Contract Events

Introduction

Contract events is a way for contracts to deliver event-like information to third-party (off-chain) applications. It can be emitted by using the EMIT instruction in Michelson.

For more details and examples of how the EMIT instruction works, refer to this article.

Getting contract events in Taquito

You can get the events in a contract through the eventSchema field of the ContractAbstraction.

Example

Usage

const Tezos = new TezosToolkit(RPC_URL);
const contractAbstraction = Tezos.contract.at('KT1...');
console.log(contractAbstraction.eventSchema);

If the contract has events this might log something similar to:

[
  {
    "tag": "%tag1",
    "type": {
      "prim": "int"
    }
  },
  {
    "tag": "%tag2",
    "type": {
      "prim": "int"
    }
  },
  {
    "tag": "%tag3",
    "type": {
      "prim": "string"
    }
  }
]

Subscribing to Events in Taquito

Taquito provides a simple way for users to subscribe to certain events on the blockchain via the PollingSubscribeProvider.

Example

Usage

const Tezos = new TezosToolkit(RPC_URL);

Tezos.setStreamProvider(
  Tezos.getFactory(PollingSubscribeProvider)({
    shouldObservableSubscriptionRetry: true,
    pollingIntervalMilliseconds: 1500
  })
);

try {
  const sub = Tezos.stream.subscribeEvent({
    tag: 'tagName',
    address: 'KT1_CONTRACT_ADDRESS',
    excludeFailedOperations: true
  });

  sub.on('data', console.log);

} catch (e) {
  console.log(e);
}

• tag is the tag string that was defined in the smart contract with the EMIT instruction
• address is the address of the smart contract that was called
• excludeFailedOperations: In rare cases, events from failed operations can be received by the subscriber. You can use this field to filter out these events (if you pass true to this field, only events from successful operations will be received)

info

If you would like to subscribe to any event that goes through, you can call subscribeEvent() as is without any parameters

Output

The output of the subscribeEvent method will look something like this:

{
  "opHash": "oopRTC5iNxssoC5dAz54u7uthUz6xBayaSPcLXhkLwHGjuS7Bos",
  "blockHash": "BLCTEDjZDtuUcYxmSPXHn3XrKruub4NF4mzTgR2EbpPRFN7JzDV",
  "level": 313647,
  "kind": "event",
  "source": "KT1ACmSCoRsA69zHnv5mMBC4vdcxbFJpHRoo",
  "nonce": 0,
  "type": {
    "prim": "or",
    "args": [
      {
        "prim": "nat"
      },
      {
        "prim": "string"
      }
    ]
  },
  "tag": "first",
  "payload": {
    "prim": "Left",
    "args": [
      {
        "int": "10"
      }
    ]
  },
  "result": {
    "status": "applied",
    "consumed_milligas": "1000000"
  }
}