Ethereum smart contract event trigger notification

Question

I am using a Ethereum smart contract which contains a trigger mechanism that is triggered every time a transaction is made. What I want is to get notified whenever the event is triggered. How can I implement a notification mechanism?

The event code is as follows:

event Transfer(address indexed from, address indexed to, uint to)

Answer 1

I have an example here for web3js 1.0.0.beta:

function handler (event) {

   console.log(event.returnValues);

}

function errorCallback (err) {

   console.error(err);

}

let subscription = contractObj.events.TestEvent().subscription;

subscription.on('data', handler).on('error', errorCallback);

/////// code to unsubscribe

subscription.unsubscribe(function (result) {

   console.log(result)

});

///// code for usage class

class Listener {

  constructor(event, handler) {

    this.subscription = event;

    this.subscription.on('data', handler).on('error', this.errorCallback);

  }

  errorCallback(err) {

    console.log(err);

  }

}

class Test {

  constructor(contractObj) {

    this.contractObj = contractObj;

    this.createListener();

  }\

  createListener() {

    let self = this;

    this.listener = new Listener(this.contractObj.events.TestEvent(), function (event) {

      self.returnValues = event.returnValues;

    });

  }

}