Drizzle is the newest member of the Truffle Suite and our first front-end development tool. At its core, Drizzle takes care of synchronizing your contract data, transaction data and more from the blockchain to a Redux store. There are also higher-level abstractions on top of the base drizzle library; tools for React compatibility ( drizzle-react ) and a set of ready-to-use React components ( drizzle-react-components ).

We're going to focus on the lower levels today, taking you through setting up a Truffle project with React and Drizzle from scratch. This way we can gain the best understanding of how Drizzle works under the hood. With this knowledge, you can leverage the full power of Drizzle with any front-end framework of your choosing, or use the higher-level React abstractions with confidence.

This will be a very minimal tutorial focused on setting and getting a simple string stored in a contract. It's meant for those with a basic knowledge of Truffle, who have some knowledge of JavaScript and React.js, but who are new to using Drizzle.

Note: For Truffle basics, please read through the Truffle [Pet Shop](/tutorials/pet-shop) tutorial before proceeding.

In this tutorial we will be covering:

Setting up the development environment Creating a Truffle project from scratch Writing the smart contract Compiling and migrating the smart contract Testing the smart contract Creating our React.js project Wiring up the front-end client Wire up the React app with Drizzle Write a component to read from Drizzle Write a component to write to the smart contract

Setting up the development environment

There are a few technical requirements before we start. Please install the following:

Truffle

Once we have those installed, we only need one command to install Truffle:

npm install -g truffle

To verify that Truffle is installed properly, type truffle version on a terminal. If you see an error, make sure that your npm modules are added to your path.

Finally, since this is a React.js tutorial, we will be creating our React project with Create-React-App.

You won't have to do anything if you have NPM version 5.2 or above. You can check your NPM version by running npm --version . If you do not, then you will need to install the tool globally with this command:

npm install -g create-react-app

Creating a Truffle project

Truffle initializes in the current directory, so first create a directory in your development folder of choice and then move inside it. mkdir drizzle-react-tutorial cd drizzle-react-tutorial Now we're ready to spawn our empty Truffle project by running the following command: truffle init

Let's take a brief look at the directory structure that was just generated.

Directory structure

The default Truffle directory structure contains the following:

contracts/ : Contains the Solidity source files for our smart contracts. There is an important contract in here called Migrations.sol , which we'll talk about later.

: Contains the Solidity source files for our smart contracts. There is an important contract in here called , which we'll talk about later. migrations/ : Truffle uses a migration system to handle smart contract deployments. A migration is an additional special smart contract that keeps track of changes.

: Truffle uses a migration system to handle smart contract deployments. A migration is an additional special smart contract that keeps track of changes. test/ : Contains both JavaScript and Solidity tests for our smart contracts.

: Contains both JavaScript and Solidity tests for our smart contracts. truffle-config.js : Truffle configuration file.

Writing our smart contract

We'll add a simple smart contract called MyStringStore.

Create a new file named MyStringStore.sol in the contracts/ directory. Add the following content to the file: pragma solidity ^0.5.0; contract MyStringStore { string public myString = "Hello World"; function set(string memory x) public { myString = x; } }

Since this isn't a Solidity tutorial, all you need to know about this is:

We've created a public string variable named myString and initialized it to "Hello World". This automatically creates a getter (since it's a public variable) so we don't have to write one ourselves.

and initialized it to "Hello World". This automatically creates a getter (since it's a public variable) so we don't have to write one ourselves. We've created a setter method that simply sets the myString variable with whatever string is passed in.

Launching a test blockchain with Truffle Develop

Before we move ahead, let's first launch our test blockchain with the Truffle Develop console.

Open up a new terminal, navigate to the project directory, and run the following command:

truffle develop

This will spawn a new blockchain that listens on 127.0.0.1:9545 by default.

Compiling and migrating the smart contract

Now we are ready to compile and migrate our contract.

Compilation

In the Truffle Develop console, type the following command:

compile

Note: If you're on Windows and encountering problems running this command, please see the documentation on [resolving naming conflicts on Windows](/docs/advanced/configuration#resolving-naming-conflicts-on-windows).

You should see output similar to the following:

Compiling ./contracts/Migrations.sol... Compiling ./contracts/MyStringStore.sol... Writing artifacts to ./build/contracts

Migration

Now that we've successfully compiled our contracts, it's time to migrate them to the blockchain!

