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

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 with difference things like filter, watch, subscription and etc. But nothing works as per need.
I also have an another query
What does filter, subscribe, and watch exactly do. I am always getting confuse between these terms. Can someone give a clear idea.
Note: I am using WEB3JS 1.0.0.26 version.

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;
});
}
}