## Retrieving Events
In this section, we will learn to read events released by smart contracts using the FMZ Quant Trading Platform. The events released by smart contracts are stored in the logs of the Ethereum virtual machine.
### eth_getLogs
To query the event of smart contract release, you need to use Ethereum's RPC method ```eth_getLogs``` to get the on-chain log data, for how to call Ethereum RPC nodes, we have explained in our previous course.
For example, we get the event of ```WETH``` contract can write code and test it using FMZ's **debugging tool**, the RPC node configured by the exchange object is an Ethereum mainnet node, and in calling the ```eth_getLogs``` method, we specify three parameters ```fromBlock```, ```toBlock```, ```address```, and we use the fromBlock and toBlock parameters to limit the query to data within a block:
```javascript
function toAmount(s, decimals) {
return Number((BigDecimal(BigInt(s)) / BigDecimal(Math.pow(10, decimals))).toString())
}
function toInnerAmount(n, decimals) {
return (BigDecimal(n) * BigDecimal(Math.pow(10, decimals))).toFixed(0)
}
function main() {
// getBlockNumber
var blockNumber = exchange.IO("api", "eth", "eth_blockNumber")
Log("blockNumber:", blockNumber)
// get logs
var fromBlock = "0x" + (toAmount(blockNumber, 0) - 1).toString(16)
var toBlock = "0x" + toAmount(blockNumber, 0).toString(16)
var params = {
"fromBlock" : fromBlock,
"toBlock" : toBlock,
"address" : "0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2" // Address of WETH contract
}
var logs = exchange.IO("api", "eth", "eth_getLogs", params)
// Due to the large amount of data, if you use the Log function to print, the data will be truncated. Use return to return the complete data in the "Function Result" edit box on the page
return logs
}
```
The logs data were obtained, and we omitted some of them due to their large content:
```javascript
[{
"data": "0x00000000000000000000000000000000000000000000000001c1a55000000000",
"topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef", "0x0000000000000000000000006b75d8af000000e20b7a7ddf000ba900b4009a80", "0x000000000000000000000000bcb095c1f9c3dc02e834976706c87dee5d0f1fb6"],
"transactionHash": "0x27f9bf5abe3148169b4b85a83e1de32bd50eb81ecc52e5af006157d93353e4c4",
"transactionIndex": "0x0",
"removed": false,
"address": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
"blockHash": "0x847be24a7b159c292bda030a011dfec89487b70e71eed486969b032d6ef04bad",
"blockNumber": "0x109b1cc",
"logIndex": "0x0"
}, {
"data": "0x00000000000000000000000000000000000000000000000008ea20cdea027c00",
"logIndex": "0x5",
"topics": ["0xe1fffcc4923d04b559f4d29a8bfc6cda04eb5b0d3c460751c2402c5c5cc9109c", "0x0000000000000000000000007a250d5630b4cf539739df2c5dacb4c659f2488d"],
"address": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
"blockHash": "0x847be24a7b159c292bda030a011dfec89487b70e71eed486969b032d6ef04bad",
"blockNumber": "0x109b1cc",
"removed": false,
"transactionHash": "0xace3afa02e8af5d1ef6fc1635fbdf7bee37624547937ea5272c23968dd034c09",
"transactionIndex": "0x1"
},
...
{
"blockNumber": "0x109b1cd",
"address": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
"data": "0x00000000000000000000000000000000000000000000000002c053531ab8a000",
"logIndex": "0xd3",
"removed": false,
"topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef", "0x0000000000000000000000001111111254eeb25477b68fb85ed929f73a960582", "0x000000000000000000000000252ba9b5916171dbdadd2cec7f91875a006955d0"],
"transactionHash": "0x3012b82891f85b077cfe1c12cb9722b93c696ef2c37d67981ccddcc9c3396aca",
"transactionIndex": "0x8d",
"blockHash": "0xcd3d567c9bd02a4549b1de0dc638ab5523e847c3c156b096424f56c633000fd9"
}, {
"topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef", "0x00000000000000000000000012b791bb27b3a4ee958b5a435fea7d49ec076e9c", "0x000000000000000000000000ef1c6e67703c7bd7107eed8303fbe6ec2554bf6b"],
"transactionIndex": "0x91",
"logIndex": "0xdb",
"removed": false,
"blockNumber": "0x109b1cd",
"data": "0x0000000000000000000000000000000000000000000000000164f2434262e1cc",
"transactionHash": "0x6aa8d80daf42f442591e7530e31323d05e1d6dd9f9f9b9c102e157d89810c048",
"address": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
"blockHash": "0xcd3d567c9bd02a4549b1de0dc638ab5523e847c3c156b096424f56c633000fd9"
}, {
"address": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
"blockHash": "0xcd3d567c9bd02a4549b1de0dc638ab5523e847c3c156b096424f56c633000fd9",
"blockNumber": "0x109b1cd",
"logIndex": "0xde",
"removed": false,
"topics": ["0x7fcf532c15f0a6db0bd6d0e038bea71d30d808c7d98cb3bf7268a95bf5081b65", "0x000000000000000000000000ef1c6e67703c7bd7107eed8303fbe6ec2554bf6b"],
"data": "0x0000000000000000000000000000000000000000000000000164f2434262e1cc",
"transactionHash": "0x6aa8d80daf42f442591e7530e31323d05e1d6dd9f9f9b9c102e157d89810c048",
"transactionIndex": "0x91"
}]
```
We can see that there are various events in the logs data, if we only care about ```Transfer``` events, we need to filter out the ```Transfer``` events in these data.
### Retrieving Logs
The Ethereum log is divided into two parts: 1. ```topics```; 2. ```data```.
- ```topics```
Taking the results of the code run for the ```eth_getLogs``` section test as an example, the data in the ```topics``` field is:
```desc
"topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef", "0x00000000000000000000000012b791bb27b3a4ee958b5a435fea7d49ec076e9c", "0x000000000000000000000000ef1c6e67703c7bd7107eed8303fbe6ec2554bf6b"],
```
The value of the ```topics``` field is an array structure used to describe the event. It is specified that its (array) length cannot exceed 4 and the first element is the signature hash of the event.
In the FMZ Quant Trading Platform, we can calculate this signature hash using the ```Encode``` function, using the following code:
```javascript
function main() {
var eventFunction = "Transfer(address,address,uint256)"
var eventHash = Encode("keccak256", "string", "hex", eventFunction)
Log("eventHash:", "0x" + eventHash)
// eventHash: 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef
}
```
Calculate the ```keccak256``` hash value (hex encoding) of ```Transfer(address,address,uint256)``` is ```0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef```.
The value of the ```topics``` field is an array structure, with the second element, and the third element, respectively:
- Sending address ```from```
- Receiving address ```to```
- ```data```
The data in the ```data``` field are:
```desc
"data": "0x0000000000000000000000000000000000000000000000000164f2434262e1cc",
```
Certain parameters in the event (parameters without indexed declarations in the Solidity code of the smart contract) are stored in the ```data``` section.
Parse the data ```0x0000000000000000000000000000000000000000000000000164f2434262e1cc```
```javascript
function toAmount(s, decimals) {
return Number((BigDecimal(BigInt(s)) / BigDecimal(Math.pow(10, decimals))).toString())
}
function main() {
var value = "0x0000000000000000000000000000000000000000000000000164f2434262e1cc"
Log(toAmount(value, 0) / 1e18) // 0.10047146239950075
}
```
This data is obtained as 0.10047146239950075 and the ```data``` is the corresponding transfer amount.
--------------------------------------------------
The above was explained, practiced and ready to go. We can start retrieving the logs at:
```javascript
function toAmount(s, decimals) {
return Number((BigDecimal(BigInt(s)) / BigDecimal(Math.pow(10, decimals))).toString())
}
function toInnerAmount(n, decimals) {
return (BigDecimal(n) * BigDecimal(Math.pow(10, decimals))).toFixed(0)
}
function main() {
// getBlockNumber
var blockNumber = exchange.IO("api", "eth", "eth_blockNumber")
Log("blockNumber:", blockNumber)
// get logs
var fromBlock = "0x" + (toAmount(blockNumber, 0) - 1).toString(16)
var toBlock = "0x" + toAmount(blockNumber, 0).toString(16)
var params = {
"fromBlock" : fromBlock,
"toBlock" : toBlock,
"address" : "0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2"
}
var logs = exchange.IO("api", "eth", "eth_getLogs", params)
// Traverse logs
var eventFunction = "Transfer(address,address,uint256)"
var eventHash = "0x" + Encode("keccak256", "string", "hex", eventFunction)
Log("eventHash:", eventHash)
var counter = 0
for (var i = logs.length - 1; i >= 0 && counter < 10; i--) {
if (logs[i].topics[0] == eventHash) {
Log("Event Transfer, data:", toAmount(logs[i].data, 0) / 1e18, ", blockNumber:", toAmount(logs[i].blockNumber, 0), ", transactionHash:", logs[i].transactionHash,
", log:", logs[i])
counter++
}
}
}
```
Check on ```https://etherscan.io/```:
https://stocksharp.ru/file/150274
Results of the test code run in FMZ debugging tool:
https://stocksharp.ru/file/150275
Data in the ```from```, ```to``` fields can also be parsed depending on the needs at the time of retrieving, e.g:
```javascript
function main() {
var from = "0x00000000000000000000000012b791bb27b3a4ee958b5a435fea7d49ec076e9c"
var address = "0x" + exchange.IO("encodePacked", "address", from)
Log("address:", address)
}
```
Running results:
> address: 0x12b791bb27b3a4ee958b5a435fea7d49ec076e9c
### Listening to Contract Events
Since the **debugging tool** can only test the code for a short time and output the content only after the code execution is completed, it cannot display and output the log in real time. In this section, we use the FMZ Quant Trading Platform to create live trading to test.
Here we use the Ethereum mainnet, and we listen to the ```Transfer(address,address,uint256)``` event of the ```USDT``` cryptocurrency contract. Based on what we learned in the last lesson, we designed and wrote an example of continuously listening to the events of a certain smart contract:
```javascript
function toAmount(s, decimals) {
return Number((BigDecimal(BigInt(s)) / BigDecimal(Math.pow(10, decimals))).toString())
}
function toInnerAmount(n, decimals) {
return (BigDecimal(n) * BigDecimal(Math.pow(10, decimals))).toFixed(0)
}
function addEventListener(contractAddress, event, callBack) {
var self = {}
self.eventHash = "0x" + Encode("keccak256", "string", "hex", event)
self.contractAddress = contractAddress
self.latestBlockNumber = 0
self.fromBlockNumber = 0
self.firstBlockNumber = 0
/* TODO: test
self.isFirst = true
*/
self.getBlockNumber = function() {
var maxTry = 10
for (var i = 0; i < maxTry; i++) {
var ret = exchange.IO("api", "eth", "eth_blockNumber")
if (ret) {
return toAmount(ret, 0)
}
Sleep(5000)
}
throw "getBlockNumber failed"
}
self.run = function() {
var currBlockNumber = self.getBlockNumber()
var fromBlock = "0x" + self.fromBlockNumber.toString(16)
var toBlock = "0x" + currBlockNumber.toString(16)
var params = {
"fromBlock" : fromBlock,
"toBlock" : toBlock,
"address" : self.contractAddress,
"topics" : [self.eventHash]
}
// Log("fromBlockNumber:", self.fromBlockNumber, ", currBlockNumber:", currBlockNumber, "#FF0000")
var logs = exchange.IO("api", "eth", "eth_getLogs", params)
if (!logs) {
return
}
for (var i = 0; i < logs.length; i++) {
if (toAmount(logs[i].blockNumber, 0) > self.latestBlockNumber) {
/* TODO: test
if (self.isFirst) {
self.firstBlockNumber = toAmount(logs[i].blockNumber, 0)
Log("firstBlockNumber:", self.firstBlockNumber)
self.isFirst = false
}
*/
callBack(logs[i])
}
}
self.latestBlockNumber = currBlockNumber
self.fromBlockNumber = self.latestBlockNumber - 1
}
self.latestBlockNumber = self.getBlockNumber()
self.fromBlockNumber = self.latestBlockNumber - 1
return self
}
var listener = null
function main() {
var event = "Transfer(address,address,uint256)"
var contractAddress = "0xdac17f958d2ee523a2206206994597c13d831ec7"
var decimals = exchange.IO("api", contractAddress, "decimals")
Log(exchange.IO("api", contractAddress, "name"), " decimals:", decimals)
listener = addEventListener(contractAddress, event, function(log) {
var fromAddress = "0x" + exchange.IO("encodePacked", "address", log.topics[1])
var toAddress = "0x" + exchange.IO("encodePacked", "address", log.topics[2])
Log("Transfer:", fromAddress, "->", toAddress, ", value:", toAmount(log.data, decimals), ", blockNumber:", toAmount(log.blockNumber, 0))
/* TODO: test
arrLog.push(log)
*/
})
while (true) {
listener.run()
Sleep(5000)
}
}
/* TODO: test
var arrLog = []
function onexit() {
Log("End the run and verify the record")
var firstBlockNumber = listener.firstBlockNumber
var endBlockNumber = listener.latestBlockNumber
Log("getLogs, from:", firstBlockNumber, " -> to:", endBlockNumber)
var fromBlock = "0x" + (firstBlockNumber).toString(16)
var toBlock = "0x" + (endBlockNumber).toString(16)
var params = {
"fromBlock" : fromBlock,
"toBlock" : toBlock,
"topics" : ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"],
"address" : "0xdac17f958d2ee523a2206206994597c13d831ec7"
}
var logs = exchange.IO("api", "eth", "eth_getLogs", params)
Log("arrLog:", arrLog.length)
Log("logs:", logs.length)
if (arrLog.length != logs.length) {
Log("Length varies!")
return
}
for (var i = 0; i < arrLog.length; i++) {
Log("Determine the blockNumber:", logs[i].blockNumber == arrLog[i].blockNumber, ", Determine from:", logs[i].topics[1] == arrLog[i].topics[1],
"Determine to:", logs[i].topics[2] == arrLog[i].topics[2])
}
}
*/
```
Running on live trading:
https://stocksharp.ru/file/150276
For the execution results, a validation section (TODO: test) is also written in the code. After a simple validation it can be seen that the ```Transfer``` event of the USDT contract is continuously monitored and data is recorded, and a comparison between this data and the event data obtained at once can be observed that the data is consistent with:
https://stocksharp.ru/file/150277
### Event Filtering
Based on the previous lesson ```Listening to contract events```, we expand on it by adding filters to the listening process to listen for transfers to and from specified addresses. When a smart contract creates a log (i.e. releases an event), the log data ```topics``` contains up to 4 pieces of information. So we design a filter rule with ```[[A1, A2, ...An], null, [C1], D]``` as an example.
1. ```[A1, A2, ...An]``` corresponds to the data at position ```topics[0]```.
2. ```Null``` corresponds to the data at position ```topics[1]```.
3. ```[C1]``` corresponds to data at position ```topics[2]```.
4. ```D``` corresponds to the data at position ```topics[3]```.
- If an element in the condition structure is set ```null``` means it is not filtered, e.g. ```null``` corresponds to ```topics[1]``` and any value matches.
- If the element in the condition structure sets a single value indicating that the position must match, e.g. ```[C1]``` corresponds to ```topics[2]``` or ```D``` corresponds to ```topics[3]```, and unmatched logs are filtered.
- If the element in the condition structure is an array, it means that at least one of the elements in the array should match, e.g. ```[A1, A2, ...An]``` corresponds to ```topics[0]```, ```[A1, A2, ...An]``` with any one of them matching ```topics[0]```, then the logs will not be filtered.
**Listening to USDT transfers from exchanges**
Monitoring of ```USDT``` transactions transferred from and to the Binance Exchange:
```javascript
function toAmount(s, decimals) {
return Number((BigDecimal(BigInt(s)) / BigDecimal(Math.pow(10, decimals))).toString())
}
function toInnerAmount(n, decimals) {
return (BigDecimal(n) * BigDecimal(Math.pow(10, decimals))).toFixed(0)
}
function addEventListener(contractAddress, event, callBack) {
var self = {}
self.eventHash = "0x" + Encode("keccak256", "string", "hex", event)
self.contractAddress = contractAddress
self.latestBlockNumber = 0
self.fromBlockNumber = 0
self.firstBlockNumber = 0
self.filters = []
self.setFilter = function(filterCondition) {
if (filterCondition.length > 4) {
throw "filterCondition error"
}
self.filters.push(filterCondition)
Log("Set filter conditions:", filterCondition)
}
self.getTokenBalanceOfWallet = function(walletAddress, tokenAddress, tokenDecimals) {
var balance = exchange.IO("api", tokenAddress, "balanceOf", walletAddress)
if (balance) {
return toAmount(balance, tokenDecimals)
}
return null
}
self.getBlockNumber = function() {
var maxTry = 10
for (var i = 0; i < maxTry; i++) {
var ret = exchange.IO("api", "eth", "eth_blockNumber")
if (ret) {
return toAmount(ret, 0)
}
Sleep(5000)
}
throw "getBlockNumber failed"
}
self.run = function() {
var currBlockNumber = self.getBlockNumber()
var fromBlock = "0x" + self.fromBlockNumber.toString(16)
var toBlock = "0x" + currBlockNumber.toString(16)
var params = {
"fromBlock" : fromBlock,
"toBlock" : toBlock,
"address" : self.contractAddress,
"topics" : [self.eventHash]
}
var logs = exchange.IO("api", "eth", "eth_getLogs", params)
if (!logs) {
return
}
for (var i = 0; i < logs.length; i++) {
if (toAmount(logs[i].blockNumber, 0) > self.latestBlockNumber) {
// Check the filter condition, and execute the judgment if the filter condition is set
if (self.filters.length != 0) {
// Initial filter marker
var isFilter = true
// Traverse filter condition setting
for (var j = 0; j < self.filters.length; j++) {
// Take a filter setting, e.g: [[A1, A2, ...An], null, [C1], D]
var cond = self.filters[j]
// Traverse the filter setting
var final = true
for (var topicsIndex = 0; topicsIndex < cond.length; topicsIndex++) {
// Take one of the conditions in the filter setting, if it is the first condition: i.e. the data to be compared with topics[0]
var condValue = cond[topicsIndex]
// Data in the logs
if (topicsIndex > logs[i].topics.length - 1) {
continue
}
var topicsEleValue = logs[i].topics[topicsIndex]
// If it's a Transfer event, you need to handle the from and to
if (logs[i].topics[0] == "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef") {
if (topicsIndex == 1 || topicsIndex == 2) {
topicsEleValue = "0x" + exchange.IO("encodePacked", "address", topicsEleValue)
}
}
// If the condValue type is an array, it means that there are multiple comparison conditions in this position, and the multiple condition comparison is a logical or relationship
if (Array.isArray(condValue) && condValue.length > 1) {
// Determine condValue[0] == topicsEleValue || condValue[1] == topicsEleValue
final = final && condValue.some(element => element === topicsEleValue)
}else if (condValue === null) {
final = final && true
} else {
final = final && (condValue === topicsEleValue)
}
}
if (final) {
isFilter = false
}
}
if (isFilter) {
continue
}
}
callBack(logs[i])
}
}
self.latestBlockNumber = currBlockNumber
self.fromBlockNumber = self.latestBlockNumber - 1
}
self.latestBlockNumber = self.getBlockNumber()
self.fromBlockNumber = self.latestBlockNumber - 1
return self
}
var listener = null
function main() {
// Initial clean-up log
LogReset(1)
LogProfitReset()
var event = "Transfer(address,address,uint256)" // Listening to events
var contractAddress = "0xdac17f958d2ee523a2206206994597c13d831ec7" // USDT contract address
var decimals = exchange.IO("api", contractAddress, "decimals") // Get the precision information of USDT token
var accountBinanceAddress = "0x28C6c06298d514Db089934071355E5743bf21d60" // Binance hot wallet address
accountBinanceAddress = accountBinanceAddress.toLowerCase() // Addresses are handled in lowercase
Log(exchange.IO("api", contractAddress, "name"), " decimals:", decimals)
// Creating a listener object
listener = addEventListener(contractAddress, event, function(log) {
var fromAddress = "0x" + exchange.IO("encodePacked", "address", log.topics[1])
var toAddress = "0x" + exchange.IO("encodePacked", "address", log.topics[2])
if (fromAddress == accountBinanceAddress) {
Log("Binance transfer out - ", " Transfer:", fromAddress, "->", toAddress, ", value:", toAmount(log.data, decimals), ", blockNumber:", toAmount(log.blockNumber, 0), "#CD32CD")
} else if (toAddress == accountBinanceAddress) {
Log("Binance transfer in - ", " Transfer:", fromAddress, "->", toAddress, ", value:", toAmount(log.data, decimals), ", blockNumber:", toAmount(log.blockNumber, 0), "#FF0000")
}
})
// Set up event filtering
listener.setFilter([null, accountBinanceAddress, null]) // Binance -> USDT
listener.setFilter([null, null, accountBinanceAddress]) // USDT -> Binance
var preBalance = 0
while (true) {
listener.run()
var balance = listener.getTokenBalanceOfWallet(accountBinanceAddress, contractAddress, decimals)
if (balance) {
var direction = ""
if (preBalance != 0 && preBalance > balance) {
direction = " ↓ " + (preBalance - balance) + "#CD32CD"
} else if (preBalance != 0 && preBalance < balance) {
direction = " ↑ " + (balance - preBalance) + "#FF0000"
}
Log("Binance wallet address:", accountBinanceAddress, " balance:", balance, direction)
LogProfit(balance, "&") // Drawing only, no log printing
preBalance = balance
}
LogStatus(_D(), "Binance wallet address:", accountBinanceAddress, ", balance:", balance)
Sleep(5000 * 3)
}
}
```
The above code running in live trading:
https://stocksharp.ru/file/150278
In this lesson, we introduced how to design an event filter. And used it to listen for ```USDT``` transactions associated with the Binance Exchange hot wallet. You can modify and extend this sample program to listen to any event you are interested in, to see what new transactions ```smart money``` has made, what new items the ```NFT``` Tycoons have rushed, etc.
To be continued...