Listener: Subscribe an internal service to any event

Overview

A Listener is a midpoint that listen for an event from a contract and makes an HTTP request based on the values from the event values.

As an example of a Listener, we will define a midpoint that sends Slack notification when a user borrows a token on Aave V2.

EVMEventEmittedSource:

1. Listen for the deposit event to be emitted

2. Extract the sender, recipient address and amount to be send to the MakeHttpRequestTask

MakeHttpRequestTask:

1. Notify the user via Slack using a Slack webhook

Midpoint

$ npm install -g midpoint-cli

$ midpoint init blank my-listener

1 . EVMEventEmittedSource

$ midpoint add-source evmEventEmittedSource listener-source

We define the EVM Event Emitted source so that Midpoint can listen for the borrow event.

{
    "evmEventEmittedSource": {
      "chainId": "1",
      "contractAddress": "0x7d2768dE32b0b80b7a3454c06BdAc94A69DDc7A9",
      "eventName": "Borrow",
      "arguments": [
        {
          "name": "reserve",
          "datatype": "address",
          "indexed": true
        },
        {
          "name": "user",
          "datatype": "address",
          "indexed": true
        },
        {
          "name": "onBehalfOf",
          "datatype": "address",
          "indexed": false
        },
        {
          "name": "amount",
          "datatype": "uint256",
          "indexed": false
        },
        {
          "name": "borrowRateMode",
          "datatype": "uint256",
          "indexed": false
        },
        {
          "name": "borrowRate",
          "datatype": "uint256",
          "indexed": false
        },
        {
          "name": "referral",
          "datatype": "uint16",
          "indexed": true
        }
      ]
    }
}

2. CallEvmFunction Task

$ midpoint add-task callEvmFunction get-erc20-name

We define the Call EVM Function task to retrieve the ERC20 name of the token that was borrowed.

{
  "callEvmFunction": {
      "chainId": "1",
      "contractAddress": "{{reserve}}",
      "functionName": "name",
      "arguments": [],
      "returnValues": [
        {
          "name": "erc20-name",
          "datatype": "string"
        }
      ]
    }
}

3. MakeHttpRequest Task

$ midpoint add-task makeHttpRequest send-to-slack

We define the Make HTTP Request task to notify users via Slack message using the Slack Webhook.

To add WEB_HOOK_URL as a secret:

$ midpoint add-secret <slack webhook url> WEB_HOOK_URL

{
    "makeHttpRequest": {
        "urlRaw": "https://hooks.slack.com/services/[[WEB_HOOK_URL]]",
        "method": "POST",
        "urlType": "raw",
        "bodyType": "json",
        "body": "{\"text\": \"The deposit function was called by {{user}} for {{amount}} on the Apecoin contract\"}"
    }
}

4. Define Path

We define our path file.

listener-source => get-erc20-name
get-erc20-name => send-to-slack

5. Publish Midpoint

$ midpoint publish