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Contract Events

The Sophia language also provides you the possibility to emit Events in your functions. On this page you will learn how to access and decode the event log of a specific transaction.

EventEmitter contract

This example contract that emits events will be used in the following examples:

contract EventEmitter =

    datatype event =
        FirstEvent(int)
        | AnotherEvent(indexed address, string)

    entrypoint emitEvents(value: int, msg: string) =
        Chain.event(FirstEvent(value))
        Chain.event(AnotherEvent(Call.caller, msg))

When initializing a contract instance using the source code and providing the ACI or obtaining it via http compiler (default) you will be able to access the emitEvents entrypoint of the Sophia contract above as follows: 

// events emitted by contract calls are automatically decoded
const tx = await contractInstance.methods.emitEvents(1337, "this message is not indexed")
console.log(tx.decodedEvents)

/*
[
  {
    address: 'ct_6y3N9KqQb74QsvR9NrESyhWeLNiA9aJgJ7ua8CvsTuGot6uzh',
    data: 'cb_dGhpcyBtZXNzYWdlIGlzIG5vdCBpbmRleGVkdWmUpw==',
    topics: [
      '101640830366340000167918459210098337687948756568954742276612796897811614700269',
      '39519965516565108473327470053407124751867067078530473195651550649472681599133'
    ],
    name: 'AnotherEvent',
    decoded: [
      'fUq2NesPXcYZ1CcqBcGC3StpdnQw3iVxMA3YSeCNAwfN4myQk',
      'this message is not indexed'
    ]
  },
  {
    address: 'ct_6y3N9KqQb74QsvR9NrESyhWeLNiA9aJgJ7ua8CvsTuGot6uzh',
    data: 'cb_Xfbg4g==',
    topics: [
      '59505622142252318624300825714684802559980671551955787864303522023309554554980',
      1337
    ],
    name: 'FirstEvent',
    decoded: [ '1337' ]
  }
]
*/

Note:

  • As you can see the event log will be automatically decoded in case you perform a ContractCallTx directly

Of course it is also possible to decode the event log if you request the transaction details from the node for a transaction that has been mined already. You can request the transaction details by providing the tx-hash and then decode the event log using the contractInstance as follows: 

const txHash = 'th_2YV3AmAz2kXdTnQxXtR2uxQi3KuLS9wfvXyqKkQQ2Y6dE6RnET';
// client is an instance of the Universal Stamp
const tx = await client.tx(txHash)

// decode events using contract instance
const decodedUsingInstance = contractInstance.decodeEvents('emitEvents', tx.log)

// OR decode of events using contract instance ACI methods
const decodedUsingInstanceMethods = contractInstance.methods.emitEvents.decodeEvents(tx.log)
console.log(decodedUsingInstanceMethods || decodedUsingInstance)

/*
[
  {
    address: 'ct_fKhQBiNQkDfoZcVF1ZzPzY7Lig6FnHDCLyFYBY33ZjfzGYPps',
    data: 'cb_dGhpcyBtZXNzYWdlIGlzIG5vdCBpbmRleGVkdWmUpw==',
    topics: [
      '101640830366340000167918459210098337687948756568954742276612796897811614700269',
      '39519965516565108473327470053407124751867067078530473195651550649472681599133'
    ],
    name: 'AnotherEvent',
    decoded: [
      'fUq2NesPXcYZ1CcqBcGC3StpdnQw3iVxMA3YSeCNAwfN4myQk',
      'this message is not indexed'
    ]
  },
  {
    address: 'ct_fKhQBiNQkDfoZcVF1ZzPzY7Lig6FnHDCLyFYBY33ZjfzGYPps',
    data: 'cb_Xfbg4g==',
    topics: [
      '59505622142252318624300825714684802559980671551955787864303522023309554554980',
      1337
    ],
    name: 'FirstEvent',
    decoded: [ '1337' ]
  }
]
*/

Decode events without ACI (low-level)

As an alternative you can make use of the low-level API which allows you to decode events for a given transaction by providing the log of the transaction as well as the correct schema of the events to the decodeEvents function manually:

import { decodeEvents, SOPHIA_TYPES } from '@aeternity/aepp-sdk/es/contract/aci/transformation'

const txHash = 'th_2tMWziKAQR1CwK2PMfvMhKZgEVLmcxsPYkRXey97s9SdXj4zat'
// client is an instance of the Universal Stamp
const tx = await client.tx(txHash)

const eventsSchema = [
  { name: 'FirstEvent', types: [SOPHIA_TYPES.int] },
  { name: 'AnotherEvent', types: [SOPHIA_TYPES.address, SOPHIA_TYPES.string] },
]

const decodedEvents = decodeEvents(tx.log, { schema: eventsSchema })
console.log(decodedEvents)
/*
[
  {
    address: 'ct_2dE7Xd7XCg3cwpKWP18VPDwfhz5Miji9FoKMTZN7TYvGt64Kc',
    data: 'cb_dGhpcyBtZXNzYWdlIGlzIG5vdCBpbmRleGVkdWmUpw==',
    topics: [
      '101640830366340000167918459210098337687948756568954742276612796897811614700269',
      '39519965516565108473327470053407124751867067078530473195651550649472681599133'
    ],
    name: 'AnotherEvent',
    decoded: [
      'fUq2NesPXcYZ1CcqBcGC3StpdnQw3iVxMA3YSeCNAwfN4myQk',
      'this message is not indexed'
    ]
  },
  {
    address: 'ct_2dE7Xd7XCg3cwpKWP18VPDwfhz5Miji9FoKMTZN7TYvGt64Kc',
    data: 'cb_Xfbg4g==',
    topics: [
      '59505622142252318624300825714684802559980671551955787864303522023309554554980',
      1337
    ],
    decoded: []
  }
]
*/