Note: Read more about migrations in the [Truffle documentation](/docs/getting_started/migrations).

To create our own migration script.

Create a new file named 2_deploy_contracts.js in the migrations/ directory. Add the following content to the 2_deploy_contracts.js file: const MyStringStore = artifacts.require("MyStringStore"); module.exports = function(deployer) { deployer.deploy(MyStringStore); }; Back in our Truffle Develop console, migrate the contract to the blockchain. migrate You should see the migrations being executed in order, with the details of each migration listed.

Testing the smart contract

Before we proceed, we should write a couple tests to ensure that our contract works as expected.

Create a new file named MyStringStore.js in the test/ directory. Add the following content to the MyStringStore.js file: const MyStringStore = artifacts.require("./MyStringStore.sol"); contract("MyStringStore", accounts => { it("should store the string 'Hey there!'", async () => { const myStringStore = await MyStringStore.deployed(); // Set myString to "Hey there!" await myStringStore.set("Hey there!", { from: accounts[0] }); // Get myString from public variable getter const storedString = await myStringStore.myString.call(); assert.equal(storedString, "Hey there!", "The string was not stored"); }); });

Running the tests

Back in the Truffle Develop console, run the tests: test If all the tests pass, you'll see console output similar to this: Using network 'develop'. Contract: MyStringStore ✓ should store the string 'Hey there!' (98ms) 1 passing (116ms)

Awesome! Now we know that the contract actually works.

Creating our React.js project

Now that we are done with the smart contract, we can write our front-end client with React.js! In order to do this, open another terminal, navigate to your project directory, and simply run this command (if you have NPM version 5.2 or above):

npx create-react-app client

If you have an older version of NPM, make sure Create-React-App is installed globally as per the instructions in the Setting up the development environment section and then run the following:

create-react-app client

This should create a client directory in your Truffle project and bootstrap a barebones React.js project for you to start building your front-end with.

Wiring up the front-end client

Since Create-React-App's default behavior disallows importing files from outside of the src folder, we need to bring the contracts in our build folder inside src . We can copy and paste them every time we compile our contracts, but a better way is to simply configure Truffle to put the files there.

In the truffle-config.js file, replace the contents with the following:

const path = require("path"); module.exports = { contracts_build_directory: path.join(__dirname, "client/src/contracts") };

This will make sure to output the contract build artifacts directory inside your React project. But this also means we'll have to restart our Truffle Develop console. Press CTRL + C to exit out of the Truffle Develop console and then start it again with truffle develop .

From there, make sure you run the compile and migrate commands again so that the new build artifacts will be output into the new folder. If you are encountering issues, try migrate --reset for a clean migration from scratch.

Install Drizzle

This is the most delicious part, we install Drizzle. Make sure you are in the client directory and then run the following:

npm install @drizzle/store

And that's it for dependencies! Note that we don't need to install Web3.js or @truffle/contract ourselves. Drizzle contains everything we need to work reactively with our smart contracts.

Wire up the React app with Drizzle

Before we go further, let's start our React app by running the follow command inside our client directory:

npm start

This will serve the front-end under localhost:3000 , so open that up in your browser.

Note: Make sure to use an incognito window if you already have MetaMask installed (or disable MetaMask for now). Otherwise, the app will try to use the network specified in MetaMask rather than the develop network under `localhost:9545`.

If the default Create-React-App page loaded without any issues, you may proceed.

Setup the store

The first thing we need to do is to setup and instantiate the Drizzle store. We are going add the following code to client/src/index.js :

// import drizzle functions and contract artifact import { Drizzle } from "@drizzle/store"; import MyStringStore from "./contracts/MyStringStore.json"; // let drizzle know what contracts we want and how to access our test blockchain const options = { contracts: [MyStringStore], web3: { fallback: { type: "ws", url: "ws://127.0.0.1:9545", }, }, }; // setup drizzle const drizzle = new Drizzle(options);

First, we imported the tools from Drizzle as well as the contract definition.

We then built our options object for Drizzle, which in this case is just specifying the specific contract we want to be loaded by passing in the JSON build artifact.

