Many objects in a Node
emit events for example, a net.Server emits an event each time a peer connects
to it, an fs.readStream emits an event when the file is opened. All objects
which emit events are the instances of events.EventEmitter.
The event emitter allows us
to decouple the producers and consumers of events using a standard interface.
EventEmitter class lies in the events module. EventEmitter provides
multiple properties like on and emit. on property
is used to bind a function with the event and emit is used to
fire an event.
var http = require("http");
var events = require('events');
var eventEmitter = new events.EventEmitter();
var serviceDownAlert = function(){
console.log('The Ping Client -->
xxx.xxx.xxx.xxx is not responding..');
}
//attach listener
eventEmitter.on('serviceDown', serviceDownAlert);
function pingClient(){
var _host="35.185.129.118",
_path="/isalive.html",_port="5432";
httpGet(_host,_port,_path,function (err,res){
console.log("Pinging..");
console.log(res);
if(res.statusCode.toString() != "200"){
eventEmitter.emit('serviceDown');
}
});
}
function httpGet(host,port,path,callback){
var options = {
"method": "GET",
"hostname": host,
"port": port,
"path": path,
"headers": {
"cache-control": "no-cache"
}
};
var req = http.request(options, function (res) {
var chunks = [];
var result =
res.on("data", function (chunk) {
chunks.push(chunk);
});
res.on("end", function () {
var body = Buffer.concat(chunks);
result = {"response":body.toString(),"statusCode":res.statusCode};
callback(null,result);
});
res.on('error', function (err) {
result = {"response":null,"statusCode":res.statusCode};
callback(err, result);
});
}).on('error', function(err) {
result = {"response":null,"statusCode":"-1"};
callback(err, result);
});
req.end();
}
//start ping client
pingClient();
Here a Ping-Client hits a server
URL and expects HTTP 200 status code in response, if it doesn’t receives 200 http
status code it emits “serviceDown”
event.
“serviceDownAlert” function is registered as event listener, after “serviceDown” event is emitted listeners
get called.
We can attach multiple
event listener functions to a single event even a anonymous function can also
be attached as listener function to an event.
Anonymous function as Event Listener:
eventEmitter.on('serviceDown', function()
{
console.log('Anonymous Fuction as Event
Listener:: The Ping Client --> xxx.xxx.xxx.xxx is not responding..');
}
Methods of EventEmitter class:
1.
emitter.addListener(event, listener):Adds a listener to the end of the listeners array for
the specified event. No checks are made to see if the listener has already been
added.
2.
emitter.on(event,
listener):Adds a listener to the end of the listeners array for the specified
event. No checks are made to see if the listener has already been added. It can
also be called as an alias of emitter.addListener()
3.
emitter.once(event,
listener):Adds a one time listener for the event. This listener is invoked only
the next time the event is fired, after which it is removed.
4.
emitter.removeListener(event, listener):Removes a listener from the listener
array for the specified event. Caution: changes array indices in the listener
array behind the listener.
5.
emitter.removeAllListeners([event]):Removes all listeners, or those of the
specified event.
6.
emitter.setMaxListeners(n):By default EventEmitters will print a warning if
more than 10 listeners are added for a particular event.
7.
emitter.getMaxListeners():Returns the current maximum listener value for the
emitter which is either set by emitter.setMaxListeners(n) or defaults to
EventEmitter.defaultMaxListeners.
8.
emitter.listeners(event):Returns a copy of the array of listeners for the specified
event.
9.
emitter.emit(event[,
arg1][, arg2][, ...]):Raise the specified events with the supplied arguments.
10.
emitter.listenerCount(type):
Returns the number of listeners listening to the type of event.
References
