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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
Oracle: Call any API from a smart contract
Data Feed: Stream a data feed on-chain at regular intervals
Last modified 8mo ago