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Token transfer_from

View this samples code on GitHub.

Overview​

token_transfer_from_backend is a canister that can transfer ICRC-1 tokens on behalf of accounts to other accounts. It is an example of a canister that uses an ICRC-1 ledger canister that supports the ICRC-2 approve and transfer from standard. 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 or the caller has not approved the canister to spend their tokens. In case of success, a unique identifier of the transaction is returned. The example code assumes the caller of transfer has already approved the token transfer canister to spend their tokens.

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 4-6 and 8-10 below.

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

dfx new --type=motoko token_transfer_from --no-frontend
cd token_transfer_from

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.

[!IMPORTANT] Don't forget to add the icrc1_ledger_canister as a dependency for token_transfer_from_backend, otherwise the build will fail.

{
    "canisters": {
        "token_transfer_from_backend": {
            "main": "src/token_transfer_from_backend/main.mo",
            "type": "motoko",
            "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: Deploy the ICRC-1 ledger locally:​

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

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 anonymous principal (2vxsx-fae)
  • Minting 100 tokens to the default identity
  • Setting the transfer fee to 0.0001 tokens
  • Naming the token Local ICRC1 / L-ICRC1
  • Enabling the ICRC-2 standard for the ledger
dfx deploy icrc1_ledger_canister --argument "(variant {
  Init = record {
    token_symbol = \"ICRC1\";
    token_name = \"L-ICRC1\";
    minting_account = record {
      owner = principal \"$(dfx identity --identity anonymous get-principal)\"
    };
    transfer_fee = 10_000;
    metadata = vec {};
    initial_balances = vec {
      record {
        record {
          owner = principal \"$(dfx identity --identity default get-principal)\";
        };
        10_000_000_000;
      };
    };
    archive_options = record {
      num_blocks_to_archive = 1000;
      trigger_threshold = 2000;
      controller_id = principal \"$(dfx identity --identity anonymous get-principal)\";
    };
    feature_flags = opt record {
      icrc2 = true;
    };
  }
})"

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 6: 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 \"$(dfx identity --identity default get-principal)\";
})"
```

The output should be:

```bash
(10_000_000_000 : nat)

Step 7: Prepare the token transfer canister:​

Replace the contents of the src/token_transfer_from_backend/main.mo file with the following:

import Icrc1Ledger "canister:icrc1_ledger_canister";
import Debug "mo:base/Debug";
import Result "mo:base/Result";
import Error "mo:base/Error";

actor {

  type TransferArgs = {
    amount : Nat;
    toAccount : Icrc1Ledger.Account;
  };

  public shared ({ caller }) func transfer(args : TransferArgs) : async Result.Result<Icrc1Ledger.BlockIndex, Text> {
    Debug.print(
      "Transferring "
      # debug_show (args.amount)
      # " tokens to account"
      # debug_show (args.toAccount)
    );

    let transferFromArgs : Icrc1Ledger.TransferFromArgs = {
      // the account we want to transfer tokens from (in this case we assume the caller approved the canister to spend funds on their behalf)
      from = {
        owner = caller;
        subaccount = null;
      };
      // can be used to distinguish between transactions
      memo = null;
      // the amount we want to transfer
      amount = args.amount;
      // the subaccount we want to spend the tokens from (in this case we assume the default subaccount has been approved)
      spender_subaccount = null;
      // if not specified, the default fee for the canister is used
      fee = null;
      // we take the principal and subaccount from the arguments and convert them into an account identifier
      to = args.toAccount;
      // 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 = null;
    };

    try {
      // initiate the transfer
      let transferFromResult = await Icrc1Ledger.icrc2_transfer_from(transferFromArgs);

      // check if the transfer was successfull
      switch (transferFromResult) {
        case (#Err(transferError)) {
          return #err("Couldn't transfer funds:\n" # debug_show (transferError));
        };
        case (#Ok(blockIndex)) { return #ok blockIndex };
      };
    } catch (error : Error) {
      // catch any errors that might occur during the transfer
      return #err("Reject message: " # Error.message(error));
    };
  };
};

Step 8: Deploy the token transfer canister:​

dfx deploy token_transfer_from_backend

Step 9: Approve the canister to transfer funds on behalf of the user:​

[!IMPORTANT] Make sure that you are using the default dfx account that we minted tokens to in step 5 for the following steps.

Make the following call to approve the token_transfer_from_backend canister to transfer 100 tokens on behalf of the default identity:

dfx canister call --identity default icrc1_ledger_canister icrc2_approve "(
  record {
    spender= record {
      owner = principal \"$(dfx canister id token_transfer_from_backend)\";
    };
    amount = 10_000_000_000: nat;
  }
)"

If successful, the output should be:

(variant { Ok = 1 : nat })

Step 10: Let the canister transfer funds on behalf of the user:​

Now that the canister has an approval for the default identities tokens on the ledger, the canister can transfer 1 token on behalf of the default identity to another account, in this case to the canisters own account.

dfx canister call token_transfer_from_backend transfer "(record {
  amount = 100_000_000;
  toAccount = record {
    owner = principal \"$(dfx canister id token_transfer_from_backend)\";
  };
})"

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: