viem
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JavaScript
import { DecodeLogDataMismatch, DecodeLogTopicsMismatch, } from '../../errors/abi.js';
import { InvalidInputRpcError } from '../../errors/rpc.js';
import { decodeEventLog } from '../../utils/abi/decodeEventLog.js';
import { encodeEventTopics, } from '../../utils/abi/encodeEventTopics.js';
import { formatLog } from '../../utils/formatters/log.js';
import { getAction } from '../../utils/getAction.js';
import { observe } from '../../utils/observe.js';
import { poll } from '../../utils/poll.js';
import { stringify } from '../../utils/stringify.js';
import { createContractEventFilter } from './createContractEventFilter.js';
import { getBlockNumber } from './getBlockNumber.js';
import { getContractEvents, } from './getContractEvents.js';
import { getFilterChanges } from './getFilterChanges.js';
import { uninstallFilter } from './uninstallFilter.js';
/**
* Watches and returns emitted contract event logs.
*
* - Docs: https://viem.sh/docs/contract/watchContractEvent
*
* This Action will batch up all the event logs found within the [`pollingInterval`](https://viem.sh/docs/contract/watchContractEvent#pollinginterval-optional), and invoke them via [`onLogs`](https://viem.sh/docs/contract/watchContractEvent#onLogs).
*
* `watchContractEvent` will attempt to create an [Event Filter](https://viem.sh/docs/contract/createContractEventFilter) and listen to changes to the Filter per polling interval, however, if the RPC Provider does not support Filters (e.g. `eth_newFilter`), then `watchContractEvent` will fall back to using [`getLogs`](https://viem.sh/docs/actions/public/getLogs) instead.
*
* @param client - Client to use
* @param parameters - {@link WatchContractEventParameters}
* @returns A function that can be invoked to stop watching for new event logs. {@link WatchContractEventReturnType}
*
* @example
* import { createPublicClient, http, parseAbi } from 'viem'
* import { mainnet } from 'viem/chains'
* import { watchContractEvent } from 'viem/contract'
*
* const client = createPublicClient({
* chain: mainnet,
* transport: http(),
* })
* const unwatch = watchContractEvent(client, {
* address: '0xFBA3912Ca04dd458c843e2EE08967fC04f3579c2',
* abi: parseAbi(['event Transfer(address indexed from, address indexed to, uint256 value)']),
* eventName: 'Transfer',
* args: { from: '0xc961145a54C96E3aE9bAA048c4F4D6b04C13916b' },
* onLogs: (logs) => console.log(logs),
* })
*/
export function watchContractEvent(client, parameters) {
const { abi, address, args, batch = true, eventName, fromBlock, onError, onLogs, poll: poll_, pollingInterval = client.pollingInterval, strict: strict_, } = parameters;
const enablePolling = (() => {
if (typeof poll_ !== 'undefined')
return poll_;
if (typeof fromBlock === 'bigint')
return true;
if (client.transport.type === 'webSocket')
return false;
if (client.transport.type === 'fallback' &&
client.transport.transports[0].config.type === 'webSocket')
return false;
return true;
})();
const pollContractEvent = () => {
const strict = strict_ ?? false;
const observerId = stringify([
'watchContractEvent',
address,
args,
batch,
client.uid,
eventName,
pollingInterval,
strict,
fromBlock,
]);
return observe(observerId, { onLogs, onError }, (emit) => {
let previousBlockNumber;
if (fromBlock !== undefined)
previousBlockNumber = fromBlock - 1n;
let filter;
let initialized = false;
const unwatch = poll(async () => {
if (!initialized) {
try {
filter = (await getAction(client, createContractEventFilter, 'createContractEventFilter')({
abi,
address,
args: args,
eventName: eventName,
strict: strict,
fromBlock,
}));
}
catch { }
initialized = true;
return;
}
try {
let logs;
if (filter) {
logs = await getAction(client, getFilterChanges, 'getFilterChanges')({ filter });
}
else {
// If the filter doesn't exist, we will fall back to use `getLogs`.
// The fall back exists because some RPC Providers do not support filters.
// Fetch the block number to use for `getLogs`.
const blockNumber = await getAction(client, getBlockNumber, 'getBlockNumber')({});
// If the block number has changed, we will need to fetch the logs.
// If the block number doesn't exist, we are yet to reach the first poll interval,
// so do not emit any logs.
if (previousBlockNumber && previousBlockNumber < blockNumber) {
logs = await getAction(client, getContractEvents, 'getContractEvents')({
abi,
address,
args,
eventName,
fromBlock: previousBlockNumber + 1n,
toBlock: blockNumber,
strict,
});
}
else {
logs = [];
}
previousBlockNumber = blockNumber;
}
if (logs.length === 0)
return;
if (batch)
emit.onLogs(logs);
else
for (const log of logs)
emit.onLogs([log]);
}
catch (err) {
// If a filter has been set and gets uninstalled, providers will throw an InvalidInput error.
// Reinitialize the filter when this occurs
if (filter && err instanceof InvalidInputRpcError)
initialized = false;
emit.onError?.(err);
}
}, {
emitOnBegin: true,
interval: pollingInterval,
});
return async () => {
if (filter)
await getAction(client, uninstallFilter, 'uninstallFilter')({ filter });
unwatch();
};
});
};
const subscribeContractEvent = () => {
const strict = strict_ ?? false;
const observerId = stringify([
'watchContractEvent',
address,
args,
batch,
client.uid,
eventName,
pollingInterval,
strict,
]);
let active = true;
let unsubscribe = () => (active = false);
return observe(observerId, { onLogs, onError }, (emit) => {
;
(async () => {
try {
const transport = (() => {
if (client.transport.type === 'fallback') {
const transport = client.transport.transports.find((transport) => transport.config.type === 'webSocket');
if (!transport)
return client.transport;
return transport.value;
}
return client.transport;
})();
const topics = eventName
? encodeEventTopics({
abi: abi,
eventName: eventName,
args,
})
: [];
const { unsubscribe: unsubscribe_ } = await transport.subscribe({
params: ['logs', { address, topics }],
onData(data) {
if (!active)
return;
const log = data.result;
try {
const { eventName, args } = decodeEventLog({
abi: abi,
data: log.data,
topics: log.topics,
strict: strict_,
});
const formatted = formatLog(log, {
args,
eventName: eventName,
});
emit.onLogs([formatted]);
}
catch (err) {
let eventName;
let isUnnamed;
if (err instanceof DecodeLogDataMismatch ||
err instanceof DecodeLogTopicsMismatch) {
// If strict mode is on, and log data/topics do not match event definition, skip.
if (strict_)
return;
eventName = err.abiItem.name;
isUnnamed = err.abiItem.inputs?.some((x) => !('name' in x && x.name));
}
// Set args to empty if there is an error decoding (e.g. indexed/non-indexed params mismatch).
const formatted = formatLog(log, {
args: isUnnamed ? [] : {},
eventName,
});
emit.onLogs([formatted]);
}
},
onError(error) {
emit.onError?.(error);
},
});
unsubscribe = unsubscribe_;
if (!active)
unsubscribe();
}
catch (err) {
onError?.(err);
}
})();
return () => unsubscribe();
});
};
return enablePolling ? pollContractEvent() : subscribeContractEvent();
}
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