Deferred.js

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

	// module:
	//		dojo/Deferred
	// summary:
	//		Deferred base class.

	var PROGRESS = 0,
			RESOLVED = 1,
			REJECTED = 2;
	var FULFILLED_ERROR_MESSAGE = "This deferred has already been fulfilled.";

	var noop = function(){};
	var freezeObject = Object.freeze || noop;
	var signalQueue = [];

	function signalWaiting(waiting, type, result){
		for(var i = 0; i < waiting.length; i++){
			signalListener(waiting[i], type, result);
		}
	}

	function signalListener(listener, type, result){
		var func = listener[type];
		var deferred = listener.deferred;
		if(func){
			try{
				var newResult = func(result);
				if(newResult && typeof newResult.then === "function"){
					listener.cancel = newResult.cancel;
					newResult.then(
							// Only make resolvers if they're actually going to be used
							makeDeferredSignaler(deferred, RESOLVED),
							makeDeferredSignaler(deferred, REJECTED),
							makeDeferredSignaler(deferred, PROGRESS));
					return;
				}
				signalDeferred(deferred, RESOLVED, newResult);
			}catch(error){
				signalDeferred(deferred, REJECTED, error);
			}
		}else{
			signalDeferred(deferred, type, result);
		}
	}

	function makeDeferredSignaler(deferred, type){
		return function(value){
			signalDeferred(deferred, type, value);
		};
	}

	function signalDeferred(deferred, type, result){
		signalQueue.push(deferred, type, result);
		if(signalQueue.length > 3){
			return;
		}

		for(var i = 0; i < signalQueue.length; i += 3){
			if(!signalQueue[i].isCanceled()){
				switch(signalQueue[i + 1]){
					case PROGRESS:
						signalQueue[i].progress(signalQueue[i + 2]);
						break;
					case RESOLVED:
						signalQueue[i].resolve(signalQueue[i + 2]);
						break;
					case REJECTED:
						signalQueue[i].reject(signalQueue[i + 2]);
						break;
				}
			}
		}
		signalQueue = [];
	}

	return compose(function Deferred(/*Function?*/ canceler){
		// summary:
		//		Constructor for a deferred.
		// description:
		//		Creates a new Deferred. This API is preferred over dojo/_base/Deferred.
		//
		// canceler:
		//		Will be invoked if the deferred is canceled. The canceler receives the
		//		reason the deferred was canceled as its argument. The deferred is
		//		rejected with its return value, if any.
		var promise = this.promise = new Promise;
		var fulfilled, result;
		var canceled = false;
		var waiting = [];

		this.isResolved = promise.isResolved = function(){
			// summary:
			//		Checks whether the deferred has been resolved.
			// returns: Boolean
			return fulfilled === RESOLVED;
		};

		this.isRejected = promise.isRejected = function(){
			// summary:
			//		Checks whether the deferred has been rejected.
			// returns: Boolean
			return fulfilled === REJECTED;
		};

		this.isFulfilled = promise.isFulfilled = function(){
			// summary:
			//		Checks whether the deferred has been resolved or rejected.
			// returns: Boolean
			return !!fulfilled;
		};

		this.isCanceled = promise.isCanceled = function(){
			// summary:
			//		Checks whether the deferred has been canceled.
			// returns: Boolean
			return canceled;
		};

		var progress = this.progress = function(update, /*Boolean?*/ strict){
			// summary:
			//		Emit a progress update on the deferred.
			// returns: dojo/promise/Promise
			//		Returns the original promise for the deferred.
			//
			// update:
			//		The progress update
			// strict:
			//		If strict, will throw an error if the deferred is already fulfilled.
			if(!fulfilled){
				signalWaiting(waiting, PROGRESS, update);
				return promise;
			}else if(strict === true){
				throw new Error(FULFILLED_ERROR_MESSAGE);
			}else{
				return promise;
			}
		};

