: the amount of computational time used for execution, in picoseconds;
* `proofSize`: the amount of storage in bytes that a transaction is allowed to read.

```typescript
import { BN, BN_ONE } from '@polkadot/util';
import type { WeightV2 } from '@polkadot/types/interfaces';
...
const MAX_CALL_WEIGHT = new BN(5_000_000_000_000).isub(BN_ONE);

const readOnlyGasLimit = api.registry.createType('WeightV2', {
    refTime: new BN(1_000_000_000_000),
    proofSize: MAX_CALL_WEIGHT,
  }) as WeightV2;
```

In this example, we call the contract method `getByAccount` which takes one argument `accountId`. For simplicity, we are using the contract address as the caller.

```typescript
...
const {
  gasConsumed,
  gasRequired,
  storageDeposit,
  result,
  output,
  debugMessage,
} = await contract.query.getByAccount(
  contract.address, // caller address
  {
    gasLimit: readOnlyGasLimit,
  },
  accountId
);
if (result.isOk && output) {
  console.log(output.toHuman());
}
if (result.isErr) {
  console.log(result.toHuman());
}
```

Contract query results in [`ContractCallOutcome`](https://github.com/polkadot-js/api/blob/22d06b93a6c936563aab7466c09ee09cde3adc9c/packages/api-contract/src/types.ts#L55C1-L62C2). If the query is successful, you can extract the return value from the output object. See how it can be done [here](https://github.com/Cardinal-Cryptography/bulletin-board-example/blob/d4262aff974630a2dbe4cfa86bdbb5a067f76acb/frontend/src/utils/getPostByAccount.ts#L39-L41).

## Send signed transaction to a contract

{% hint style="info" %}
**For beginners**: we can assume that, in the context of smart contracts, a signed transaction will be needed whenever we want to call a method that modifies the smart contract's state. These will be the methods that use `&mut self` as the first argument.
{% endhint %}

### Signing account

To sign a transaction, we need a wallet. To keep this example simple, we will use [`@polkadot/extension-dapp`](https://polkadot.js.org/docs/extension) to retrieve wallet providers added to the app page and assume that there’s at least one account. See the [Aleph Zero Signer Integration](https://docs.alephzero.org/aleph-zero/build/aleph-zero-signer-integration) tutorial for more information.

```typescript
import {
  web3Enable,
  web3Accounts,
} from "@polkadot/extension-dapp";

const APP_NAME = 'Aleph Zero The Bulletin Board';
const injectedExtensions = await web3Enable(APP_NAME);

// Return list of accounts imported from extensions
const accounts: InjectedAccountWithMeta[] = await web3Accounts();
// For simplicity, here we assume that there's at least one account in the connected wallet extensions 
const userAccount = accounts[0];
```

### Gas estimation

To sign and send a transaction to a contract, we should estimate values for `gasLimit`. Although the unused gas is refunded after the call, it is good practice to specify a reasonable `gasLimit` for transactions. As shown [here](#query-a-contract-state), this (`gasRequired`) can be estimated with `contract.query.[method]`. Here is an alternative way of how to do it using `api.call.contractsApi.call<ContractExecResult>`.

{% hint style="warning" %}
In practice, when estimating `gasLimit` using `gasRequired` you may want to add some margin to ensure the transaction won't be reverted due to insufficient `gasLimit`.&#x20;
{% endhint %}

{% hint style="info" %}
**For beginners**: the term `message` below refers to the methods of the smart contract marked with the `#[ink(message)]` macro: these will be the contract's methods that are callable from outside of the contract. In most cases, the terms \[contract] `message` and `method` will be used interchangeably.
{% endhint %}

The first step is to get [`AbiMessage`](https://github.com/polkadot-js/api/blob/22d06b93a6c936563aab7466c09ee09cde3adc9c/packages/api-contract/src/types.ts#L33C1-L34C1) for the contract method. The function below searches for the method in the contract ABI and returns it if found.

```typescript
...
type Result<T, E = Error> = { ok: true; value: T } | { ok: false; error: E };

const toContractAbiMessage = (
  contractPromise: ContractPromise,
  message: string
): Result<AbiMessage, string> => {
  const value = contractPromise.abi.messages.find((m) => m.method === message);

  if (!value) {
    const messages = contractPromise?.abi.messages
      .map((m) => m.method)
      .join(", ");

    const error = `"${message}" not found in metadata.spec.messages: [${messages}]`;
    console.error(error);

    return { ok: false, error };
  }

  return { ok: true, value };
};
```

The `getGasLimit(...)` function below estimates the `gasRequired` for this contract call which should be sufficient to cover the gas fees when submitting the actual extrinsic. There are also other contract call options which can be adjusted:

* `storageDepositLimit` - The maximum amount of balance that can be charged/reserved for the storage consumed.
* `value` - The balance (in native currency, `AZERO` in our case) to transfer from the caller to the contract. Non-zero values here apply only to methods marked with the `#[ink(message, payable)]` macro.

```typescript
import type { Weight, ContractExecResult } from "@polkadot/types/interfaces";
import { AbiMessage, ContractOptions } from "@polkadot/api-contract/types";
...
const getGasLimit = async (
  api: ApiPromise,
  userAddress: string,
  message: string,
  contract: ContractPromise,
  options = {} as ContractOptions,
  args = [] as unknown[]
  // temporarily type is Weight instead of WeightV2 until polkadot-js type `ContractExecResult` will be changed to WeightV2
): Promise<Result<Weight, string>> => {
  const abiMessage = toContractAbiMessage(contract, message);
  if (!abiMessage.ok) return abiMessage;

  const { value, gasLimit, storageDepositLimit } = options;

  const { gasConsumed, gasRequired, storageDeposit, debugMessage, result } =
    await api.call.contractsApi.call<ContractExecResult>(
      userAddress,
      contract.address,
      value ?? new BN(0),
      gasLimit ?? null,
      storageDepositLimit ?? null,
      abiMessage.value.toU8a(args)
    );
 
  return { ok: true, value: gasRequired };
};
```

### Sign and send the transaction

Function `sendPost(...)` puts all the pieces together. It calls the payable method on the contract `post(expiresAfter: u32, postText: String)` and handles the result.

```typescript
...
const sendPost = async (
  expiresAfter: number,
  postText: string,
  totalPrice: number,
  api: ApiPromise,
  userAccount: InjectedAccountWithMeta
): Promise<void> => {
  if (!userAccount.meta.source) return;

  const contract = new ContractPromise(
    api,
    bulletinBoardMetadata,
    addresses.bulletin_board_address
  );

  const injector = await web3FromSource(userAccount.meta.source);

  const options: ContractOptions = {
    value: totalPrice,
  };
  const gasLimitResult = await getGasLimit(
    api,
    userAccount.address,
    "post",
    contract,
    options,
    [expiresAfter, postText]
  );

  if (!gasLimitResult.ok) {
    console.log(gasLimitResult.error);
    return;
  }

  const { value: gasLimit } = gasLimitResult;

  const tx = contract.tx.post(
    {
      value: totalPrice, // amount of native token to be transferred
      gasLimit,
    },
    expiresAfter,
    postText
  );

  await tx
    .signAndSend(
      userAccount.address,
      { signer: injector.signer },
      ({ events = [], status }) => {
        events.forEach(({ event }) => {
          const { method } = event;
          if (method === "ExtrinsicSuccess" && status.type === "InBlock") {
            console.log("Success!");
          } else if (method === "ExtrinsicFailed") {
            console.log(`An error occured: ${method}.`);
          }
        });
      }
    )
    .catch((error) => {
      console.log(`An error occured: ${error}.`);
    });
};
```

To learn more about how to handle transaction events see [Transaction Subscriptions](https://polkadot.js.org/docs/api/start/api.tx.subs). Also, see how it is done in the [Bulletin Board Example](https://github.com/Cardinal-Cryptography/bulletin-board-example/blob/main/frontend/src/utils/sendPost.ts).

{% hint style="warning" %}
When running [substrate-contract-node](https://github.com/paritytech/substrate-contracts-node), the blocks are never finalized since it does not use a finality mechanism by default. On a live network such as Aleph Zero Testnet, you should expect a ‘Finalized’ transaction status soon after the ‘InBlock’ status.

If you want to run a local chain that will work exactly as the Testnet, you can use [`the ./scripts/run_nodes.sh`](https://github.com/Cardinal-Cryptography/aleph-node/blob/main/scripts/run_nodes.sh) script, which will bootstrap a small chain locally.
{% endhint %}

## Closing remarks

We hope that this tutorial helped you learn more about how to interact with smart contracts in the front-end application. You shouldn't stop here! Check out other resources that may help you extend your dApp:

* [Nightly-connect](https://connect.nightly.app/docs/): A permissionless, open-source solution that serves as both a wallet adapter and a bridge wallet, enabling connections through QR codes or deep links.
* [Typechain](https://github.com/Brushfam/typechain-polkadot): Generate Typescript wrappers around your smart contract!
* [ink!athon](https://github.com/scio-labs/inkathon): A full-stack dApp boilerplate for ink! smart contracts with an integrated front-end.