Understudy

A means to provide interceptors (i.e. hooks) when performing asynchronous actions.

All hooks are asynchronous
- All logic done by perform-based actions is asynchronous.
- Error first helpers.
- Fail fast.
Consistency in arguments provided to hooks & actions
- Hooks can only mutate arguments that they are passed, not the number of arguments.
- Use of callback arguments to pass information removes focus on return values to mitigate this potentially odd behavior.
Opt-in behavior.
- Only calls to .perform(action ... enable hooking.

By depending on understudy you are exposed to three methods: perform, before and after.

`.perform(action, arg0, /* arg1, ... */, work, callback)`

This is the core API for invoking hooks provided by Understudy. Each call to perform for the same action should have a consistent argument signature because this is what will be expected by each of the before and after hooks for the action. The overall flow control is:

Call all before hooks for action.
Call work function for action.
Call all after hooks for action`.
Call callback with results from work function.

`.before(action, arg0, /* arg1, ... */, next)`

Called before the work function is executed in perform with exactly the arguments passed to .perform. Nothing passed to next have an impact on the flow control above except any error is supplied short-circuits execution to the callback.

`.after(action, arg0, /* arg1, ... */, next)`

Called after the work function is executed in perform with exactly the arguments passed to .perform. Nothing passed to next have an impact on the flow control above except any error is supplied short-circuits execution to the callback.

Real-world Usage

Let's consider a real-world application with two interceptable actions:

start: Application has started
http:request: Application has received an incoming HTTP request.

We could easily implement this App behavior in Understudy:

var Understudy = require('understudy');

var App = module.exports = function App() {
  Understudy.call(this);
};

//
// Starts the application after running before and
// after hooks.
//
App.prototype.start = function (options, callback) {
  this.perform('start', options, function (next) {
    //
    // Here, `options` may have been mutated from the execution
    // of the before hooks.
    // ...
    // Do some other async thing
    // ...
    // These arguments are passed to the final callback unless
    // short-circuited by an error here, or in an after hook.
    //
    next(null, options);
  }, callback);
};

App.prototype.handle = function (req, res) {
  req.times = {
    start: process.hrtime()
  };

  this.perform('http:request', req, res, function (next) {
    req.times.middle = process.hrtime();
    req.times.begin  = process.hrtime(req.times.start);
    next();
  }, function (err) {
    if (err) {
      //
      // Do some error handling.
      //
    }

    req.times.total = process.hrtime(req.times.start);
    req.times.after = process.hrtime(req.times.middle);
    console.log([
      'Total time: %s',
      '  Before hooks: %s',
      '  After  hooks: %s'
    ].join('\n'), format(req.times.total), format(req.times.begin), format(req.times.after));

    res.end();
  });
}

//
// Now we consume a new app with hooks.
//
var http = require('http');

var app = new App();
app.before('start', function (options, next) {
  var server = options.server = http.createServer(function (req, res) {
    app.handle(req, res);
  });

  server.listen(options.port, next);
});

app.after('start', function (options, next) {
  console.log('App started on %s', options.port);
});

app.before('http:request', function (req, res, next) {
  //
  // Do something asynchronous.
  //
  next();
});

app.start({ port: 8080 }, function () {
  console.log('ok');
});

//
// Format process.hrtime()
//
function format(s) {
  return (s[0] * 1e3 + s[1] / 1e6) / 1e3;
}

More Examples

See the example directory

Error handling when no callback is provided

Each before and after hook can provide an optional error to short-circuit evaluation of the flow that would normally follow it. This error will be provided to your callback, when supplied. In the event that you DO NOT provide a callback and a before, after or work function responds with an Error IT WILL BE IGNORED AND FLOW WILL CONTINUE. e.g.

var Understudy = require('understudy');
var actor = new Understudy();

actor.before('always', function (next) {
  next(new Error('I always fail'));
});

actor.after('always', function (next) {
  console.log('I always get called. NO MATTER WHAT');
  console.log('BUT, only when no callback is supplied.');
  next(new Error('Another swallowed error'));
});

actor.perform('always', function (done) {
  done(new Error('Errors are ignored here too.'));
});

In other words (as in the above example): if you do not supply a callback to your .perform then understudy will consider all of your before, after and work functions as "fire and forget".

NealEhardt/understudy