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Token transfer

View this samples code on GitHub.

Overview​

Token transfer is a canister that can transfer ICRC-1 tokens from its account to other accounts. It is an example of a canister that uses an ICRC-1 ledger canister. Sample code is available in Motoko and Rust.

Architecture​

The sample code revolves around one core transfer function which takes as input the amount of tokens to transfer, the Account to which to transfer tokens and returns either success or an error in case e.g. the token transfer canister doesn’t have enough tokens to do the transfer. In case of success, a unique identifier of the transaction is returned.

This sample will use the Rust variant.

Prerequisites​

This example requires an installation of:

How to get there​

The following steps will guide you through the process of setting up the token transfer canister for your own project.

[!TIP] If you just want to interact with this example, follow steps 4-8 and 10-12 below.

Step 1: Create a new dfx project and navigate into the project's directory.​

dfx new --type=rust token_transfer --no-frontend
cd token_transfer

Step 2: Determine ICRC-1 ledger file locations​

[!NOTE] You can read more about how to setup the ICRC-1 ledger locally here.

Go to the releases overview and copy the latest replica binary revision. At the time of writing, this is d87954601e4b22972899e9957e800406a0a6b929.

The URL for the ledger Wasm module is https://download.dfinity.systems/ic/<REVISION>/canisters/ic-icrc1-ledger.wasm.gz, so with the above revision it would be https://download.dfinity.systems/ic/d87954601e4b22972899e9957e800406a0a6b929/canisters/ic-icrc1-ledger.wasm.gz.

The URL for the ledger .did file is https://raw.githubusercontent.com/dfinity/ic/<REVISION>/rs/rosetta-api/icrc1/ledger/ledger.did, so with the above revision it would be https://raw.githubusercontent.com/dfinity/ic/d87954601e4b22972899e9957e800406a0a6b929/rs/rosetta-api/icrc1/ledger/ledger.did.

OPTIONAL: If you want to make sure, you have the latest ICRC-1 ledger files you can run the following script.

curl -o download_latest_icrc1_ledger.sh "https://raw.githubusercontent.com/dfinity/ic/326df23607fc8280a047daba2d8462f1dfc57466/rs/rosetta-api/scripts/download_latest_icrc1_ledger.sh"
chmod +x download_latest_icrc1_ledger.sh
./download_latest_icrc1_ledger.sh

Step 3: Configure the dfx.json file to use the ledger :​

Replace its contents with this but adapt the URLs to be the ones you determined in step 2. Note that we are deploying the ICRC-1 ledger to the same canister id the ckBTC ledger uses on mainnet. This will make it easier to interact with it later.

{
    "canisters": {
        "token_transfer_backend": {
            "candid": "src/token_transfer_backend/token_transfer_backend.did",
            "package": "token_transfer_backend",
            "type": "rust",
            "dependencies": ["icrc1_ledger_canister"]
        },
        "icrc1_ledger_canister": {
            "type": "custom",
            "candid": "https://raw.githubusercontent.com/dfinity/ic/d87954601e4b22972899e9957e800406a0a6b929/rs/rosetta-api/icrc1/ledger/ledger.did",
            "wasm": "https://download.dfinity.systems/ic/d87954601e4b22972899e9957e800406a0a6b929/canisters/ic-icrc1-ledger.wasm.gz",
            "specified_id": "mxzaz-hqaaa-aaaar-qaada-cai"
        }
    },
    "defaults": {
        "build": {
            "args": "",
            "packtool": ""
        }
    },
    "output_env_file": ".env",
    "version": 1
}

If you chose to download the ICRC-1 ledger files with the script, you need to replace the Candid and Wasm file entries:

...
"candid": icrc1_ledger.did,
"wasm" : icrc1_ledger.wasm.gz,
  ...

Step 4: Start a local replica:​

dfx start --background --clean

Step 5: Create a new identity that will work as a minting account:​

dfx identity new minter --storage-mode plaintext
dfx identity use minter
export MINTER=$(dfx identity get-principal)

[!IMPORTANT] Transfers from the minting account will create Mint transactions. Transfers to the minting account will create Burn transactions.

Step 6: Switch back to your default identity and record its principal to mint an initial balance to when deploying the ledger:​

dfx identity use default
export DEFAULT=$(dfx identity get-principal)

Step 7: Deploy the ICRC-1 ledger locally:​

Take a moment to read the details of the call made below. Not only are you deploying an ICRC-1 ledger canister, you are also:

  • Setting the minting account to the principal you saved in a previous step (MINTER)
  • Minting 100 tokens to the DEFAULT principal
  • Setting the transfer fee to 0.0001 tokens
  • Naming the token Local ICRC1 / L-ICRC1
dfx deploy icrc1_ledger_canister --argument "(variant { Init =
record {
     token_symbol = \"ICRC1\";
     token_name = \"L-ICRC1\";
     minting_account = record { owner = principal \"${MINTER}\" };
     transfer_fee = 10_000;
     metadata = vec {};
     initial_balances = vec { record { record { owner = principal \"${DEFAULT}\"; }; 10_000_000_000; }; };
     archive_options = record {
         num_blocks_to_archive = 1000;
         trigger_threshold = 2000;
         controller_id = principal \"${MINTER}\";
     };
 }
})"

If successful, the output should be:

Deployed canisters.
URLs:
  Backend canister via Candid interface:
    icrc1_ledger_canister: http://127.0.0.1:4943/?canisterId=bnz7o-iuaaa-aaaaa-qaaaa-cai&id=mxzaz-hqaaa-aaaar-qaada-cai

