Web3 event filters

Overview

The Web3.js library allows developers to easily listen for events from the EVM contained on zeknd DAppChain. Also, is possible to create filters for indexed values.

#Filtering

Let's create a filter to get the latest block created on zeknd DAppChain and continuously print the block hash on the console:

const {
  Client, CryptoUtils, zekndProvider
} = require('zeknd-js')
const Web3 = require('web3')

// Create the client
const client = new Client(
  'default',
  'ws://127.0.0.1:46658/websocket',
  'ws://127.0.0.1:46658/queryws',
);

// Create private key for first account
const privateKey = CryptoUtils.generatePrivateKey()

// Instantiate web3 client using zekndProvider as provider
const web3 = new Web3(new zekndProvider(client, privateKey));

// Create filter to get the latest block
const filter = web3.eth.filter('latest');

// Watch filter will return the hash of the latest block continuously
filter.watch(function (error, result) {
  if (error) {
    console.error(error)
  } else {
    console.log('Block hash', result)

#Filtering by indexed values

Another great feature is filtering by indexed values, which can be used to trigger event handlers when a specific indexed value is emitted.

For the following contract:

pragma solidity ^0.4.22;

contract SimpleStore {
  uint value;

  constructor() {
      value = 10;
  }

  event NewValueSet(uint indexed _value);

  function set(uint _value) public {
    value = _value;
    emit NewValueSet(value);
  }
}

It's possible to set up an event handler for the NewValueSet event that's only triggered when the value emitted is 10, and it won't be triggered if the contract emits any other value.

// Generate public and private keys
const privateKey = CryptoUtils.generatePrivateKey()
const publicKey = CryptoUtils.publicKeyFromPrivateKey(privateKey)

// Create the client
const client = new Client(
  'default',
  'ws://127.0.0.1:46658/websocket',
  'ws://127.0.0.1:46658/queryws',
)

// The address for the caller of the function
const from = LocalAddress.fromPublicKey(publicKey).toString()

// Instantiate web3 client using zekndProvider
const web3 = new Web3(new zekndProvider(client, privateKey))

// ABI for the contract
const ABI = [
  {
    constant: false,
    inputs: [{ name: '_value', type: 'uint256' }],
    name: 'set',
    outputs: [],
    payable: false,
    stateMutability: 'nonpayable',
    type: 'function'
  },
  { inputs: [], payable: false, stateMutability: 'nonpayable', type: 'constructor' },
  {
    anonymous: false,
    inputs: [{ indexed: true, name: '_value', type: 'uint256' }],
    name: 'NewValueSet',
    type: 'event'
  }
]

// Address of the contract deployed
const contractAddress = '0x...'

// Instantiate the contract and let it ready to be used
const contract = new web3.eth.Contract(ABI, contractAddress, {from})

// Subscribe for listen the event NewValueSet
contract.events.NewValueSet({ filter: { _value: 10 } }, (err: Error, event: any) => {
  if (err) t.error(err)
  else {
    // Print on terminal only when value set is equal to 10
    console.log('The value set is 10')
  }
})