Cadence Owned Accounts (COAs) are EVM accounts owned by a Cadence resource and
are used to interact with Flow EVM from Cadence.
COAs expose two interfaces for interaction: one on the Cadence side and one on the EVM side. In this guide, we will
focus on how to interact with COAs with Cadence.
In this guide we will walk through some basic examples creating and interacting with a COA in Cadence. Your specific
usage of the COA resource will depend on your own application's requirements (e.g. the COA resource may not live
directly in /storage/evm as in these examples, but may instead be a part of a more complex resource structure).
The CadenceOwnedAccount resource is a part of the EVM system contract, so to use any of these functions, you will
need to begin by importing the EVM contract into your Cadence code.
To import the EVM contract into your Cadence code using the simple import syntax, you can use the following format
(learn more about configuring contracts in flow.jsonhere):
// This assumes you are working in the in the Flow CLI, FCL, or another tool that supports this syntax
// The contract address should be configured in your project's `flow.json` file
import "EVM"
// ...
However, if you wish to use manual address imports instead, you can use the following format:
// Must use the correct address based on the network you are interacting with
To create a COA, we can use the createCadenceOwnedAccount function from the EVM contract. This function takes no
arguments and returns a new CadenceOwnedAccount resource which represents this newly created EVM account.
For example, we can create this COA in a transaction, saving it to the user's storage and publishing a public capability
to its reference:
create_coa.cdc
import "EVM"
// Note that this is a simplified example & will not handle cases where the COA already exists
To get the EVM address of a COA, you can use the address function from the EVM contract. This function returns the
EVM address of the COA as an EVM.Address struct. This struct is used to represent addresses within Flow EVM and can
also be used to query the balance, code, nonce, etc. of an account.
For our example, we could query the address of the COA we just created with the following script:
get_coa_address.cdc
import "EVM"
access(all)
fun main(address: Address): EVM.EVMAddress {
// Get the desired Flow account holding the COA in storage
let account = getAuthAccount<auth(Storage) &Account>(address)
// Borrow a reference to the COA from the storage location we saved it to
let coa = account.storage.borrow<&EVM.CadenceOwnedAccount>(
from: /storage/evm
) ?? panic("Could not borrow reference to the signer's CadenceOwnedAccount (COA). "
.concat("Ensure the signer account has a COA stored in the canonical /storage/evm path"))
// Return the EVM address of the COA
return coa.address()
}
If you'd prefer the hex representation of the address, you instead return using the EVMAddress.toString() function:
return coa.address().toString()
The above will return the EVM address as a string; however note that Cadence does not prefix hex strings with 0x.
Like any other Flow EVM or Cadence account, COAs possess a balance of FLOW tokens. To get the current balance of our
COA, we can use the COA's balance function. It will return a EVM.Balance struct for the account - these are used to
represent balances within Flow EVM.
This script will query the current balance of our newly created COA:
get_coa_balance.cdc
import "EVM"
access(all)
fun main(address: Address): EVM.Balance {
// Get the desired Flow account holding the COA in storage
let account = getAuthAccount<auth(Storage) &Account>(address)
// Borrow a reference to the COA from the storage location we saved it to
let coa = account.storage.borrow<&EVM.CadenceOwnedAccount>(
from: /storage/evm
) ?? panic("Could not borrow reference to the signer's CadenceOwnedAccount (COA). "
.concat("Ensure the signer account has a COA stored in the canonical /storage/evm path"))
// Get the current balance of this COA
return coa.balance()
}
You can also easily get the UFix64 FLOW balance of any EVM address with this script:
get_coa_balance_as_ufix64.cdc
import "EVM"
access(all)
fun main(addressHex: String): UFix64 {
let addr = EVM.addressFromString(addressHex)
return addr.balance().inFLOW()
}
The above script is helpful if you already know the COA address and can provide the hex representation directly.
Tokens can be seamlessly transferred between the Flow EVM and Cadence environment using the deposit and withdraw
functions provided by the COA resource. Anybody with a valid reference to a COA may deposit Flow tokens into a it,
however only someone with the Owner or Withdraw entitlements can withdraw tokens.
The deposit function takes a FlowToken.Vault resource as an argument, representing the tokens to deposit. It will
transfer the tokens from the vault into the COA's balance.
This transaction will withdraw Flow tokens from a user's Cadence vault and deposit them into their COA:
deposit_to_coa.cdc
import "EVM"
import "FungibleToken"
import "FlowToken"
transaction(amount: UFix64) {
let coa: &EVM.CadenceOwnedAccount
let sentVault: @FlowToken.Vault
prepare(signer: auth(BorrowValue) &Account) {
// Borrow the public capability to the COA from the desired account
// This script could be modified to deposit into any account with a `EVM.CadenceOwnedAccount` capability
This is a basic example which only transfers tokens between a single user's COA & Flow account. It can be easily
modified to transfer these tokens between any arbitrary accounts.
You can also deposit tokens directly into other types of EVM accounts using the EVM.EVMAddress.deposit function. See
the EVM contract documentation for more information.
The withdraw function takes a EVM.Balance struct as an argument, representing the amount of Flow tokens to withdraw,
and returns a FlowToken.Vault resource with the withdrawn tokens.