		var resolve = this.resolve = function(value, /*Boolean?*/ strict){
			// summary:
			//		Resolve the deferred.
			// returns: dojo/promise/Promise
			//		Returns the original promise for the deferred.
			//
			// value:
			//		The promise result value.
			// strict:
			//		If strict, will throw an error if the deferred is already fulfilled.
			if(!fulfilled){
				// Set fulfilled, store value. After signaling waiting listeners unset
				// waiting.
				signalWaiting(waiting, fulfilled = RESOLVED, result = value);
				waiting = null;
				return promise;
			}else if(strict === true){
				throw new Error(FULFILLED_ERROR_MESSAGE);
			}else{
				return promise;
			}
		};

		var reject = this.reject = function(error, /*Boolean?*/ strict){
			// summary:
			//		Reject the deferred.
			// returns: dojo/promise/Promise
			//		Returns the original promise for the deferred.
			//
			// error:
			//		The promise error value.
			// strict:
			//		If strict, will throw an error if the deferred is already fulfilled.
			if(!fulfilled){
				signalWaiting(waiting, fulfilled = REJECTED, result = error);
				waiting = null;
				return promise;
			}else if(strict === true){
				throw new Error(FULFILLED_ERROR_MESSAGE);
			}else{
				return promise;
			}
		};

		this.then = promise.then = function(/*Function?*/ callback, /*Function?*/ errback, /*Function?*/ progback){
			// summary:
			//		Add new callbacks to the deferred.
			// returns: dojo/promise/Promise
			//		Returns a new promise for the result of the callback(s).
			//
			// callback: 
			//		Callback to be invoked when the promise is resolved.
			// errback: 
			//		Callback to be invoked when the promise is rejected.
			// progback: 
			//		Callback to be invoked when the promise emits a progress update.
			var listener = [progback, callback, errback];
			// Ensure we cancel the promise we're waiting for, or if callback/errback
			// have returned a promise, cancel that one.
			listener.cancel = promise.cancel;
			listener.deferred = new Deferred(function(reason){
				// Check whether cancel is really available, returned promises are not
				// required to expose `cancel`
				return listener.cancel && listener.cancel(reason);
			});
			if(fulfilled && !waiting){
				signalListener(listener, fulfilled, result);
			}else{
				waiting.push(listener);
			}
			return listener.deferred.promise;
		};

		/**
		* promise.Deferred#cancel([reason, strict]) -> Boolean | reason
		* - reason (?): A message that may be sent to the deferred's canceler, explaining why it's being canceled.
		* - strict (Boolean): if strict, will throw an error if the deferred has already been fulfilled.
		*
		* Signal the deferred that we're no longer interested in the result.
		* The deferred may subsequently cancel its operation and reject the
		* promise. Can affect other promises that originate with the same
		* deferred. Returns the rejection reason if the deferred was canceled
		* normally.
		**/
		this.cancel = promise.cancel = function(reason, /*Boolean?*/ strict){
			// summary:
			//		Signal the deferred that we're no longer interested in the result.
			// description:
			//		Signal the deferred that we're no longer interested in the result.
			//		The deferred may subsequently cancel its operation and reject the
			//		promise. Can affect other promises that originate with the same
			//		deferred. Returns the rejection reason if the deferred was canceled
			//		normally.
			//
			// reason:
			//		A message that may be sent to the deferred's canceler, explaining why
			//		it's being canceled.
			// strict:
			//		If strict, will throw an error if the deferred is already fulfilled.
			if(!fulfilled){
				// Cancel can be called even after the deferred is fulfilled
				if(canceler){
					var returnedReason = canceler(reason);
					reason = typeof returnedReason === "undefined" ? reason : returnedReason;
				}
				canceled = true;
				if(!fulfilled){
					// Allow canceler to provide its own reason, but fall back to a CancelError
					if(typeof reason === "undefined"){
						reason = new CancelError;
					}
					signalWaiting(waiting, fulfilled = REJECTED, result = reason);
					waiting = null;
					return reason;
				}else if(fulfilled === REJECTED && result === reason){
					return reason;
				}
			}else if(strict === true){
				throw new Error(FULFILLED_ERROR_MESSAGE);
			}
		};

		freezeObject(promise);
	},{
		// Define the shape of Deferred instances
		promise: null,
		resolve: noop,
		reject: noop,
		progress: noop,
		then: noop,
		cancel: noop,
		isResolved: noop,
		isRejected: noop,
		isFulfilled: noop,
		isCanceled: noop
	});
});