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Flow EVM Example

Welcome to Flow EVM! This is an example project to help you get started working with Flow EVM using familiar EVM tooling as well as interacting with EVM from the Cadence environment

ℹ️ If you haven't already, check out the HelloWorld quick start guide to get rolling with Flow EVM using the EVM tooling you know and love. This guide will take you a step further, introducing you to the Cadence environment and how to orchestrate EVM interactions from Cadence.

Overview

In this example, we're going to a set of smart contracts that codify a simple onchain lottery. The lottery "admin" will be CadenceOwnedAccount (COA) which resides in a Cadence contract. This Cadence contract will have a single public method that enables committment of a verifiably random number retrieved from the Cadence execution environment to the lottery contract running in EVM.

Throughout this example, you'll learn how to:

  • Deploy a Cadence contract
  • Deploy an EVM contract using a CadenceOwnedAccount
  • Query EVM state from Cadence
  • Conditionally execute EVM state change from Cadence
  • Orchestrate arbitrary calls to EVM from the Cadence runtime

Walkthrough

Pre-requisites

You'll need Foundry to interact with Flow EVM from the CLI as well as Flow CLI to interact with the Flow blockchain.

Step 1: Create a Flow account

Generate a public/private key pair from which we'll create a Flow account using the PreviewNet faucet.

flow keys generate

Resulting in something like:

🔴️ Store private key safely and don't share with anyone!
Private Key             <PRIVATE_KEY>
Public Key 		        <PUBLIC_KEY>
Mnemonic 		        <MNEMONIC_PHRASE>
Derivation Path 	    m/44'/539'/0'/0/0
Signature Algorithm     ECDSA_P256

Next copy the public key, go to the PreviewNet faucet and paste the public key value. Complete the Captcha (assuming you're not a robot) and click "Create Account". You should see a message indicating the created address and your new account's PreviewNet $FLOW balance like so:

PreviewNet Faucet Confirmation

With that account created, you'll need to copy the private key you generated to the previewnet-lottery.pkey, so run the following command, also pasting your private key where named:

cp previewnet-lottery.pkey.example previewnet-lottery.pkey && echo <YOUR_PRIVATE_KEY> > previewnet-lottery.pkey

Lastly, configure this account in your flow.json which will tell Flow CLI how to use execute transactions using this account. Under the "accounts" field, replace the address under "previewnet-lottery"

"previewnet-lottery": {
    "address": "<YOUR_PREVIEWNET_ADDRESS>",
    "key": {
        "type": "file",
        "location": "./previewnet-lottery.pkey"
    }
}

ℹ️ Note, PreviewNet Flow account creation will soon be integrated into Flow CLI for a smoother experience

Step 2: Deploy the lottery contracts

Now that we have a Flow account to interact with on PreviewNet, we can deploy our contracts.

In this process, we'll be deploying both a Cadence contract and a Solidity contract in Flow EVM. This will occur in a single Cadence transaction, and will result in the SimpleLotteryVRF Cadence contract "owning" the SimpleLottery EVM contract. Before we execute the deployment, let's take a closer look at the SimpleLotteryVRF initialization block:

init() {
    // SimpleLottery compiled bytecode object - note we removed the 0x prefix
    let bytecode: String = "608...033"
    // Create a new CadenceOwnedAccount and assign to this contract
    self.coa = EVM.createCadenceOwnedAccount()

    // Fund the COA with some initial balance from the deployment account's
    // FlowToken Vault
    let fundVault = self.account.storage
        .borrow<auth(FungibleToken.Withdraw) &{FungibleToken.Vault}>(
            from: /storage/flowTokenVault
        ) ?? panic("Could not borrow FlowToken Vault")
    self.coa.deposit(
        from: fundVault.withdraw(amount: 10.0)
    )

    // Deploy the EVM SimpleLottery contract & preserve the deployment
    // address
    self.lotteryContractAddress = self.coa.deploy(
        code: bytecode,
        gasLimit: 60000,
        value: EVM.Balance(attoflow: 0)
    )
}

Let's now deploy this contract on PreviewNet using Flow CLI:

flow accounts add-contract cadence/contracts/SimpleLotteryVRF.cdc \
    --signer previewnet-lottery
    --network previewnet

When this contract is deployed, a CadenceOwnedAccount will be created, funded with EVM $FLOW, before finally deploying the SimpleLottery EVM contract from its compiled bytecode. Because SimpleLottery is Ownable, this means that the onlyOwner methods within that contract can only be called by the SimpleLotteryVRF Cadence contract via the encapsulated COA.

Since the resolve method in SimpleLotteryVRF retrieves a random UInt64 from Cadence's secure revertibleRandom method, the commitment of a random number to the lottery contract is gated by Cadence smart contract logic.