Step 8: Verify that the ledger canister is healthy and working as expected by using the command:​

[!NOTE] You can find more information on how to interact with the ICRC-1 ledger here

dfx canister call icrc1_ledger_canister icrc1_balance_of "(record {
  owner = principal \"${DEFAULT}\";
  }
)"
```

The output should be:

```bash
(10_000_000_000 : nat)

Step 9: Prepare the token transfer canister:​

Replace the contents of the src/token_transfer_backend/Cargo.toml file with the following:

[package]
name = "token_transfer_backend"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[lib]
crate-type = ["cdylib"]

[dependencies]
candid = "0.10"
ic-cdk = "0.12"
ic-cdk-timers = "0.6" # Feel free to remove this dependency if you don't need timers
icrc-ledger-types = "0.1.5"
serde = "1.0.197"
serde_derive = "1.0.197"

Replace the contents of the src/token_transfer_backend/src/lib.rs file with the following:

use candid::{CandidType, Deserialize, Principal};
use icrc_ledger_types::icrc1::account::Account;
use icrc_ledger_types::icrc1::transfer::{BlockIndex, NumTokens, TransferArg, TransferError};
use serde::Serialize;

#[derive(CandidType, Deserialize, Serialize)]
pub struct TransferArgs {
    amount: NumTokens,
    to_account: Account,
}

#[ic_cdk::update]
async fn transfer(args: TransferArgs) -> Result<BlockIndex, String> {
    ic_cdk::println!(
        "Transferring {} tokens to account {}",
        &args.amount,
        &args.to_account,
    );

    let transfer_args: TransferArg = TransferArg {
        // can be used to distinguish between transactions
        memo: None,
        // the amount we want to transfer
        amount: args.amount,
        // we want to transfer tokens from the default subaccount of the canister
        from_subaccount: None,
        // if not specified, the default fee for the canister is used
        fee: None,
        // the account we want to transfer tokens to
        to: args.to_account,
        // a timestamp indicating when the transaction was created by the caller; if it is not specified by the caller then this is set to the current ICP time
        created_at_time: None,
    };

    // 1. Asynchronously call another canister function using `ic_cdk::call`.
    ic_cdk::call::<(TransferArg,), (Result<BlockIndex, TransferError>,)>(
        // 2. Convert a textual representation of a Principal into an actual `Principal` object. The principal is the one we specified in `dfx.json`.
        //    `expect` will panic if the conversion fails, ensuring the code does not proceed with an invalid principal.
        Principal::from_text("mxzaz-hqaaa-aaaar-qaada-cai")
            .expect("Could not decode the principal."),
        // 3. Specify the method name on the target canister to be called, in this case, "icrc1_transfer".
        "icrc1_transfer",
        // 4. Provide the arguments for the call in a tuple, here `transfer_args` is encapsulated as a single-element tuple.
        (transfer_args,),
    )
    .await // 5. Await the completion of the asynchronous call, pausing the execution until the future is resolved.
    // 6. Apply `map_err` to transform any network or system errors encountered during the call into a more readable string format.
    //    The `?` operator is then used to propagate errors: if the result is an `Err`, it returns from the function with that error,
    //    otherwise, it unwraps the `Ok` value, allowing the chain to continue.
    .map_err(|e| format!("failed to call ledger: {:?}", e))?
    // 7. Access the first element of the tuple, which is the `Result<BlockIndex, TransferError>`, for further processing.
    .0
    // 8. Use `map_err` again to transform any specific ledger transfer errors into a readable string format, facilitating error handling and debugging.
    .map_err(|e| format!("ledger transfer error {:?}", e))
}

// Enable Candid export (see https://internetcomputer.org/docs/current/developer-docs/backend/rust/generating-candid)
ic_cdk::export_candid!();

Replace the contents of the src/token_transfer_backend/token_transfer_backend.did file with the following:

[!NOTE] The token_transfer_backend.did file is a Candid file that describes the service interface of the canister. It was generated from the Rust code using the candid-extractor tool. You can read more about the necessary steps here.

type Account = record { owner : principal; subaccount : opt vec nat8 };
type Result = variant { Ok : nat; Err : text };
type TransferArgs = record { to_account : Account; amount : nat };
service : { transfer : (TransferArgs) -> (Result) }

Step 10: Deploy the token transfer canister:​

dfx deploy token_transfer_backend

Step 11: Transfer funds to your canister:​

[!IMPORTANT] Make sure that you are using the default dfx account that we minted tokens to in step 7 for the following steps.

Make the following call to transfer 10 tokens to the canister:

dfx canister call icrc1_ledger_canister icrc1_transfer "(record {
  to = record {
    owner = principal \"$(dfx canister id token_transfer_backend)\";
  };
  amount = 1_000_000_000;
})"

If successful, the output should be:

(variant { Ok = 1 : nat })

Step 12: Transfer funds from the canister:​

Now that the canister owns tokens on the ledger, you can transfer 1 token from the canister to another account, in this case back to the default account:

dfx canister call token_transfer_backend transfer "(record {
  amount = 100_000_000;
  to_account = record {
    owner = principal \"$(dfx identity get-principal)\";
  };
})"

Security considerations and best practices​

If you base your application on this example, we recommend you familiarize yourself with and adhere to the security best practices for developing on the Internet Computer. This example may not implement all the best practices.

For example, the following aspects are particularly relevant for this app: