Below are the common methods to monitor swap events:

For production environments, it’s highly recommended to use a paid Mirror Node provider for commercial and high-traffic purposes. While Hedera’s public mirror node offers free REST API and JSON API endpoints, they have global rate limits. These are best suited for development or low rate usage scenarios.

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Polling Swap Events for all Pairs

⛽ No gas cost

Every time a user executes a swap, the contract emits a ‘Swap’ event with the updated reserve values for the token pair. The following code demonstrates how to listen to these ‘Swap’ events for all pairs using either the REST API or JSON RPC.

Listening to ‘Swap’ events without specifying an address in the filter data will return logs for all pairs on SaucerSwap, as well as other DEXs on Hedera that share the same ‘topic0’ hash signature for the ‘Swap’ event. To identify and filter specific pairs, extract the pair’s EVM address from the log.

When a swap involves multiple liquidity pairs, a successful smart contract call will emit multiple ‘Swap’ events. To determine the route used, as well as the initial input amount and the final output amount, aggregate all the ‘Swap’ event logs.

Resources:

Typescript
import * as ethers from 'ethers'; //V6

//Set one of Hedera's JSON RPC Relay as the provider
const provider = new ethers.JsonRpcProvider(hederaJsonRelayUrl, '', {
  batchMaxCount: 1, //workaround for V6
});
  
const latestBlock = await provider.getBlockNumber();
const fromBlock = latestBlock - 1000;

//load ABI data containing the Swap event
const abiInterfaces = new ethers.Interface(abi);

const filter = {
  topics: [abiInterfaces.getEvent('Swap')!.topicHash], //topic0 filter
  fromBlock: fromBlock,
  toBlock: latestBlock,
};

//group logs on transaction hash
const groupedLogs:any = {};
const logs = await provider.getLogs(filter);
for (const log of logs) {      
  const tnxHash = log.transactionHash;     
  if (!groupedLogs[tnxHash]) {
    groupedLogs[tnxHash] = [];
  }
  groupedLogs[tnxHash].push(log);
}

Object.keys(groupedLogs).forEach(tnxHash => {
  console.log(`\nTransaction hash: ${tnxHash}`);
  for (const log of groupedLogs[tnxHash]) {
    const pairEvmAddress = log.address; //use this to get token0 and token1 data

    const parsedLog = abiInterfaces.parseLog({ topics: log.topics.slice(), data: log.data });
    const result = parsedLog!.args;

    let output = '';
    output += `Pair: ${pairEvmAddress}`;
    output += `, amountIn: ${result.amount0}`;
    output += `, amountOut: ${result.amount1}`;
    output += `, sqrtPriceX96: ${result.sqrtPriceX96}`;
    output += `, liquidity: ${result.liquidity}`;
    output += `, tick: ${result.tick}`;

    console.log(output);
  }
});