Events

PreviousListen to Changes NextVET Transfers

Last updated 23 days ago

Was this helpful?

Events

Example Data

This example used below will utilize the VTHO contract, which manages VeChain's VTHO Token.

Smart Contract Address: 0x0000000000000000000000000000456e65726779
The contract's source code can be found on GitHub at:
Its Application Binary Interface (ABI) is shared on b32, a repository that gathers publicly available interfaces for VeChain projects:

Connection

The connection is managed using WebSockets, which connect directly to a VeChain node.

A simple connection can be established with this snippet:

import WebSocket from 'ws';
const ws = new WebSocket('wss://mainnet.vechain.org/subscriptions/event');
ws.onmessage = (message) => {
    console.log('New event', message.data);
}

This will receive all events on the blockchain as JSON-encoded strings.

Filter Specific Events

Using the subscriptions helper, a custom subscription URL can be built that listens only to the specified event:

const wsUrl = subscriptions.getEventSubscriptionUrl(
  'https://mainnet.vechain.org',
  "event Transfer(address indexed from, address indexed to, uint256 value)"
);

Event Parameters

Filters can be adjusted to only show events that meet specific criteria. These parameters must be indexed and are given as a list.

The transfer event uses two indexed parameters: from and to.

For instance, to filter all transfers originating from a specific address, you would supply that address as the first parameter:

const wsUrl = subscriptions.getEventSubscriptionUrl(
  'https://mainnet.vechain.org',
  "event Transfer(address indexed from, address indexed to, uint256 value)",
  ['0x0000000000000000000000000000456e65726779']
);

To filter for all transfers to a specific address, provide only the second value in the list:

const wsUrl = subscriptions.getEventSubscriptionUrl(
  'https://mainnet.vechain.org',
  "event Transfer(address indexed from, address indexed to, uint256 value)",
  [null, '0x0000000000000000000000000000456e65726779']
);

When using filters, all provided values must match for the filters to apply.

Blockchain Parameters

Emitter

Many contracts emit similar events, which sometimes necessitates restricting listening to a specific contract. An optional options argument can be used to filter for a particular contract address.

For example, to listen exclusively for VTHO transfers from the VTHO contract at 0x0000000000000000000000000000456e65726779, it would be defined as follows:

const wsUrl = subscriptions.getEventSubscriptionUrl(
  'https://mainnet.vechain.org',
  "event Transfer(address indexed from, address indexed to, uint256 value)",
  [],
  { address: '0x0000000000000000000000000000456e65726779' }
);

Blocks

To resume listening from a specific block position, the options can define a blockID to continue from where a previous listener may have disconnected.

Decoding Events

The events are received as JSON-encoded strings. These strings must be parsed and decoded into usable objects.

Using the contract interface defined earlier, you can decode these events with the parseLog() function:

import WebSocket from "ws";
import { ABIContract } from "@vechain/sdk-core";
import { subscriptions } from "@vechain/sdk-network";

const ws = new WebSocket(
  subscriptions.getEventSubscriptionUrl(
    "https://mainnet.vechain.org",
    "event Transfer(address indexed from, address indexed to, uint256 value)",
    [],
    {
        address: "0x0000000000000000000000000000456e65726779"
    }
  )
);

ws.onmessage = (message) => {
    console.log(message.data);
  const eventData = JSON.parse(message.data as any);

  // Ensure topics is an array of strings
  const topics = Array.isArray(eventData.topics)
    ? eventData.topics.map(String)
    : [String(eventData.topics)];

  // Ensure data is a single hex string
  const data = Array.isArray(eventData.data)
    ? eventData.data.map(String).join("")
    : String(eventData.data) || "0x";

  const abiContract = new ABIContract([
    {
      type: "event",
      name: "Transfer",
      inputs: [
        { type: "address", name: "from", indexed: true },
        { type: "address", name: "to", indexed: true },
        { type: "uint256", name: "value", indexed: false },
      ],
    },
  ]);

  const decoded = abiContract.parseLog<"Transfer">(data, topics);

  if (!decoded || decoded.eventName !== "Transfer") {
    throw new Error("Transfer event not detected");
  }
};

Example Project