viem
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JavaScript
import { encodeEventTopics, } from '../../utils/abi/encodeEventTopics.js';
import { observe } from '../../utils/observe.js';
import { poll } from '../../utils/poll.js';
import { stringify } from '../../utils/stringify.js';
import { DecodeLogDataMismatch, DecodeLogTopicsMismatch, } from '../../errors/abi.js';
import { InvalidInputRpcError } from '../../errors/rpc.js';
import { decodeEventLog } from '../../utils/abi/decodeEventLog.js';
import { formatLog } from '../../utils/formatters/log.js';
import { getAction } from '../../utils/getAction.js';
import { createEventFilter, } from './createEventFilter.js';
import { getBlockNumber } from './getBlockNumber.js';
import { getFilterChanges } from './getFilterChanges.js';
import { getLogs } from './getLogs.js';
import { uninstallFilter } from './uninstallFilter.js';
/**
* Watches and returns emitted [Event Logs](https://viem.sh/docs/glossary/terms#event-log).
*
* - Docs: https://viem.sh/docs/actions/public/watchEvent
* - JSON-RPC Methods:
* - **RPC Provider supports `eth_newFilter`:**
* - Calls [`eth_newFilter`](https://ethereum.org/en/developers/docs/apis/json-rpc/#eth_newfilter) to create a filter (called on initialize).
* - On a polling interval, it will call [`eth_getFilterChanges`](https://ethereum.org/en/developers/docs/apis/json-rpc/#eth_getfilterchanges).
* - **RPC Provider does not support `eth_newFilter`:**
* - Calls [`eth_getLogs`](https://ethereum.org/en/developers/docs/apis/json-rpc/#eth_getlogs) for each block between the polling interval.
*
* This Action will batch up all the Event Logs found within the [`pollingInterval`](https://viem.sh/docs/actions/public/watchEvent#pollinginterval-optional), and invoke them via [`onLogs`](https://viem.sh/docs/actions/public/watchEvent#onLogs).
*
* `watchEvent` will attempt to create an [Event Filter](https://viem.sh/docs/actions/public/createEventFilter) and listen to changes to the Filter per polling interval, however, if the RPC Provider does not support Filters (e.g. `eth_newFilter`), then `watchEvent` will fall back to using [`getLogs`](https://viem.sh/docs/actions/public/getLogs) instead.
*
* @param client - Client to use
* @param parameters - {@link WatchEventParameters}
* @returns A function that can be invoked to stop watching for new Event Logs. {@link WatchEventReturnType}
*
* @example
* import { createPublicClient, http } from 'viem'
* import { mainnet } from 'viem/chains'
* import { watchEvent } from 'viem/public'
*
* const client = createPublicClient({
* chain: mainnet,
* transport: http(),
* })
* const unwatch = watchEvent(client, {
* onLogs: (logs) => console.log(logs),
* })
*/
export function watchEvent(client, { address, args, batch = true, event, events, fromBlock, onError, onLogs, poll: poll_, pollingInterval = client.pollingInterval, strict: strict_, }) {
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 strict = strict_ ?? false;
const pollEvent = () => {
const observerId = stringify([
'watchEvent',
address,
args,
batch,
client.uid,
event,
pollingInterval,
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, createEventFilter, 'createEventFilter')({
address,
args,
event: event,
events,
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, getLogs, 'getLogs')({
address,
args,
event: event,
events,
fromBlock: previousBlockNumber + 1n,
toBlock: blockNumber,
});
}
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 subscribeEvent = () => {
let active = true;
let unsubscribe = () => (active = false);
(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 events_ = events ?? (event ? [event] : undefined);
let topics = [];
if (events_) {
const encoded = events_.flatMap((event) => encodeEventTopics({
abi: [event],
eventName: event.name,
args,
}));
// TODO: Clean up type casting
topics = [encoded];
if (event)
topics = topics[0];
}
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: events_ ?? [],
data: log.data,
topics: log.topics,
strict,
});
const formatted = formatLog(log, { args, eventName });
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,
});
onLogs([formatted]);
}
},
onError(error) {
onError?.(error);
},
});
unsubscribe = unsubscribe_;
if (!active)
unsubscribe();
}
catch (err) {
onError?.(err);
}
})();
return () => unsubscribe();
};
return enablePolling ? pollEvent() : subscribeEvent();
}
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