zero_coding zero_coding - 5 months ago 19
Node.js Question

write async function with EventEmitter

I am new in node and try to use async and event behavior advantages in node. I used to understand from node, everything that handle with Event object, it gonna be async execution.

Then i had try this, consider following code:

var events = require("events");

var event = new events.EventEmitter();


event.on("work", function () {

for (var i = 0; i <= 10; i++) {
console.log("I do my work " + i);
}

event.emit("done");
});

var async = function (cb) {

event.on("done", cb);
event.emit("work");
for (var i = 0; i <= 10; i++) {
console.log("Async " + i);
}
}


async(function () {
console.log("I am done callback!");
});


This is async execution? In my opinion no! Why, because i had read this sentence many:


An event is fired, so go and do something and then when you have finished it, come
back and tell me, but in meanwhile i will do something else.


Like a fast food restaurant scenario. But the code above, when the event work gonna fired, following sequence will happen:


  1. go into the callback

  2. let through the loop

  3. output I do my work n

  4. fired the done event

  5. output I am done callback!

  6. output Async n



why I am done callback! gonna output before Async n? Why is here not like fast food restaurant scenario, like


the work event is fired, go and do you stuff and come back when you
done, in meanwhile i will output Async n


This is i used to understand about event driven behavior and async in node. But now i am very confused. I know, javascript works on single thread. How can i write an async function with event emitter? But i think is not possible, because when i emit an event, it will immediately execute the handler.

Answer

I used to understand from node, everything that handle with Event object, it gonna be async execution.

This is incorrect. Events are synchronous. When you add a listener, you're just saving the callback in an object:

this._events[type].push(listener);

When you emit an event, you're just iterating an array and calling each listener:

for (i = 0; i < len; i++)
      listeners[i].apply(this, args);

Source code: https://github.com/joyent/node/blob/master/lib/events.js

This is async execution? In my opinion no!

You are correct. It's async if you call any I/O function or use setImmediate, nextTick or a timer—otherwise, it's synchronous. A synchronous code being written asynchrously doesn't convert it to an asynchrous code.

why I am done callback! gonna output before Async n?

Because when you receive the "done" callback you call to "cb":

event.on("done", cb);

When cb returns, the "Async n" loop is executed.

How can i write an async function with event emitter?

Using setImmediate or process.nextTick.

If you want to postpone the "I do my work" loop execution, you can wrap the line events.emit ("work") with nextTick.

var events = require("events");

var event = new events.EventEmitter();


event.on("work", function () {

    for (var i = 0; i <= 10; i++) {
        console.log("I do my work " + i);
    }

    event.emit("done");
});

var async = function (cb) {

    event.on("done", cb);
    process.nextTick (function () {         //<-----
        event.emit("work");
    });                                     //<-----
    for (var i = 0; i <= 10; i++) {
        console.log("Async " + i);
    }
}


async(function () {
    console.log("I am done callback!");
});  

This will print:

Async 0
Async 1
Async 2
Async 3
Async 4
Async 5
Async 6
Async 7
Async 8
Async 9
Async 10
I do my work 0
I do my work 1
I do my work 2
I do my work 3
I do my work 4
I do my work 5
I do my work 6
I do my work 7
I do my work 8
I do my work 9
I do my work 10
I am done callback!