Group of Asynchronous Workers

worker-group.js

/**
 * Starts all workers at once and executes the given afterFinish
 * callback when all workers have completed. Workers are expected
 * to signal when they are finished.
 *
 * @example
 *
 *   WorkerGroup(
 *     [ function(whenDone) { $.ajax({url: 'http://jquery.com/a/', error: whenDone, success: whenDone}); }
 *     , function(whenDone) { $.ajax({url: 'http://jquery.com/b/', error: whenDone, success: whenDone}); }
 *     , function(whenDone) { $.ajax({url: 'http://jquery.com/c/', error: whenDone, success: whenDone}); } ],
 *     function(returnVals) { console.log('results are in:', returnVals); });
 *
 *   // WorkerGroup returns immediately and execution resumes here.
 *   console.log('collecting data');
 */
function WorkerGroup(workers, afterFinish) {
  var activeCount = workers.length;
  var returnVals  = [];

  var observeWorker = function(workerId) {
    // Called by a worker when it's finished
    return function(returnVal) {
      returnVals[workerId] = returnVal;

      if (-- activeCount == 0 && afterFinish)
        afterFinish(returnVals);
    };
  };

  for (var n = 0; n < workers.length; n ++) {
    (function(workerId) {
      setTimeout(function() {
        // Hmm... this worker may punt by calling setTimeout,
        // so we can't really know if he finishes unless he
        // calls the given observeWorker value
        workers[workerId](observeWorker(workerId));
      }, 0);
    })(n);
  }
}