And finally, we created the drizzleStore and used that to create our drizzle instance which we will pass in as a prop to our App component.

Once that is complete, your index.js should look something like this:

import React from 'react'; import ReactDOM from 'react-dom'; import './index.css'; import App from './App'; import * as serviceWorker from './serviceWorker'; // import drizzle functions and contract artifact import { Drizzle, generateStore } from "@drizzle/store"; import MyStringStore from "./contracts/MyStringStore.json"; // let drizzle know what contracts we want and how to access our test blockchain const options = { contracts: [MyStringStore], web3: { fallback: { type: "ws", url: "ws://127.0.0.1:9545", }, }, }; // setup the drizzle store and drizzle const drizzle = new Drizzle(options); ReactDOM.render(<App drizzle={drizzle} />, document.getElementById('root'));

Note again that the drizzle instance is passed into the App component as props.

Wire up the App component

Now that we have a drizzle instance to play around with, we can go into client/src/App.js to start working with the React API.

Adding state variables

First thing we will do is to add the following line inside our App component:

state = { loading: true, drizzleState: null };

We are going to be using two state variables here:

loading — Indicates if Drizzle has finished initializing and the app is ready. The initialization process includes instantiating web3 and our smart contracts, fetching any available Ethereum accounts and listening (or, in cases where subscriptions are not supported: polling) for new blocks. drizzleState — This is where we will store the state of the Drizzle store in our top-level component. If we can keep this state variable up-to-date, then we can simply use simple props and state to work with Drizzle (i.e. you don't have to use any Redux or advanced React patterns).

Adding some initialization logic

Next we will add in our componentDidMount method into the component class so that we can run some initialization logic.

componentDidMount() { const { drizzle } = this.props; // subscribe to changes in the store this.unsubscribe = drizzle.store.subscribe(() => { // every time the store updates, grab the state from drizzle const drizzleState = drizzle.store.getState(); // check to see if it's ready, if so, update local component state if (drizzleState.drizzleStatus.initialized) { this.setState({ loading: false, drizzleState }); } }); }

First, we grab the drizzle instance from the props, then we call drizzle.store.subscribe and pass in a callback function. This callback function is called whenever the Drizzle store is updated. Note that this store is actually a Redux store under the hood, so this might look familiar if you've used Redux previously.

Whenever the store is updated, we will try to grab the state with drizzle.store.getState() and then if Drizzle is initialized and ready, we will set loading to false, and also update the drizzleState state variable.

By doing this, drizzleState will always be up-to-date and we also know exactly when Drizzle is ready so we can use a loading component to let the user know.

Unsubscribing from the store

Note that we assign the return value of the subscribe() to a class variable this.unsubscribe . This is because it is always good practice to unsubscribe from any subscriptions you have when the component un-mounts. In order to do this, we save a reference to that subscription (i.e. this.unsubscribe ), and inside componentWillUnmount , we have the following:

componentWillUnmount() { this.unsubscribe(); }

This will safely unsubscribe when the App component un-mounts so we can prevent any memory leaks.

Replace the render method

Finally, we can replace the boilerplate render method with something that applies to us better:

render() { if (this.state.loading) return "Loading Drizzle..."; return <div className="App">Drizzle is ready</div>; }

In the next section, we will replace "Drizzle is ready" with an actual component that will read from the store. If you refresh your browser and run this app now, you should see "Loading Drizzle..." briefly flash on screen and then subsequently "Drizzle is ready".

Full component code

When you are done this section, your App component should look like the following:

class App extends Component { state = { loading: true, drizzleState: null }; componentDidMount() { const { drizzle } = this.props; // subscribe to changes in the store this.unsubscribe = drizzle.store.subscribe(() => { // every time the store updates, grab the state from drizzle const drizzleState = drizzle.store.getState(); // check to see if it's ready, if so, update local component state if (drizzleState.drizzleStatus.initialized) { this.setState({ loading: false, drizzleState }); } }); } componentWillUnmount() { this.unsubscribe(); } render() { if (this.state.loading) return "Loading Drizzle..."; return <div className="App">Drizzle is ready</div>; } }

