An EventEmitter-like interface for promises.

Like EventEmitter, it handles triggering sequences of asynchronous actions on a common location. Unlike EventEmitter, it uses promise composition to support return values and late binding of dependencies between tasks.

Suppose you have a stream of mogs to process, and you want them to share an event bus.

var PromiseBus = require('promisebus');

var bus = new PromiseBus();

Given a mog, we can go fetch its name from the database and take its picture with a camera we have set up in the enclosure.

bus.register('name', [], function(eid) {
  return knex.select('name').from('mogs').where({ enclosure_id: eid });
})

bus.register('picture', [], function(eid) {
  return cameras[eid].takePicture();
});

Given its picture, we can approximate its age and gender, and given its name and age, we can estimate how likely someone is to adopt it.

bus.register('age', ['picture'], function(eid, data) {
  return estimateAgeByFacialProportions(data.picture);
});

bus.register('gender', ['picture'], function(eid, data) {
  // mogs are highly gender-normative
  var colour = getPredominantColour(data.picture);
  return colour === 'pink' ? 'female' : 'male';
});

bus.register('adoptionProbability', ['name', 'age'], function(eid, data) {
  return data.age < 2 || data.name.match(/fluff/) ? 0.9 : 0.2;
});

So now, when we receive a mog to process, we can run all these disparate pieces of code in one step.

bus.run(5).then(function(mog) {
  display mog.picture;
  console.log(mog.name, " is very happy to be adopted! ", mog.gender === 'female' ? "She" : "He", " hopes to see you soon!");
  if (mog.adoptionProbability < 0.5) {
    console.debug("We totally didn't call this one.")
  }
});

Or, we could run only one such task.

bus.runTask('adoptionProbability', 6).then(function(probability) {
  if (probability > 0.5) {
    console.debug('Guys, this should be an easy one, why is it still around?')
  }
});

And our mogs go happy to good homes.

PromiseBus#register() Registers a task to run on the bus.

• name, String naming this task. Optional, but if it isn't specified then function.name will be used instead.
• dependencies, Array of Strings listing the tasks this task depends on
• task, Function implementing the task itself. Will be passed the event's arguments, then its dependencies.
• Returns the PromiseBus instance for chaining.

PromiseBus#unregister() Unregisters an existing task.

• name, Name of the task to unregister
• Returns the PromiseBus instance for chaining.

PromiseBus#tasks() The list of tasks on the bus

• Returns an object of the form { name: { dependencies, task } }

PromiseBus#run() Run the bus's tasks

• ...args, Arguments to pass to the tasks
• Returns a Promise for an object of the form { name: return value }

PromiseBus#runTask() Run a specific task (and its dependencies). Does not run disconnected tasks.

• name, The task to run
• ...args, Arguments to pass to the tasks

PromiseBus#runTasks() Plural form of the above.

• names, Array with names of tasks to run
• ...args, Arguments to pass to the tasks