We can run the following transaction to withdraw Flow tokens from a user's COA and deposit them into their Flow vault:
withdraw_from_coa.cdc
import "EVM"
import "FungibleToken"
import "FlowToken"
transaction(amount: UFix64) {
let sentVault: @FlowToken.Vault
let receiver: &{FungibleToken.Receiver}
prepare(signer: auth(BorrowValue) &Account) {
// Borrow a reference to the COA from the storage location we saved it to with the `EVM.Withdraw` entitlement
let coa = signer.storage.borrow<auth(EVM.Withdraw) &EVM.CadenceOwnedAccount>(
from: /storage/evm
) ?? panic("Could not borrow reference to the signer's CadenceOwnedAccount (COA). "
.concat("Ensure the signer account has a COA stored in the canonical /storage/evm path"))
// We must create a `EVM.Balance` struct to represent the amount of Flow tokens to withdraw
let withdrawBalance = EVM.Balance(attoflow: 0)
withdrawBalance.setFLOW(flow: amount)
// Withdraw the balance from the COA, we will use this later to deposit into the receiving account
// Deposit the withdrawn tokens into the receiving vault
self.receiver.deposit(from: <-self.sentVault)
}
}
info
This is a basic example which only transfers tokens between a single user's COA & Flow account. It can be easily
modified to transfer these tokens between any arbitrary accounts.
To interact with smart contracts on the EVM, you can use the call function provided by the COA resource. This function
takes the EVM address of the contract you want to call, the data you want to send, the gas limit, and the value you want
to send. It will return a EVM.Result struct with the result of the call - you will need to handle this result in your
Cadence code.
This transaction will use the signer's COA to call a contract method with the defined signature and args at a given EVM
address, executing with the provided gas limit and value:
call.cdc
import "EVM"
/// Calls the function with the provided signature and args at the target contract address using
/// the defined gas limit and transmitting the provided value.
) ?? panic("Could not borrow reference to the signer's CadenceOwnedAccount (COA). "
.concat("Ensure the signer account has a COA stored in the canonical /storage/evm path"))
}
execute {
// Deserialize the EVM address from the hex string
let contractAddress = EVM.addressFromString(evmContractHex)
// Construct the calldata from the signature and arguments
let calldata = EVM.encodeABIWithSignature(
signature,
args
)
// Define the value as EVM.Balance struct
let value = EVM.Balance(attoflow: flowValue)
// Call the contract at the given EVM address with the given data, gas limit, and value
// These values could be configured through the transaction arguments or other means
// however, for simplicity, we will hardcode them here
let result: EVM.Result = self.coa.call(
to: contractAddress,
data: calldata,
gasLimit: gasLimit,
value: value
)
// Revert the transaction if the call was not successful
// Note: a failing EVM call will not automatically revert the Cadence transaction
// and it is up to the developer to use this result however it suits their application
assert(
result.status == EVM.Status.successful,
message: "EVM call to ".concat(evmContractHex)
.concat(" and signature ").concat(signature)
.concat(" failed with error code ").concat(result.errorCode.toString())
.concat(": ").concat(result.errorMessage)
)
}
}
info
Notice that the calldata is encoded in the scope of the transaction. While developers can encode the calldata
outside the scope of the transaction and pass the encoded data as an argument, doing so compromises the
human-readability of Cadence transactions.
It's encouraged to either define transactions for each COA call and encoded the hardcoded EVM signature and arguments,
or to pass in the human-readable arguments and signature and encode the calldata within the transaction. This ensures a
more interpretable and therefore transparent transaction.
Similar to transferring ETH and other native value in other EVMs, you'll want to call to the target EVM address with
empty calldata and providing the transfer value.
transfer_evm_flow.cdc
import "EVM"
/// Transfers FLOW to another EVM address from the signer's COA
///
/// @param to: the serialized EVM address of the recipient
As covered in the Batched EVM transactions walkthrough, you can script multiple EVM
calls in a single Cadence transaction. Compared to the single ERC721 transfer, bulk sending multiple tokens isn't much
more code and allows for greater utility out of a single transaction. Below is an example of a bulk ERC721 token
transfer.
erc721_bulk_transfer.cdc
import "EVM"
/// Bulk transfers ERC721 tokens from the signer's COA to the named recipient. All tokens must be from
/// the same collection and sent to the same recipient.
///
/// @param erc721AddressHex: the serialized EVM address of the ERC721 contract
/// @param to: the serialized EVM address of the recipient
/// @param ids: an array of IDs to send from the signer's COA to the recipient
transaction(erc721AddressHex: String, to: String, ids: [UInt256]) {
let coa: auth(EVM.Call) &EVM.CadenceOwnedAccount
prepare(signer: auth(BorrowValue) &Account) {
// Borrow an entitled reference to the COA from the canonical storage location
To deploy a contract to the EVM, you can use the deploy function provided by the COA resource. This function takes the
contract code, gas limit, and value you want to send. It will return the EVM address of the newly deployed contract.
This transaction will deploy a contract with the given code using the signer's COA:
deploy_evm_contract.cdc
import "EVM"
transaction(bytecode: String) {
let coa: auth(EVM.Deploy) &EVM.CadenceOwnedAccount
prepare(signer: auth(BorrowValue) &Account) {
// Borrow an entitled reference to the COA from the storage location we saved it to