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Your First Dapp

In this tutorial, you will learn how to build a dapp on the Aptos Blockchain. A dapp usually consists of a user interface written in JavaScript, which interacts with one or more Move Modules.

For this tutorial, we will use the Move Module HelloBlockchain described in Your first Move Module and focus on building the user interface.

We will use:

The end result is a dapp that lets users publish and share snippets of text on the Aptos Blockchain.

The full source code for this tutorial is available here.

Prerequisites

Aptos Wallet

Before starting this tutorial, you should install the Aptos Wallet extension.

After you install it:

  1. Open the Wallet and click Create a new wallet. Then click Create account to create an Aptos Account.
  2. Copy the private key. You will need it to set up the Aptos CLI in the next section.
note

Ensure that your account has sufficient funds to perform transactions by clicking the Faucet button.

Aptos CLI

  1. Install the Aptos CLI.

  2. Run aptos init, and when it asks for your private key, paste the private key from the Aptos Wallet that you copied earlier. This will initialize the Aptos CLI to use the same account as used by the Aptos Wallet.

  3. Run aptos account list to verify that it is working.

Step 1: Set up a single page app

We will now set up the frontend user interface for our dapp. We will use create-react-app to set up the app in this tutorial, but neither React nor create-react-app are required. You can use your preferred JavaScript framework.

$ npx create-react-app first-dapp --template typescript
$ cd first-dapp
$ npm start

You will now have a basic React app up and running in your browser.

Step 2: Integrate the Aptos Wallet Web3 API

The Aptos Wallet provides a Web3 API for dapps at window.aptos. You can see how it works by opening up the browser console and running await window.aptos.account(). It will print out the address corresponding to the account you set up in the Aptos Wallet.

Next we will update our app to use this API to display the Wallet account's address.

Wait until window.aptos is defined

The first step when integrating with the window.aptos API is to delay rendering the application until the window.onload event has fired.

Open up src/index.tsx and change the following code snippet:

root.render(
<React.StrictMode>
<App />
</React.StrictMode>
);

to this:

window.addEventListener('load', () => {
root.render(
<React.StrictMode>
<App />
</React.StrictMode>
);
});

This change will ensure that the window.aptos API has been initialized by the time we render the app (if we render too early, the Wallet extension may not have had a chance to initialize the API yet and thus window.aptos will be undefined).

(Optional) TypeScript setup for window.aptos

If you are using TypeScript, you may also want to inform the compiler of the existence of the window.aptos API. Add the following to src/index.tsx:

declare global {
interface Window { aptos: any; }
}

This lets us use the window.aptos API without having to do (window as any).aptos.

Display window.aptos.account() in the app

Our app is now ready to use the window.aptos API. We will change src/App.tsx to retrieve the value of window.aptos.account() (the wallet account) on initial render, store it in state, and then display it:

import React from 'react';
import './App.css';

function App() {
// Retrieve aptos.account on initial render and store it.
const [address, setAddress] = React.useState<string | null>(null);
React.useEffect(() => {
window.aptos.account().then((data : {address: string}) => setAddress(data.address));
}, []);

return (
<div className="App">
<p><code>{ address }</code></p>
</div>
);
}

export default App;

Refresh the page and you will see your account address.

Add some CSS

Next, replace the contents of src/App.css:

a, input, textarea {
display: block;
}

textarea {
border: 0;
min-height: 50vh;
outline: 0;
padding: 0;
width: 100%;
}

Step 3: Use the SDK to get data from the blockchain

The Wallet is now integrated with our dapp. Next, we will integrate the Aptos SDK to get data from the blockchain. We will use the Aptos SDK to retrieve information about our account and display that information on the page.

Add the aptos dependency to package.json

First, add the SDK to the project's dependencies:

$ npm install --save aptos

You will now see "aptos": "^0.0.20" (or similar) in your package.json.

Create an AptosClient

Now we can import the SDK and create an AptosClient to interact with the blockchain (technically it interacts with the REST API, which interacts with the blockchain).

As our wallet account is on devnet, we will set up the AptosClient to interact with devnet as well. Add the following to src/App.tsx:

import { Types, AptosClient } from 'aptos';

// Create an AptosClient to interact with devnet.
const client = new AptosClient('https://fullnode.devnet.aptoslabs.com/v1');

function App() {
// ...

// Use the AptosClient to retrieve details about the account.
const [account, setAccount] = React.useState<Types.AccountData | null>(null);
React.useEffect(() => {
if (!address) return;
client.getAccount(address).then(setAccount);
}, [address]);

return (
<div className="App">
<p><code>{ address }</code></p>
<p><code>{ account?.sequence_number }</code></p>
</div>
);
}

Now, in addition to displaying the account address, the app will also display the account's sequence_number. This sequence_number represents the next transaction sequence number to prevent replay attacks of transactions. You will see this number increasing as you make transactions with the account.

