An example script for watching for events emitted by a Celo contract via Forno

cUSD_abi.json

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datePeriod"},{"type":"address[]","name":"initialBalanceAddresses"},{"type":"uint256[]","name":"initialBalanceValues"}],"constant":false},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"numberValidatorsInCurrentSet","inputs":[],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"getBlockNumberFromHeader","inputs":[{"type":"bytes","name":"header"}],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"address","name":""}],"name":"owner","inputs":[],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"bool","name":""}],"name":"isOwner","inputs":[],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"string","name":""}],"name":"symbol","inputs":[],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"getEpochNumber","inputs":[],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"numberValidatorsInSet","inputs":[{"type":"uint256","name":"blockNumber"}],"constant":true},{"type":"function","stateMutability":"nonpayable","payable":false,"outputs":[{"type":"bool","name":""}],"name":"decreaseAllowance","inputs":[{"type":"address","name":"spender"},{"type":"uint256","name":"value"}],"constant":false},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""},{"type":"uint256","name":""},{"type":"uint256","name":""},{"type":"uint256","name":""}],"name":"getInflationParameters","inputs":[],"constant":true},{"type":"function","stateMutability":"nonpayable","payable":false,"outputs":[{"type":"bool","name":""}],"name":"transfer","inputs":[{"type":"address","name":"to"},{"type":"uint256","name":"value"}],"constant":false},{"type":"function","stateMutability":"nonpayable","payable":false,"outputs":[],"name":"setRegistry","inputs":[{"type":"address","name":"registryAddress"}],"constant":false},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"unitsToValue","inputs":[{"type":"uint256","name":"units"}],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"allowance","inputs":[{"type":"address","name":"accountOwner"},{"type":"address","name":"spender"}],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"getEpochSize","inputs":[],"constant":true},{"type":"function","stateMutability":"nonpayable","payable":false,"outputs":[{"type":"bool","name":""}],"name":"transferWithComment","inputs":[{"type":"address","name":"to"},{"type":"uint256","name":"value"},{"type":"string","name":"comment"}],"constant":false},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""}],"name":"minQuorumSize","inputs":[{"type":"uint256","name":"blockNumber"}],"constant":true},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"uint256","name":""},{"type":"uint256","name":""}],"name":"fractionMulExp","inputs":[{"type":"uint256","name":"aNumerator"},{"type":"uint256","name":"aDenominator"},{"type":"uint256","name":"bNumerator"},{"type":"uint256","name":"bDenominator"},{"type":"uint256","name":"exponent"},{"type":"uint256","name":"_decimals"}],"constant":true},{"type":"function","stateMutability":"nonpayable","payable":false,"outputs":[],"name":"transferOwnership","inputs":[{"type":"address","name":"newOwner"}],"constant":false},{"type":"function","stateMutability":"view","payable":false,"outputs":[{"type":"bytes32","name":""}],"name":"getParentSealBitmap","inputs":[{"type":"uint256","name":"blockNumber"}],"constant":true},{"type":"event","name":"InflationFactorUpdated","inputs":[{"type":"uint256","name":"factor","indexed":false},{"type":"uint256","name":"lastUpdated","indexed":false}],"anonymous":false},{"type":"event","name":"InflationParametersUpdated","inputs":[{"type":"uint256","name":"rate","indexed":false},{"type":"uint256","name":"updatePeriod","indexed":false},{"type":"uint256","name":"lastUpdated","indexed":false}],"anonymous":false},{"type":"event","name":"Transfer","inputs":[{"type":"address","name":"from","indexed":true},{"type":"address","name":"to","indexed":true},{"type":"uint256","name":"value","indexed":false}],"anonymous":false},{"type":"event","name":"TransferComment","inputs":[{"type":"string","name":"comment","indexed":false}],"anonymous":false},{"type":"event","name":"Approval","inputs":[{"type":"address","name":"owner","indexed":true},{"type":"address","name":"spender","indexed":true},{"type":"uint256","name":"value","indexed":false}],"anonymous":false},{"type":"event","name":"RegistrySet","inputs":[{"type":"address","name":"registryAddress","indexed":true}],"anonymous":false},{"type":"event","name":"OwnershipTransferred","inputs":[{"type":"address","name":"previousOwner","indexed":true},{"type":"address","name":"newOwner","indexed":true}],"anonymous":false}]

eventWatcher.ts

// This script will watch for events emitted by contracts specified in the `watchedAddresses` array.
// Adding to the `topics` array will filter events 
// More info on topics can be found here: https://solidity.readthedocs.io/en/develop/abi-spec.html#events

// At the time of writing, Forno websocket connections are disconnected after 20 minutes
// When the websocket connection is broken, the script will stop listening for 500ms before attempting to reconnect
// If a block is created during that gap, the relevant events in that block will be missed. Running two or more listeners concurrently will reduce the chance of missed blocks 

const Web3 = require('web3')
const cUSD_abi = require('cUSD_abi.json')

let watchedAddresses: Array<string> = ["0x765DE816845861e75A25fCA122bb6898B8B1282a"] // cUSD contract
let topics: Array<null|string> = [] 
let lastSeenBlock: null|Number = null

function setupProviderAndSubscriptions() {
    
    let provider = new Web3.providers.WebsocketProvider('wss://forno.celo.org/ws')
    let web3 = new Web3(provider)
    let setupNewProvider = false
    
    // Keeps track of the number of times we've retried to set up a new provider
    // and subs without a successful header
    let sequentialRetryCount = 0
    
    const setupNewProviderAndSubs = async () => {
        // To prevent us from retrying too aggressively, wait a little if
        // we try setting up multiple times in a row
        const sleepTimeMs = sequentialRetryCount * 100
        console.log('sleeping', sleepTimeMs)
        await sleep(sleepTimeMs)
        sequentialRetryCount++
        // To avoid a situation where multiple error events are triggered
        if (!setupNewProvider) {
            setupNewProvider = true
            setupProviderAndSubscriptions()
        }
    }
    
    provider.on('error', async (error: any) => {
        console.log('WebsocketProvider encountered an error', error)
        await setupNewProviderAndSubs()
    })
    
    provider.on('end', async () => {
        console.log('WebsocketProvider has ended, will restart')
        await setupNewProviderAndSubs()
    })
    
    let headerSubscription = web3.eth.subscribe('newBlockHeaders')
    
    headerSubscription.on('data', function(blockHeader: any) {
        if (sequentialRetryCount > 0) {
          sequentialRetryCount = 0
        }
        lastSeenBlock = blockHeader.number
    })
    
    let eventSubscription = web3.eth.subscribe('logs', {
        address: watchedAddresses,
        fromBlock: lastSeenBlock,
        topics: topics
    })
    
    eventSubscription.on('data', function (data: any) {
        console.log(data)
    })
    
    eventSubscription.on('error', async (error: any) => {
        console.log('Block header subscription encountered an error', error)
        await setupNewProviderAndSubs()
    })
}

setupProviderAndSubscriptions()
    function sleep(ms: number, onSleep?: () => void): Promise<void> {
        return new Promise((resolve) => {
            setTimeout(resolve, ms)
            if (onSleep) {
                onSleep()
            }
     })
}

compile typescript and run with $ node eventWatcher.js