Write a component to read from Drizzle

First, let's create a new file at client/src/ReadString.js and paste in the following:

import React from "react"; class ReadString extends React.Component { componentDidMount() { const { drizzle, drizzleState } = this.props; console.log(drizzle); console.log(drizzleState); } render() { return <div>ReadString Component</div>; } } export default ReadString;

And then inside App.js , import the new component with this statement:

import ReadString from "./ReadString";

Now modify your App.js render method so that we pass in the drizzle instance from props as well as the drizzleState from the component state:

render() { if (this.state.loading) return "Loading Drizzle..."; return ( <div className="App"> <ReadString drizzle={this.props.drizzle} drizzleState={this.state.drizzleState} /> </div> ); }

Go back to the browser and open up your console. You should see that the two console.log statements are working and they are displaying both the drizzle instance as well as a drizzleState that is fully initialized.

What this tells us is that the drizzleState we get in this component will always be fully ready once this component mounts. At this point, you can take some time to explore the drizzle instance object as well as the drizzleState object.

drizzle instance and drizzleState

For the most part, drizzleState is there for you to read information from (i.e. contract state variables, return values, transaction status, account data, etc.), whereas the drizzle instance is what you will use to actually get stuff done (i.e. call contract methods, the Web3 instance, etc.).

Wiring up the ReadString component

Now that we have access to our drizzle instance and the drizzleState , we can put in the logic that allows us read the smart contract variable we are interested in. Here is what the full code of ReadString.js should look like:

import React from "react"; class ReadString extends React.Component { state = { dataKey: null }; componentDidMount() { const { drizzle } = this.props; const contract = drizzle.contracts.MyStringStore; // let drizzle know we want to watch the `myString` method const dataKey = contract.methods["myString"].cacheCall(); // save the `dataKey` to local component state for later reference this.setState({ dataKey }); } render() { // get the contract state from drizzleState const { MyStringStore } = this.props.drizzleState.contracts; // using the saved `dataKey`, get the variable we're interested in const myString = MyStringStore.myString[this.state.dataKey]; // if it exists, then we display its value return <p>My stored string: {myString && myString.value}</p>; } } export default ReadString;

If everything is working, your app should display "Hello World". But first, let's walk through what we did here.

When the component mounts

componentDidMount() { const { drizzle } = this.props; const contract = drizzle.contracts.MyStringStore; // let drizzle know we want to watch the `myString` method const dataKey = contract.methods["myString"].cacheCall(); // save the `dataKey` to local component state for later reference this.setState({ dataKey }); }

When the component mounts, we first grab a reference to the contract we are interested in and assign it to contract .

We then need to tell Drizzle to keep track of a variable we are interested in. In order to do that, we call the .cacheCall() function on the myString getter method.

What we get in return is a dataKey that allows us to reference this variable. We save this to the component's state so we can use it later.

The render method

render() { // get the contract state from drizzleState const { MyStringStore } = this.props.drizzleState.contracts; // using the saved `dataKey`, get the variable we're interested in const myString = MyStringStore.myString[this.state.dataKey]; // if it exists, then we display its value return <p>My stored string: {myString && myString.value}</p>; }

From the drizzleState , we grab the slice of the state we are interested in, which in this case is the MyStringStore contract. From there, we use the dataKey we saved from before in order to access the myString variable.

Finally, we write myString && myString.value to show the value of the variable if it exists, or nothing otherwise. And in this case, it should show "Hello World" since that is the string the contract is initialized with.

Quick Recap

The most important thing to get out of this section here is that there are two steps to reading a value with Drizzle:

First, you need to let Drizzle know what variable you want to watch for. Drizzle will give you a dataKey in return and you need to save it for later reference. Second, due to the asynchronous nature of how Drizzle works, you should be watching for changes in drizzleState . Once the variable accessed by the dataKey exists, you will be able to get the value you are interested in.

Write a component to write to the smart contract

Of course, simply reading a pre-initialized variable is no fun at all; we want something that we can interact with. In this section, we will create an input box where you can type a string of your choice and have it save to the blockchain forever!

