@metamask/eth-block-tracker

@metamask/eth-block-tracker

This module walks the Ethereum blockchain, keeping track of the latest block. It uses a web3 provider as a data source and will continuously poll for the next block.

Installation

yarn add @metamask/eth-block-tracker

or

npm install @metamask/eth-block-tracker

Usage

const createInfuraProvider = require('@metamask/eth-json-rpc-infura');
const { PollingBlockTracker } = require('@metamask/eth-block-tracker');

const provider = createInfuraProvider({
  network: 'mainnet',
  projectId: process.env.INFURA_PROJECT_ID,
});
const blockTracker = new PollingBlockTracker({ provider });

blockTracker.on('sync', ({ newBlock, oldBlock }) => {
  if (oldBlock) {
    console.log(`sync #${Number(oldBlock)} -> #${Number(newBlock)}`);
  } else {
    console.log(`first sync #${Number(newBlock)}`);
  }
});

API

Methods

new PollingBlockTracker({ provider, pollingInterval, retryTimeout, keepEventLoopActive, usePastBlocks })

• Creates a new block tracker with provider as a data source and pollingInterval (ms) timeout between polling for the latest block.
• If an error is encountered when fetching blocks, it will wait retryTimeout (ms) before attempting again.
• If keepEventLoopActive is false, in Node.js it will unref the polling timeout, allowing the process to exit during the polling interval. Defaults to true, meaning the process will be kept alive.
• If usePastBlocks is true, block numbers less than the current block number can used and emitted. Defaults to false, meaning that only block numbers greater than the current block number will be used and emitted.

getCurrentBlock()

Synchronously returns the current block. May be null.

console.log(blockTracker.getCurrentBlock());

async getLatestBlock()

Asynchronously returns the latest block. if not immediately available, it will fetch one.

async checkForLatestBlock()

Tells the block tracker to ask for a new block immediately, in addition to its normal polling interval. Useful if you received a hint of a new block (e.g. via tx.blockNumber from getTransactionByHash). Will resolve to the new latest block when done polling.

Events

latest

The latest event is emitted for whenever a new latest block is detected. This may mean skipping blocks if there were two created since the last polling period.

blockTracker.on('latest', (newBlock) => console.log(newBlock));

sync

The sync event is emitted the same as "latest" but includes the previous block.

blockTracker.on('sync', ({ newBlock, oldBlock }) =>
  console.log(newBlock, oldBlock),
);

error

The error event means an error occurred while polling for the latest block.

blockTracker.on('error', (err) => console.error(err));

Contributing

This package is part of a monorepo. Instructions for contributing can be found in the monorepo README.