Step 4: Publish a Move Module

Our dapp is now set up to read from the blockchain. The next step is to write to the blockchain. To do so, we will publish a Move Module to our account.

The Move Module provides a location for this data to be stored. Specifically, we will use the HelloBlockchain module from Your first Move Module, which provides a resource called MessageHolder that holds a string (called message).

Publish the HelloBlockchain module with the Aptos CLI

We will use the Aptos CLI to compile and publish the HelloBlockchain module.

  1. Download the hello_blockchain package.

  2. Next, use the aptos move publish command (replacing /path/to/hello_blockchain/ and <address>):

$ aptos move publish --package-dir /path/to/hello_blockchain/ --named-addresses HelloBlockchain=<address>

For example:

$ aptos move publish --package-dir ~/code/aptos-core/aptos-move/move-examples/hello_blockchain/ --named-addresses HelloBlockchain=0x5af503b5c379bd69f46184304975e1ef1fa57f422dd193cdad67dc139d532481

The --named-addresses replaces the named address HelloBlockchain in HelloBlockchain.move with the specified address. For example, if we specify --named-addresses HelloBlockchain=0x5af503b5c379bd69f46184304975e1ef1fa57f422dd193cdad67dc139d532481, then the following:

module HelloBlockchain::Message {

becomes:

module 0x5af503b5c379bd69f46184304975e1ef1fa57f422dd193cdad67dc139d532481::Message {

This makes it possible to publish the module for the given account (in this case our wallet account, 0x5af503b5c379bd69f46184304975e1ef1fa57f422dd193cdad67dc139d532481).

Assuming that your account has enough funds to execute the transaction, you can now publish the HelloBlockchain module in your account. If you refresh the app, you will see that the account sequence number has increased from 0 to 1.

You can also verify that the module was published by going to the Aptos Explorer and looking up your account. If you scroll down to the Account Modules section, you should see something like the following:

{
"address": "0x5af503b5c379bd69f46184304975e1ef1fa57f422dd193cdad67dc139d532481",
"name": "Message",
"friends": [],
"exposedFunctions": [
{
"name": "get_message",
"visibility": "public",
"genericTypeParams": [],
"params": [
"address"
],
"_return": [
"0x1::string::String"
]
},
{
"name": "set_message",
"visibility": "script",
"genericTypeParams": [],
"params": [
"signer",
"vector"
],
"_return": []
}
],
"structs": [
{
"name": "MessageChangeEvent",
"isNative": false,
"abilities": [
"drop",
"store"
],
"genericTypeParams": [],
"fields": [
{
"name": "from_message",
"type": "0x1::string::String"
},
{
"name": "to_message",
"type": "0x1::string::String"
}
]
},
{
"name": "MessageHolder",
"isNative": false,
"abilities": [
"key"
],
"genericTypeParams": [],
"fields": [
{
"name": "message",
"type": "0x1::string::String"
},
{
"name": "message_change_events",
"type": "0x1::event::EventHandle<0x5af503b5c379bd69f46184304975e1ef1fa57f422dd193cdad67dc139d532481::Message::MessageChangeEvent>"
}
]
}
]
}

Make a note of "name": "Message", we will use it in the next section.

Add module publishing instructions to the dapp

As a convenience to the users, we can have the app display the aptos move publish command if the module does not exist. To do so, we will use the Aptos SDK to retrieve the account modules and look for one where module.abi.name equals "Message" (i.e., the "name": "Message" we saw in the Aptos Explorer).

Update src/App.tsx:

function App() {
// ...

// Check for the module; show publish instructions if not present.
const [modules, setModules] = React.useState<Types.MoveModule[]>([]);
React.useEffect(() => {
if (!address) return;
client.getAccountModules(address).then(setModules);
}, [address]);

const hasModule = modules.some((m) => m.abi?.name === 'Message');
const publishInstructions = (
<pre>
Run this command to publish the module:
<br />
aptos move publish --package-dir /path/to/hello_blockchain/
--named-addresses HelloBlockchain={address}
</pre>
);

return (
<div className="App">
{!hasModule && publishInstructions}
</div>
);
}

New users will be able to use this command to create a page for their account.

Step 5: Write a message on the blockchain

Now that the module has been published, we are ready to use it to write a message on the blockchain. For this step we will use the set_message function exposed by the module.

A transaction that calls the set_message function

The signature for set_message looks like this:

public(script) fun set_message(account: signer, message_bytes: vector<u8>)

To call this function, we need to use the window.aptos API provided by the wallet to submit a transaction. Specifically, we will create a script_function_payload transaction that looks like this:

{
type: "script_function_payload",
function: "<address>::Message::set_message",
arguments: ["<hex encoded utf-8 message>"],
type_arguments: []
}

There is no need to provide the account: signer argument. Aptos provides it automatically.