First, let's create a new file client/src/SetString.js and paste in the following:

import React from "react"; class SetString extends React.Component { state = { stackId: null }; handleKeyDown = e => { // if the enter key is pressed, set the value with the string if (e.keyCode === 13) { this.setValue(e.target.value); } }; setValue = value => { const { drizzle, drizzleState } = this.props; const contract = drizzle.contracts.MyStringStore; // let drizzle know we want to call the `set` method with `value` const stackId = contract.methods["set"].cacheSend(value, { from: drizzleState.accounts[0] }); // save the `stackId` for later reference this.setState({ stackId }); }; getTxStatus = () => { // get the transaction states from the drizzle state const { transactions, transactionStack } = this.props.drizzleState; // get the transaction hash using our saved `stackId` const txHash = transactionStack[this.state.stackId]; // if transaction hash does not exist, don't display anything if (!txHash) return null; // otherwise, return the transaction status return `Transaction status: ${transactions[txHash] && transactions[txHash].status}`; }; render() { return ( <div> <input type="text" onKeyDown={this.handleKeyDown} /> <div>{this.getTxStatus()}</div> </div> ); } } export default SetString;

At this point, import and include it inside App.js just like you did with the ReadString component:

import SetString from "./SetString"; // ... render() { if (this.state.loading) return "Loading Drizzle..."; return ( <div className="App"> <ReadString drizzle={this.props.drizzle} drizzleState={this.state.drizzleState} /> <SetString drizzle={this.props.drizzle} drizzleState={this.state.drizzleState} /> </div> ); }

At this point, the app should work and you should try it out. You should be able to type something into the input text box, hit Enter, and Drizzle's react store will automatically display the new string.

Next, let's go through SetString.js step-by-step.

General structure

First let's take a look at the general React.js boilerplate that we need.

class SetString extends React.Component { state = { stackId: null }; handleKeyDown = e => { // if the enter key is pressed, set the value with the string if (e.keyCode === 13) { this.setValue(e.target.value); } }; setValue = value => { ... }; getTxStatus = () => { ... }; render() { return ( <div> <input type="text" onKeyDown={this.handleKeyDown} /> <div>{this.getTxStatus()}</div> </div> ); } }

In this component, we will have an input text box for the user to type in a string, and when the Enter key is pressed, the setValue method will be called with the string as a parameter.

Also, we will display the status of the transaction. The getTxStatus method will return a string displaying the status of the transaction by referencing a stackId state variable (more on this later).

Submitting the transaction

setValue = value => { const { drizzle, drizzleState } = this.props; const contract = drizzle.contracts.MyStringStore; // let drizzle know we want to call the `set` method with `value` const stackId = contract.methods["set"].cacheSend(value, { from: drizzleState.accounts[0] }); // save the `stackId` for later reference this.setState({ stackId }); };

We first assign the contract from the drizzle instance into contract , and then we call cacheSend() on the method we are interested in (i.e. set ). Then we pass in the string we want to set (i.e. value ) as well as our transaction options (in this case, just the from field). Note that we can get our current account address from drizzleState.accounts[0] .

What we get in return is a stackId , which is a reference to the transaction that we want to execute. Ethereum transactions don't receive a hash until they're broadcast to the network. In case an error occurs before broadcast, Drizzle keeps track of these transactions by giving each it's own ID. Once successfully broadcasted, the stackId will point to the transaction hash, so we save it in our local component state for later usage.

Tracking transaction status

getTxStatus = () => { // get the transaction states from the drizzle state const { transactions, transactionStack } = this.props.drizzleState; // get the transaction hash using our saved `stackId` const txHash = transactionStack[this.state.stackId]; // if transaction hash does not exist, don't display anything if (!txHash) return null; // otherwise, return the transaction status return `Transaction status: ${transactions[txHash] && transactions[txHash].status}`; };

Now that we have a stackId saved into our local component state, we can use this to check the status of our transaction. First, we need the transactions and transactionStack slices of state from drizzleState .

Then, we can get the transaction hash (assigned to txHash ) via transactionStack[stackId] . If the hash does not exist, then we know that the transaction has not been broadcasted yet and we return null.

Otherwise, we display a string to show the status of our transaction. Usually, this will either be "pending" or "success".

The End

Congratulations! You have taken a huge step to understanding how Drizzle works. Of course, this is only the beginning, you can use tools like drizzle-react to help you integrate Drizzle into your dapp, reducing the necessary boilerplate that you would have to write.

Alternatively, you could also bootstrap your Drizzle dapp with our Truffle box.