eventflow

EventFlow

Flow control for your event emitters.

About

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.

Usage

Creating an EventFlow emitter

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

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();
  });
});

Invoke listeners

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.
});

Errors

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.
});

Advanced

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' ]
});

Waterfall

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
});

Invoke

EventFlow also attaches the method emitter.invoke(event, [args...], callback). Invoke executes using the following rules:

1. There must be EXACTLY one listener for the event. Otherwise the callback is called with an error.
2. The listener can return a value and if so, callback is called with callback(err, value).
3. 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
});

Example Use Case: Model API

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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Developed by Terra Eclipse

Terra Eclipse, Inc. is a nationally recognized political technology and strategy firm located in Aptos, CA and Washington, D.C.

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License: MIT

Copyright (C) 2012 Terra Eclipse, Inc. (http://www.terraeclipse.com)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.