Watch Token Approvals

token.watchApprove

Watches TIP-20 Approval events for a token.

Usage

import {  } from './viem.config'
 
const  = ..({
  : '0x20c0000000000000000000000000000000000000',
})
 
.(() => {
  for (const  of ) .(.)
{  owner: '0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266',  spender: '0x70997970C51812dc3A010C7d01b50e0d17dc79C8',  amount: 100000000n}
})
 
// Later, tear down the watcher.
.()

import { ,  } from 'viem/tempo'
 
export const  = .({
  : .('0x...'),
})

Also callable standalone: Actions.token.watchApprove(client, options), with Actions imported from 'viem/tempo'.

Recipes

Collect Funds When a Payer Approves Your Processor

Filter args.spender to your processor account, then pull the approved amount with token.transferSync and its from parameter.

import {  } from './viem.config'
 
const  = ..({
  : { : .. },
  : '0x20c0000000000000000000000000000000000000',
})
 
.(async () => {
  for (const  of ) {
    // Collect the approved amount from the payer.
    const {  } = await ..({
      : ..,
      : ..,
      : ..,
      : '0x20c0000000000000000000000000000000000000',
    })
  }
})

import { ,  } from 'viem/tempo'
 
export const  = .({
  : .('0x...'),
})

Alert on Approvals From Treasury Accounts

Filter args.owner to accounts you control and flag any approval that names a spender outside your allowlist, an early signal of a compromised key.

import {  } from './viem.config'
 
// Spenders authorized by your treasury policy.
const  = new <string>([
  '0x742d35Cc6634C0532925a3b844Bc9e7595f0bEbb',
])
 
const  = ..({
  : { : '0x8ba1f109551bD432803012645Ac136ddd64DBA72' },
  : '0x20c0000000000000000000000000000000000000',
})
 
.(() => {
  for (const  of ) {
    if (.(..)) continue
    .('Unexpected approval:', .., ..)
Unexpected approval: 0x70997970C51812dc3A010C7d01b50e0d17dc79C8 100000000n
  }
})

import { ,  } from 'viem/tempo'
 
export const  = .({
  : .('0x...'),
})

Return Value

Returns a watcher handle. Watching starts when the first listener (or async iterator) attaches, and stops when off is called.

type ReturnType = {
  /** Registers a log listener. Starts the watcher on first registration. Returns an unregister function. */
  onLogs: (fn: (logs: readonly Log[]) => void) => () => void
  /** Registers an error listener. Returns an unregister function. */
  onError: (fn: (error: Error) => void) => () => void
  /** Tears down the watcher: removes listeners, ends iterators, stops the poll or subscription. */
  off: () => void
  /** Async-iterates emitted log batches (latest-only stream). */
  [Symbol.asyncIterator]: () => AsyncIterableIterator<{ logs: readonly Log[] }>
}

Each log is decoded, with the event arguments on args:

type Log = {
  /** Decoded `Approval` event arguments. */
  args: {
    /** Address of the token owner. */
    owner: Address
    /** Address of the approved spender. */
    spender: Address
    /** Amount approved. */
    amount: bigint
  }
  /** Name of the emitted event. */
  eventName: 'Approval'
  // ...standard log fields (address, blockHash, blockNumber, logIndex, transactionHash, ...)
}

Parameters

args

Type: object

type Args = {
  /** Filter by owner address. */
  owner?: Address | Address[] | null
  /** Filter by spender address. */
  spender?: Address | Address[] | null
}

Indexed argument values to filter logs by.

batch

Type: boolean
Default: true

Whether to batch the logs found within a poll interval into a single emission. When false, each log is emitted on its own.

fromBlock

Type: bigint

Block number from which to start watching for logs.

poll

Type: boolean

Whether to poll for new logs instead of using a subscription. Defaults to true when the transport cannot subscribe or fromBlock is provided.

pollingInterval

Type: number
Default: client.pollingInterval

Polling frequency (in ms).

token

Type: Address | bigint

Token to operate on: a TIP-20 token id or a contract address.

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