when.js

define([
	"./Deferred",
	"./promise/Promise"
], function(Deferred, Promise){
	"use strict";

	// module:
	//		dojo/when

	return function when(valueOrPromise, callback, errback, progback){
		// summary:
		//		Transparently applies callbacks to values and/or promises.
		// description:
		//		Accepts promises but also transparently handles non-promises. If no
		//		callbacks are provided returns a promise, regardless of the initial
		//		value. Foreign promises are converted.
		//
		//		If callbacks are provided and the initial value is not a promise,
		//		the callback is executed immediately with no error handling. Returns
		//		a promise if the initial value is a promise, or the result of the
		//		callback otherwise.
		// valueOrPromise:
		//		Either a regular value or an object with a `then()` method that
		//		follows the Promises/A specification.
		// callback: Function?
		//		Callback to be invoked when the promise is resolved, or a non-promise
		//		is received.
		// errback: Function?
		//		Callback to be invoked when the promise is rejected.
		// progback: Function?
		//		Callback to be invoked when the promise emits a progress update.
		// returns: dojo/promise/Promise
		//		Promise, or if a callback is provided, the result of the callback.

		var receivedPromise = valueOrPromise && typeof valueOrPromise.then === "function";
		var nativePromise = receivedPromise && valueOrPromise instanceof Promise;

		if(!receivedPromise){
			if(arguments.length > 1){
				return callback ? callback(valueOrPromise) : valueOrPromise;
			}else{
				return new Deferred().resolve(valueOrPromise);
			}
		}else if(!nativePromise){
			var deferred = new Deferred(valueOrPromise.cancel);
			valueOrPromise.then(deferred.resolve, deferred.reject, deferred.progress);
			valueOrPromise = deferred.promise;
		}

		if(callback || errback || progback){
			return valueOrPromise.then(callback, errback, progback);
		}
		return valueOrPromise;
	};
});