Hello World
In this quickstart guide, you'll interact with your first smart contract on the Flow Testnet. Testnet
is a public instance of the Flow blockchain designed for experimentation, where you can deploy and invoke smart contracts without incurring any real-world costs.
Smart contracts on Flow are permanent pieces of code that live on the blockchain. They allow you to encode business logic, define digital assets, and much more. By leveraging smart contracts, you can create decentralized applications (dApps) that are transparent, secure, and open to anyone.
Flow supports modern smart contracts written in Cadence, a resource-oriented programming language designed specifically for smart contracts. Cadence focuses on safety and security, making it easier to write robust contracts. Flow also supports traditional EVM-compatible smart contracts written in Solidity, allowing developers to port their existing Ethereum contracts to Flow. In this guide, we'll focus on interacting with Cadence smart contracts.
Objectives
After completing this guide, you'll be able to:
- Read a public variable on a Cadence smart contract deployed on Flow.
- Understand how to interact with contracts on Flow's
testnet
. - Retrieve and display data from a deployed smart contract via scripts.
In later steps, you'll learn how to:
- Create a Flow project using the Flow CLI.
- Add an already-deployed contract to your project with the Dependency Manager.
- Deploy a smart contract locally to the Flow Emulator.
- Write and execute transactions to interact with a deployed smart contract.
- Display data from a Cadence smart contract on a React frontend using the Flow Client Library.
Calling a Contract With a Script
The HelloWorld
contract exposes a public variable named greeting
that is accessible to everything outside the contract. We can retrieve its value using a simple script written in the Cadence programming language. Scripts in Cadence are read-only operations that allow you to query data from the blockchain without changing any state.
Here's the script:
_10import HelloWorld from 0xa1296b1e2e90ca5b_10_10access(all) fun main(): String {_10 return HelloWorld.greeting_10}
Let's break down what this script does:
- Import Statement:
import HelloWorld from 0xa1296b1e2e90ca5b
tells the script to use theHelloWorld
contract deployed at the address0xa1296b1e2e90ca5b
on thetestnet
. - Main Function:
access(all) fun main(): String
defines the entry point of the script, which returns aString
. - Return Statement:
return HelloWorld.greeting
accesses the greeting variable from theHelloWorld
contract and returns its value.
Steps to Execute the Script
- Run the Script: Click the Run button to execute the script.
- View the Output: Observe the output returned by the script. You should see the current value of the
greeting
variable, which is"Hello, World!"
.
Understanding the HelloWorld
Contract
To fully grasp how the script works, it's important to understand the structure of the HelloWorld
contract. Below is the source code for the contract:
_12access(all) contract HelloWorld {_12_12 access(all) var greeting: String_12_12 access(account) fun changeGreeting(newGreeting: String) {_12 self.greeting = newGreeting_12 }_12_12 init() {_12 self.greeting = "Hello, World!"_12 }_12}
Breakdown of the Contract
- Contract Declaration:
access(all) contract HelloWorld
declares a new contract namedHelloWorld
that is accessible to everyone. - State Variable:
access(all) var greeting: String
declares a public variablegreeting
of typeString
. Theaccess(all)
modifier means that this variable can be read by anyone. - Function to Change Greeting:
access(account) fun changeGreeting(newGreeting: String)
defines a function that allows changing the value ofgreeting
. Theaccess(account)
modifier restricts this function so that only the account that deployed the contract (the owner) can call it. - Initializer: The
init()
function is called when the contract is deployed. It sets the initial value ofgreeting
to"Hello, World!"
.
Key Points
- Public Access: The
greeting
variable is public, allowing anyone to read its value without any restrictions. - Restricted Modification: Only the contract owner can modify the
greeting
variable using thechangeGreeting
function. This ensures that unauthorized accounts cannot change the contract's state. - No Read Costs: Reading data from the blockchain is free on Flow. Executing scripts like the one you ran does not incur any costs.
What's Next?
In the upcoming tutorials, you'll learn how to:
- Modify the Greeting: Invoke the
changeGreeting
function to update thegreeting
value. - Deploy Contracts: Use the Flow CLI to deploy your own smart contracts.
- Interact with Contracts Locally: Use the Flow Emulator to test contracts in a local development environment.
- Build Frontend Applications: Display data from smart contracts in a React application using the Flow Client Library.
By understanding the HelloWorld
contract and how to interact with it, you're building a solid foundation for developing more complex applications on the Flow blockchain.
Proceed to the next tutorial to learn how to create your own contracts and deploy them live using the Flow CLI.