Flow control for your event emitters
Flow control for your event emitters.
EventEmitters are an important part of well-designed node.js applications.
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'EventEmitteremitter = ;require'eventflow'emitter;
Or, extend an EventEmitter class with EventFlow functionality.
var EventEmitter = require'events'EventEmitterrequire'eventflow'EventEmitteremitter = ;
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.
emitteron'foo'// Do something synchronous;emitteron'foo'doSomethingAsynccallback;;;
Now use one of the flow control methods to invoke your handlers and respond when they are done.
emitterseries'foo'// The listeners ran in the order they were added and are all finished.;
emitterparallel'foo'// The listeners ran in parallel and are all finished.;
In synchronous listeners, you can return
emitteron'foo'return 'Something broke';;
In async listeners, you should pass an
Error as the first argument to the
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.
emitterseries'foo'// `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:emitteron'purchase'console.logname + ' just bought ' + item + ' for ' + cost;// Somwhere else in your code:emitteron'purchase'saveToDBname: name item: item cost: cost callback;;// Perhaps in a form POST handler:emitterseries'purchase' 'Brian' 'T-Shirt' '$15.00'// The purchase was logged and saved to the db.;
EventFlow uses async directly to handle the flow-control, so you can use
results just like you already do.
// Synchronous listeners can return a result.emitteron'fruit'return 'apple';;// Async listeners use the standard (err, result) callback.emitteron'fruit'callbacknull 'orange';;emitterseries'fruit'console.logresults;// [ '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:
emitteron'foo'// sync taskreturn n + 1;;emitteron'foo'// async taskcbnull n * 3;;emitterwaterfall'foo' 2// 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
returna value and if so, callback is called with
- The listener can accept a continuation callback and if so, that function should
be called with
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.
emitteron'add'return a + b;;emitterinvoke'add' 1 2console.logvalue;// 3;emitteron'subtract'callbacknull a - b;;emitterinvoke'subtract' 3 2console.logvalue;// 1;
Lets say you are designing a simple model api around redis (or whatever db you use). It has the following API:
// Constructor stuff.// Load a model from the db.// Save the model.moduleexports = 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';// Constructor stuff.eventflowModel;// Load a model from the db.Modelparallel'validate' thisif err// There was an error validating the model or it was invalid.return cberr;else// Save the model and eventually call `cb(null)`.;moduleexports = Model;
Now your app could do something like the following:
var Model = require'./path/to/model';// Simple validation.Modelon'validate'if modeltitlelength > 50return 'Titles should be 50 chars or less.';;// Async validation that hits a db or something.Modelon'validate'Modelloadmodelidif err return cberr;if model return cb'A model already exists for this id.';cbnull;;;var thing = ;thingsave// Validation errors would appear here.;
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