README.md

description
Tutorial on how to read ERC-20 Token smart contract events with Go.

Reading ERC-20 Token Event Logs

First create the ERC-20 smart contract interface for event logs as erc20.sol:

pragma solidity ^0.4.24;

contract ERC20 {
    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

Then use abigen to create the Go exchange package given the abi:

solc --abi erc20.sol
abigen --abi=erc20_sol_ERC20.abi --pkg=token --out=erc20.go

Now in our Go application let's create the struct types matching the types of the ERC-20 event log signature:

type LogTransfer struct {
	From   common.Address
	To     common.Address
	Tokens *big.Int
}

type LogApproval struct {
	TokenOwner common.Address
	Spender    common.Address
	Tokens     *big.Int
}

Initialize the ethereum client:

client, err := ethclient.Dial("https://cloudflare-eth.com")
if err != nil {
  log.Fatal(err)
}

Create a FilterQuery passing the ERC-20 smart contract address and the desired block range. We'll be using the ZRX token for this example:

// 0x Protocol (ZRX) token address
contractAddress := common.HexToAddress("0xe41d2489571d322189246dafa5ebde1f4699f498")
query := ethereum.FilterQuery{
  FromBlock: big.NewInt(6383820),
  ToBlock:   big.NewInt(6383840),
  Addresses: []common.Address{
    contractAddress,
  },
}

Query the logs with FilterLogs:

logs, err := client.FilterLogs(context.Background(), query)
if err != nil {
  log.Fatal(err)
}

Next we'll parse the JSON abi which we'll use unpack the raw log data later:

contractAbi, err := abi.JSON(strings.NewReader(string(token.TokenABI)))
if err != nil {
  log.Fatal(err)
}

In order to filter by certain log type, we need to figure out the keccak256 hash of each event log function signature. The event log function signature hash is always topic[0] as we'll see soon. Here's how to calculate the keccak256 hash using the go-ethereum crypto package:

logTransferSig := []byte("Transfer(address,address,uint256)")
LogApprovalSig := []byte("Approval(address,address,uint256)")
logTransferSigHash := crypto.Keccak256Hash(logTransferSig)
logApprovalSigHash := crypto.Keccak256Hash(LogApprovalSig)

Now we'll iterate through all the logs and set up a switch statement to filter by event log type:

for _, vLog := range logs {
  fmt.Printf("Log Block Number: %d\n", vLog.BlockNumber)
  fmt.Printf("Log Index: %d\n", vLog.Index)

  switch vLog.Topics[0].Hex() {
  case logTransferSigHash.Hex():
    //
  case logApprovalSigHash.Hex():
    //
  }
}

Now to parse the Transfer event log we'll use abi.Unpack to parse the raw log data into our log type struct. Unpack will not parse indexed event types because those are stored under topics, so for those we'll have to parse separately as seen in the example below:

fmt.Printf("Log Name: Transfer\n")

var transferEvent LogTransfer

err := contractAbi.Unpack(&transferEvent, "Transfer", vLog.Data)
if err != nil {
  log.Fatal(err)
}

transferEvent.From = common.HexToAddress(vLog.Topics[1].Hex())
transferEvent.To = common.HexToAddress(vLog.Topics[2].Hex())

fmt.Printf("From: %s\n", transferEvent.From.Hex())
fmt.Printf("To: %s\n", transferEvent.To.Hex())
fmt.Printf("Tokens: %s\n", transferEvent.Tokens.String())

Similarly for the Approval event log:

fmt.Printf("Log Name: Approval\n")

var approvalEvent LogApproval

err := contractAbi.Unpack(&approvalEvent, "Approval", vLog.Data)
if err != nil {
  log.Fatal(err)
}

approvalEvent.TokenOwner = common.HexToAddress(vLog.Topics[1].Hex())
approvalEvent.Spender = common.HexToAddress(vLog.Topics[2].Hex())

fmt.Printf("Token Owner: %s\n", approvalEvent.TokenOwner.Hex())
fmt.Printf("Spender: %s\n", approvalEvent.Spender.Hex())
fmt.Printf("Tokens: %s\n", approvalEvent.Tokens.String())

Putting it all together and running it we'll see the following output:

Log Block Number: 6383829
Log Index: 20
Log Name: Transfer
From: 0xd03dB9CF89A9b1f856a8E1650cFD78FAF2338eB2
To: 0x924CD9b60F4173DCDd5254ddD38C4F9CAB68FE6b
Tokens: 2804000000000000000000


