Flow control for your event emitters.
EventEmitters are an important part of well-designed node.js applications.
on()
and emit()
can get you pretty far, but wouldn't it be great if you
could run your event handlers asynchronously, with a continuation callback?
EventFlow adds the flow-controlly-goodness of async to your event emitters.
Create a new emitter.
var emitter = require('eventflow')();
Or, extend an existing emitter with EventFlow functionality.
var EventEmitter = require('events').EventEmitter,
emitter = new EventEmitter();
require('eventflow')(emitter);
Or, extend an EventEmitter class with EventFlow functionality.
var EventEmitter = require('events').EventEmitter,
require('eventflow')(EventEmitter),
emitter = new EventEmitter();
Or, convert any object into an EventFlow emitter.
var emitter = {
type: 'car',
name: 'Honda'
};
require('eventflow')(emitter);
Listen for some events, with or without continuation callbacks. EventFlow does some simple introspection of your listeners to see if they accept a callback or not.
emitter.on('foo', function() {
// Do something synchronous
});
emitter.on('foo', function(callback) {
doSomethingAsync(function(bar) {
callback();
});
});
Now use one of the flow control methods to invoke your handlers and respond when they are done.
series
emitter.series('foo', function() {
// The listeners ran in the order they were added and are all finished.
});
parallel
emitter.parallel('foo', function() {
// The listeners ran in parallel and are all finished.
});
In synchronous listeners, you can return Error
objects.
emitter.on('foo', function () {
return new Error('Something broke');
});
In async listeners, you should pass an Error
as the first argument to the
callback.
emitter.on('foo', function (cb) {
cb(new Error('Something broke'));
});
No matter whether your listeners are sync or async, Errors will always be passed back as the first argument in the callback of the invocation.
emitter.series('foo', function (err) {
// `err` is the first error encountered.
});
Event listeners with arguments
EventFlow supports calling your listeners with any number of arguments, as well as the optional continuation callback.
// In your logger or something:
emitter.on('purchase', function(name, item, cost) {
console.log(name + ' just bought ' + item + ' for ' + cost);
})
// Somwhere else in your code:
emitter.on('purchase', function(name, item, cost, callback) {
saveToDB({name: name, item: item, cost: cost}, callback);
});
// Perhaps in a form POST handler:
emitter.series('purchase', 'Brian', 'T-Shirt', '$15.00', function() {
// The purchase was logged and saved to the db.
});
Using async-style callback(err, results)
EventFlow uses async directly to handle the flow-control, so you can use err
and results
just like you already do.
// Synchronous listeners can return a result.
emitter.on('fruit', function() {
return 'apple';
});
// Async listeners use the standard (err, result) callback.
emitter.on('fruit', function(callback) {
callback(null, 'orange');
});
emitter.series('fruit', function(err, results) {
console.log(results);
// [ 'apple', 'orange' ]
});
The waterfall method allows listeners to modify a variable in a series. The first listener receives an initial value, and each subsequent listener modifies the return of the last listener:
emitter.on('foo', function(n) {
// sync task
return n + 1;
});
emitter.on('foo', function(n, callback) {
// async task
cb(null, n * 3);
});
emitter.waterfall('foo', 2, function(err, n) {
// n = 9
});
EventFlow also attaches the method emitter.invoke(event, [args...], callback)
.
Invoke executes using the following rules:
- There must be EXACTLY one listener for the event. Otherwise the callback is called with an error.
- The listener can
return
a value and if so, callback is called withcallback(err, value)
. - The listener can accept a continuation callback and if so, that function should
be called with
(err, [value])
.
Think of 'invoke' as in-app RPC via an EventEmitter. Instead of passing
functions around your app in options
objects, you can invoke them instead.
Example
emitter.on('add', function(a, b) {
return a + b;
});
emitter.invoke('add', 1, 2, function(err, value) {
console.log(value);
// 3
});
emitter.on('subtract', function(a, b, callback) {
callback(null, a - b);
});
emitter.invoke('subtract', 3, 2, function(err, value) {
console.log(value);
// 1
});
Lets say you are designing a simple model api around redis (or whatever db you use). It has the following API:
function Model () {
// Constructor stuff.
}
Model.prototype = {
load: function (id, cb) {
// Load a model from the db.
},
save: function (cb) {
// Save the model.
}
}
module.exports = Model;
You know your app will need to support validation, but you dont want this Model module to include any of the app-specific validation logic. Using EventFlow, you could just use a 'validate' event to abstract it away.
var eventflow = require('eventflow');
function Model () {
// Constructor stuff.
}
eventflow(Model);
Model.prototype = {
load: function (id, cb) {
// Load a model from the db.
},
save: function (cb) {
Model.parallel('validate', this, function (err) {
if (err) {
// There was an error validating the model or it was invalid.
return cb(err);
}
else {
// Save the model and eventually call `cb(null)`.
}
});
}
}
module.exports = Model;
Now your app could do something like the following:
var Model = require('./path/to/model');
// Simple validation.
Model.on('validate', function (model) {
if (model.title.length > 50) {
return new Error('Titles should be 50 chars or less.');
}
});
// Async validation that hits a db or something.
Model.on('validate', function (model, cb) {
Model.load(model.id, function (err, model) {
if (err) return cb(err);
if (model) return cb(new Error('A model already exists for this id.'));
cb(null);
});
});
var thing = new Model();
thing.save(function (err) {
// Validation errors would appear here.
});
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