However, we do need to specify the message_bytes argument: this is the "<hex encoded utf-8 message>" in the transaction. We need a way to convert a JS string to this format. We can do so by using TextEncoder to convert to utf-8 bytes and then a one-liner to hex encode the bytes.

Add this function to src/App.tsx:

/** Convert string to hex-encoded utf-8 bytes. */
function stringToHex(text: string) {
const encoder = new TextEncoder();
const encoded = encoder.encode(text);
return Array.from(encoded, (i) => i.toString(16).padStart(2, "0")).join("");
}

Using this function, our transaction payload becomes:

{
type: "script_function_payload",
function: "<address>::Message::set_message",
arguments: [stringToHex(message)],
type_arguments: []
}

Use the window.aptos API to submit the set_message transaction

Now that we understand how to use a transaction to call the set_message function, next we call this function from our app using window.aptos.signAndSubmitTransaction().

We will add:

  • A <textarea> where the user can input a message, and
  • A <button> that calls the set_message function with the contents of the <textarea>.

Update src/App.tsx:

function App() {
// ...

// Call set_message with the textarea value on submit.
const ref = React.createRef<HTMLTextAreaElement>();
const [isSaving, setIsSaving] = React.useState(false);
const handleSubmit = async (e: any) => {
e.preventDefault();
if (!ref.current) return;

const message = ref.current.value;
const transaction = {
type: "script_function_payload",
function: `${address}::Message::set_message`,
arguments: [stringToHex(message)],
type_arguments: [],
};

try {
setIsSaving(true);
await window.aptos.signAndSubmitTransaction(transaction);
} finally {
setIsSaving(false);
}
};

return (
<div className="App">
{hasModule ? (
<form onSubmit={handleSubmit}>
<textarea ref={ref} />
<input disabled={isSaving} type="submit" />
</form>
) : publishInstructions}
</div>
);
}

To test it:

  • Type something in the <textarea> and submit the form.
  • Find your account in the Aptos Explorer and you will now see a MessageHolder resource under Account Resources with the message you wrote.

If you don't see it, try a shorter message. Long messages may cause the transaction to fail because longer messages take more gas.

Step 6: Display the message in the dapp

Now that the MessageHolder resource has been created, we can use the Aptos SDK to retrieve it and display the message.

Get the wallet account's message

To retrieve the message, we will:

  • First use AptosClient.getAccountResources() function to fetch the account's resources and store them in state.

  • Then we will look for one whose type is MessageHolder. The full type is $address::Message::MessageHolder as it is part of the $address::Message module.

    In our example it is:

     0x5af503b5c379bd69f46184304975e1ef1fa57f422dd193cdad67dc139d532481::Message::MessageHolder
  • We will use this for the initial value of the <textarea>.

Update src/App.tsx:

function App() {
// ...

// Get the message from account resources.
const [resources, setResources] = React.useState<Types.AccountResource[]>([]);
React.useEffect(() => {
if (!address) return;
client.getAccountResources(address).then(setResourdces);
}, [address]);
const resourceType = `${address}::Message::MessageHolder`;
const resource = resources.find((r) => r.type === resourceType);
const data = resource?.data as {message: string} | undefined;
const message = data?.message;

return (
// ...
<textarea ref={ref} defaultValue={message} />
// ...
);
}

To test it:

  • Refresh the page and you will see the message you wrote earlier.
  • Change the text, submit the form, and refresh the page again. You will see that the contents have been updated with your new message.

This confirms that you are reading and writing messages on the Aptos blockchain.

Display messages from other accounts

So far, we have built a "single-player" dapp where you can read and write a message on your own account. Next, we will make it possible for other people to read messages, including people who do not have the Aptos Wallet installed.

We will set it up so that going to the URL /<account address> displays the message stored at <account address> (if it exists).

  • If the app is loaded at /<account address>, we will also disable editing.

  • If editing is enabled, we will show a "Get public URL" link so you can share your message.

Update src/App.tsx:

function App() {
// Retrieve aptos.account on initial render and store it.
const urlAddress = window.location.pathname.slice(1);
const isEditable = !urlAddress;
const [address, setAddress] = React.useState<string | null>(null);
React.useEffect(() => {
if (urlAddress) {
setAddress(urlAddress);
} else {
window.aptos.account().then((data : {address: string}) => setAddress(data.address));
}
}, [urlAddress]);

// ...

return (
<div className="App">
{hasModule ? (
<form onSubmit={handleSubmit}>
<textarea ref={ref} defaultValue={message} readOnly={!isEditable} />
{isEditable && (<input disabled={isSaving} type="submit" />)}
{isEditable && (<a href={address!}>Get public URL</a>)}
</form>
) : publishInstructions}
</div>
);
}

That concludes this tutorial.