Log Block Number: 6383831
Log Index: 62
Log Name: Approval
Token Owner: 0xDD3b9186Da521AbE707B48B8f805Fb3Cd5EEe0EE
Spender: 0xCf67d7A481CEEca0a77f658991A00366FED558F7
Tokens: 10000000000000000000000000000000000000000000000000000000000000000


Log Block Number: 6383838
Log Index: 13
Log Name: Transfer
From: 0xBA826fEc90CEFdf6706858E5FbaFcb27A290Fbe0
To: 0x4aEE792A88eDDA29932254099b9d1e06D537883f
Tokens: 2863452144424379687066

Compare the parsed log output to what's on etherscan: https://etherscan.io/tx/0x0c3b6cf604275c7e44dc7db400428c1a39f33f0c6cbc19ff625f6057a5cb32c0#eventlog

---

Full code

Commands

solc --abi erc20.sol
abigen --abi=erc20_sol_ERC20.abi --pkg=token --out=erc20.go

erc20.sol

pragma solidity ^0.4.24;

contract ERC20 {
    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

event_read_erc20.go

package main

import (
	"context"
	"fmt"
	"log"
	"math/big"
	"strings"

	token "./contracts_erc20" // for demo
	"github.com/ethereum/go-ethereum"
	"github.com/ethereum/go-ethereum/accounts/abi"
	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/crypto"
	"github.com/ethereum/go-ethereum/ethclient"
)

// LogTransfer ..
type LogTransfer struct {
	From   common.Address
	To     common.Address
	Tokens *big.Int
}

// LogApproval ..
type LogApproval struct {
	TokenOwner common.Address
	Spender    common.Address
	Tokens     *big.Int
}

func main() {
	client, err := ethclient.Dial("https://cloudflare-eth.com")
	if err != nil {
		log.Fatal(err)
	}

	// 0x Protocol (ZRX) token address
	contractAddress := common.HexToAddress("0xe41d2489571d322189246dafa5ebde1f4699f498")
	query := ethereum.FilterQuery{
		FromBlock: big.NewInt(6383820),
		ToBlock:   big.NewInt(6383840),
		Addresses: []common.Address{
			contractAddress,
		},
	}

	logs, err := client.FilterLogs(context.Background(), query)
	if err != nil {
		log.Fatal(err)
	}

	contractAbi, err := abi.JSON(strings.NewReader(string(token.TokenABI)))
	if err != nil {
		log.Fatal(err)
	}

	logTransferSig := []byte("Transfer(address,address,uint256)")
	LogApprovalSig := []byte("Approval(address,address,uint256)")
	logTransferSigHash := crypto.Keccak256Hash(logTransferSig)
	logApprovalSigHash := crypto.Keccak256Hash(LogApprovalSig)

	for _, vLog := range logs {
		fmt.Printf("Log Block Number: %d\n", vLog.BlockNumber)
		fmt.Printf("Log Index: %d\n", vLog.Index)

		switch vLog.Topics[0].Hex() {
		case logTransferSigHash.Hex():
			fmt.Printf("Log Name: Transfer\n")

			var transferEvent LogTransfer

			err := contractAbi.Unpack(&transferEvent, "Transfer", vLog.Data)
			if err != nil {
				log.Fatal(err)
			}

			transferEvent.From = common.HexToAddress(vLog.Topics[1].Hex())
			transferEvent.To = common.HexToAddress(vLog.Topics[2].Hex())

			fmt.Printf("From: %s\n", transferEvent.From.Hex())
			fmt.Printf("To: %s\n", transferEvent.To.Hex())
			fmt.Printf("Tokens: %s\n", transferEvent.Tokens.String())

		case logApprovalSigHash.Hex():
			fmt.Printf("Log Name: Approval\n")

			var approvalEvent LogApproval

			err := contractAbi.Unpack(&approvalEvent, "Approval", vLog.Data)
			if err != nil {
				log.Fatal(err)
			}

			approvalEvent.TokenOwner = common.HexToAddress(vLog.Topics[1].Hex())
			approvalEvent.Spender = common.HexToAddress(vLog.Topics[2].Hex())

			fmt.Printf("Token Owner: %s\n", approvalEvent.TokenOwner.Hex())
			fmt.Printf("Spender: %s\n", approvalEvent.Spender.Hex())
			fmt.Printf("Tokens: %s\n", approvalEvent.Tokens.String())
		}

		fmt.Printf("\n\n")
	}
}

solc version used for these examples

$ solc --version
0.4.24+commit.e67f0147.Emscripten.clang