ℹ️ The caveat here is that Cadence supports post-conditions on transactions, so a caller could conceivably call the resolve method from Cadence and revert if they determine they did not win the lottery. In production, this would not be favorable and a more secure solution might commit to a random source at the lottery's end block number height - see docs for further details on Flow's onchain randomness.

On deployment, we should see a number of events emitted including an EVM event containing a deployedContractAddress value. This is the EVM address of our deployed SimpleLottery Solidity contract!

Step 3: Purchasing a lottery ticket

From EVM

Now that we have our contracts deployed, let's interact with them. Since this is an EVM contract, let's use Remix to interact with it using MetaMask.

If you haven't already, install and configure MetaMask for use with Flow EVM as per the Flow EVM documentation and be sure to fund it with $FLOW from the PreviewNet faucet.

Open up Remix IDE. Then open up MetaMask and ensure Flow is selected as your active network. Next you'll want to copy the contents of SimpleLottery.sol into a contract file on Remix. Then click on "Solidity Compiler" and compile the contract. Once compiled, click on "Deploy & Run Transactions" and select the SimpleLottery contract from the dropdown.

Next, we'll refer to the deployedContractAddress value emitted from the SimpleLotteryVRF deployment event. Scroll back up in your terminal, copy this value and paste it into the At Address field in Remix. Click "At Address" and you should see the SimpleLottery contract appear in the "Deployed Contracts" section.

Finally, we can purchase our lottery ticket by calling the purchaseTickets method which is a payable method that takes a uint256 value as an argument. Enter 1 as an argument and also enter 1 ether as the value to transmit in the "value" field above to purchase your lottery ticket. Click on purchaseTickets and you should see a MetaMask transaction popup. Confirm the transaction and you should see a successful transaction in the Remix logs.

And if we want to validate that the ticket was purchased, we can call the getNumberOfTicketsPurchased method to check that the ticket count incremented. We can do this using EVM tooling - in this case Foundry's cast command:

cast call --rpc-url https://previewnet.evm.nodes.onflow.org <LOTTERY_CONTRACT_ADDRESS> "getNumberOfTicketsPurchased()"

From Cadence

We just saw how to interact with a Flow EVM contract via traditional EVM methods, but what if you want to purchase a ticket using your CadenceOwnedAccount? Let's do that now.

To do this, we can configure the previewnet-lottery account we configured earlier with a COA. We saw in the SimpleLotteryVRF contract that COAs are resources that can be stored in contracts, but they can also (and will primarily) be stored in Flow account storage.

In the purchase_tickts.cdc Cadence transaction, multiple steps occur...

  1. The signer is configured with a COA (if one is not already stored) 
    // Configure a COA in the signer's account if needed
if signer.storage.type(at: /storage/evm) == nil {
    signer.storage.save(<-EVM.createCadenceOwnedAccount(), to: /storage/evm)
    let addressable = signer.capabilities.issue<&EVM.CadenceOwnedAccount>(/storage/evm)
    signer.capabilities.unpublish(/public/evm)
    signer.capabilities.publish(addressable, at: /public/evm)
}
  2. $FLOW is transferred from the signing Flow account to the signer's COA 
    // Send $FLOW to EVM if necessary, funding from signer's Cadence FlowToken Vault
if self.coa.balance().inFLOW() < self.value.inFLOW() {
    let flowVault = signer.storage.borrow<auth(FungibleToken.Withdraw) &{FungibleToken.Vault}>(
            from: /storage/flowTokenVault
        ) ?? panic("Problem retrieving FlowToken Vault.")
    let fundVault <- flowVault.withdraw(amount: self.value.inFLOW() - self.coa.balance().inFLOW() + 0.001)
    self.coa.deposit(from: <-fundVault)
}
  3. A call is made to the lottery contract to purchase a ticket as the COA, reverting if the call was not successful 
    // Encode the known function signature and parameters for the purchaseTickets call
let calldata: [UInt8] = EVM.encodeABIWithSignature(
    "purchaseTickets(UInt256)",
    [numTickets]
)
// Execute the call to the lottery contract
let result: EVM.Result = self.coa.call(
    to: SimpleLotteryTicketVRF.lotteryContractAddress,
    data: calldata,
    gasLimit: gasLimit,
    value: self.value
)
// Ensure the call was successful
assert(result.status == EVM.Status.successful, message: "EVM call failed.")

All of the above are executed in a single Cadence transaction. You can do that with the following command:

flow transactions send ./cadence/transactions/simple-lottery-vrf/purchase_tickets.cdc 1 \
    --signer previewnet-lottery \
    --network previewnet