Emitteth

Emitteth is an event emitter for the Ethereum blockchain. Besides web3-supported events, it provides convenience APIs for subscribing to ERC20-events (Transfer, Approval) as well as 0x-related events (LogFill, LogCancel). For 0x, an event abstraction is built-in which collects bursts of LogFill events an aggregates those into traded events of the form { amount: X, price: Y, timestamp: Z }. Currently, it supports 0x v1.

Events

Supported events, which can be subscribed to via Node's EventEmitter interface:

• mined: Same as web3's logs.
• confirmed: Like mined but only emitted after a defined number of block confirmations passed.
• latereconfirm: A log that is part of a chain reorganization (reorg) but is old enough so that it fell out the block confirmation buffer. Example: If you configured emitteth in a way that it emits events after 2 block confirmations, a lateconfirm means that a log was recaptured at block depth n-3 or older. The higher the block confirmation number, the less frequent this event is emitted.
• lateunconfirm: Similar to latereconfirm, however, this event type states that a log was removed (log.removed==true) from the part of the chain effected by the reorg.
• transferred: An ERC20 Transfer event was emitted by an observed contract.
• approved: An ERC20 Approve event was emitted by an observed contract.
• traded: One or more 0x LogFill(s) were emitted that collectively represent a settled trade.
• cancelled: A 0x LogCancel was emitted that represents a hard-cancelled order.
• newblock: Same as web3's newBlockHeaders
• pending: Same as web3's pendingTransactions

Installation

npm install emitteth

Import

import { Emitteth } from 'emitteth'

How to register event subscriptions

To build event subscriptions, a web3 object has to be passed to Emitteth. For example, you can use Infura with a WebSocket provider:

import Web3 from 'web3'
import ProviderEngine from 'web3-provider-engine'
import WebsocketSubprovider from 'web3-provider-engine/subproviders/websocket'

const getInfuraWebsocketSubprovider = (networkAsString) => {
  const engine = new ProviderEngine()
  const subprovider = new WebsocketSubprovider({ rpcUrl: 'wss://' + networkAsString + '.infura.io/ws' })

  subprovider.on('data', (err, notification) => {
    engine.emit('data', err, notification)
  })

  engine.addProvider(subprovider)
  engine.start()
  return engine
}

const web3 = new Web3(getInfuraWebsocketSubprovider('mainnet'))
// pass `web3` to Emitteth

Now you can build Ethereum subscriptions. Some examples:

Example 1

Firstly, Emitteth allows you to setup event subscriptions in a generic way by specifying the contract address and the event topics you are interested in. withFields(...) limits the fields in the event object. You can specify a number of confirmation blocks after which the events should be emitted by using withConfirmation(...).

const emitteth = new Emitteth(web3)
  .forContract('0x12459c951127e0c374ff9105dda097662a027093')
  .withTopics(['0x0d0b9391970d9a25552f37d436d2aae2925e2bfe1b2a923754bada030c498cb3', null, null, null])
  .withFields('blockNumber', 'transactionHash')
  .withConfirmations(3)
  .start()

Example 2

You can subscribe to all traded and cancelled events without filtering for a specific traded token pair. withNewBlocks(...) subscribes additionally to newblock events and returns the fields that are passed as argument (in the snippet: field number).

const emitteth = new Emitteth(web3)
  .emitAllTrades()
  .filterFields('amount', 'price')
  .includeCancels()
  .filterFields('cancelledMakerTokenAmount', 'cancelledTakerTokenAmount', 'priceAdjustment')
  .withNewBlocks('number')
  .withConfirmations(3)
  .start()

Alternatively, you can subscribe to traded events for a specific token pair by using:

const emitteth = new Emitteth(web3)
  .emitTradesForTokens('0xe41d2489571d322189246dafa5ebde1f4699f498', '0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2')
  .start()

Example 3

You might be interested in all ERC20 Transfer event of a specific token contract. In this case, you can do:

const emitteth3 = new Emitteth(web3)
  .emitTransfersForToken('0xe41d2489571d322189246dafa5ebde1f4699f498')
  .start()

Example 4

Subscription for pending transactions (careful, flood of events):

const emitteth = new Emitteth(web3)
  .withPendingTransactions()
  .start()

How to consume subscriptions

The object emitteth, which was built in the previous section, exposes Node's EventEmitter interface. Thus, we can consume events with:

// Generic events
emitteth.on('newblock', (block) => {
  ...
})
emitteth.on('pending', (txHash) => {
  ...
})
emitteth.on('mined', (log) => {
  ...
})
// `mined` events confirmed with X blocks
emitteth.on('confirmed', (log) => {
  ...
})
emitteth.on('latereconfirm', (log) => {
  ...
})
emitteth.on('lateunconfirm', (log) => {
  ...
})
// ERC20 events
emitteth.on('transferred', (transfer) => {
  ...
})
emitteth.on('approved', (approval) => {
 ...
})
// 0x events
emitteth.on('traded', (trade) => {
  ...
})
emitteth.on('cancelled', (cancelledOrder) => {
  ...
})