Intro

  • When a user needs to transfer or swap from an EVM chain (e.g. Ethereum mainnet, Arbitrum, Optimism, etc…), the Skip Go API will return an EvmTx type for the developer to pass to the user for signing
  • Unlike CosmosSDK transactions, EVM transactions do not have a notion of messages, so this object doesn’t correspond 1-to-1 to a “message”, which might be a more familiar notion to Cosmos developers
  • This doc is intended for CosmosSDK developers who aren’t already familiar with the concepts of transaction construction in the EVM and need to use EvmTx to help their users move from/to EVM chains.

EvmTx Data Structure

The EvmTx has 4 fields that the developer needs to understand:

  • to: The address of the smart contract or externally owned account (EOA) with which this transaction interacts, as a hex-string prefixed with 0x (e.g. 0xfc05aD74C6FE2e7046E091D6Ad4F660D2A159762)
  • value: The amount of wei this transaction sends to the contract its interacting with (1 ETH = 1^18 WEI)
  • data: The calldata this transaction uses to call the smart contract it interacts with, as a hex string. The data bytes will be interpreted according to the application-binary-interface (ABI) of the contract that’s being interacted with. If this field is empty, it means the transaction is sending funds to an address, rather than calling a contract.
  • required_erc20_approvals: The permissions that must be granted to a specific smart contract to spend or transfer a certain amount of their ERC-20 tokens on behalf of the end user. This allows smart contracts to execute expressive flows that may involve moving some amount of the user’s ERC-20 tokens
    • If this field is non-empty, the approval must be granted, signed, and submitted before the EvmTx populated by the other fields in the response can be submitted to the network. Otherwise, it will fail to execute with a permission error.
    • Skip’s ERC20Approval object has 3 fields that define approval:
      • token_contract: The address of the ERC-20 token on which the approval is granted
      • spender: The address of the contract to which the approval will grant spend authority
      • amount: The amount of token_contract tokens the approval will grant the spender to spend
    • Check out EIP-2612 for more information on ERC-20 approvals.
  • chain_id: This is the same as in the Cosmos context (simply an identifier for the chain), but it’s an int instead of a string

For more information on transactions, check out the Ethereum foundation’s docs

Example constructing & signing an EVM Transaction

1. Install Signing Library and Skip Library

To enable EVM transactions in your application, first install an EVM developer library. The most popular options are:

The code snippets below use viem.

Shell
npm i viem
npm i @skip-go/core

1. Initialize the SkipRouter client with the EVM WalletClient object

All 3 libraries mentioned above allow you to create WalletClient “signer” objects that:

  • Use an RPC provider under the hood to query the chain for necessary data to create transactions (e.g. nonce, gas price, etc…)
  • Expose an API that allows constructing, signing, and broadcasting transactions

You need to set up the getEVMSigner function in the SkipRouter constructor to initialize this signer object for the a given EVM chain.

For example, with Viem, we do the following:

TypeScript
import { createWalletClient, custom} from 'viem';
import * as chains from 'viem/chains';
import { SkipRouter } from '@skip-go/core';

const 
const skipClient = new SkipRouter({
  getEVMSigner: async (chainID) => {
    const chain = extractChain({
  		chains: Object.values(chains), 
  		id: parseInt(chainID)
    });
    const evmWalletClient = createWalletClient({
  		chain: chain,
  		transport: custom(window.ethereum!)
  	});
    return evmWalletClient;
  }
});

2. Request Route using SkipRouter and get required chain

Next, request your route as normal:

TypeScript
const route = await skipClient.route({
  amountIn: "1000",
  sourceAssetDenom: "0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48",
  sourceAssetChainID: "1",
  destAssetDenom: "0xaf88d065e77c8cC2239327C5EDb3A432268e5831",
  destAssetChainID: "42161",
  smartRelay: true,
  smartSwapOptions: {
    splitRoutes: true
  }
};
  

3. Get User Addresses for all Required Chains

Use the route to determine the chains for which you need to supply a user address (the source, destination, and all intermediate chains that require a recovery address for receiving tokens in case of a failure)

TypeScript
let userAddresses = []
const requiredAddresses = route.requiredChainAddresses;
// iterate over chain IDs for chains that require addresses
for (const chainID of requiredAddresses) {
  	// Check that the chain is an EVM chain
    if (parseInt(chainID)) {
      // use signer library to get address from wallet
      const chain = extractChain({
        chains: Object.values(chains), 
        id: parseInt(chainID)
      });
      const evmWalletClient = createWalletClient({
        chain: chain,
        transport: custom(window.ethereum!)
      });
      const [address] = await client.requestAddresses();
      // add to map
      userAddresses.append({address: address, chainID: chainID})
    } else {
      // handle cosmos and SVM wallets -- not shown
    }
  	
});
return evmWalletClient;
}

4. Execute the Route using SkipRouter

Finally, you can use SkipRouter.executeRoute to prompt the user to sign the approval(s) and transaction, and submit the transaction on chain.

TypeScript
await skipClient.executeRoute({
  route:route,
  userAddresses: userAddresses
});

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