How to get notified when an event triggers on ethereum smart contract

Question

When someone makes a transfer from my account, an event gets trigger name

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

Now I want to get notified when this event occurs on smart contract. I tried different things like filter, watch, subscription and etc. But nothing works as per need.

I also have another query What does filter, subscribe, and watch exactly do. I am always getting confused between these terms. Can someone give a clear idea?

Answer 1

Here's a simple example 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);

To unsubscribe:

subscription.unsubscribe(function (result) {
   console.log(result)
});

Example of usage in 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;
    });
  }
}