# Name

MochiKit.Async - manage asynchronous tasks

# Synopsis

    var url = "/src/b/bo/bob/MochiKit.Async/META.json";
    /*

        META.json looks something like this:

        {"name": "MochiKit", "version": "0.5"}

    */
    var d = loadJSONDoc(url);
    var gotMetadata = function (meta) {
        if (MochiKit.Async.VERSION == meta.version) {
            alert("You have the newest MochiKit.Async!");
        } else {
            alert("MochiKit.Async "
                + meta.version
                + " is available, upgrade!");
        }
    };
    var metadataFetchFailed = function (err) {
      alert("The metadata for MochiKit.Async could not be fetched :(");
    };
    d.addCallbacks(gotMetadata, metadataFetchFailed);

# Description

MochiKit.Async provides facilities to manage asynchronous (as in AJAX [1]) tasks. The model for asynchronous computation used in this module is heavily inspired by Twisted [2].

# Dependencies

- MochiKit.Base

# Overview

## Deferred

The Deferred constructor encapsulates a single value that is not available yet. The most important example of this in the context of a web browser would be an `XMLHttpRequest` to a server. The importance of the Deferred is that it allows a consistent API to be exposed for all asynchronous computations that occur exactly once.

The producer of the Deferred is responsible for doing all of the complicated work behind the scenes. This often means waiting for a timer to fire, or waiting for an event (e.g. `onreadystatechange` of `XMLHttpRequest`). It could also be coordinating several events (e.g. `XMLHttpRequest` with a timeout, or several Deferreds (e.g. fetching a set of XML documents that should be processed at the same time).

Since these sorts of tasks do not respond immediately, the producer of the Deferred does the following steps before returning to the consumer:

1. Create a `new` Deferred(); object and keep a reference to it, because it will be needed later when the value is ready.
2. Setup the conditions to create the value requested (e.g. create a new `XMLHttpRequest`, set its `onreadystatechange`).
3. Return the Deferred object.

Since the value is not yet ready, the consumer attaches a function to the Deferred that will be called when the value is ready. This is not unlike `setTimeout`, or other similar facilities you may already be familiar with. The consumer can also attach an "errback" to the Deferred, which is a callback for error handling.

When the value is ready, the producer simply calls `myDeferred.callback(theValue)`. If an error occurred, it should call `myDeferred.errback(theValue)` instead. As soon as this happens, the callback that the consumer attached to the Deferred is called with `theValue` as the only argument.

There are quite a few additional "advanced" features baked into Deferred, such as cancellation and callback chains, so take a look at the API reference if you would like to know more!

# API Reference

## Errors

AlreadyCalledError:

> Thrown by a Deferred if `.callback` or `.errback` are called more than once.

BrowserComplianceError:

> Thrown when the JavaScript runtime is not capable of performing the given function. Currently, this happens if the browser does not support `XMLHttpRequest`.

CancelledError:

> Thrown by a Deferred when it is cancelled, unless a canceller is present and throws something else.

GenericError:

> Results passed to `.fail` or `.errback` of a Deferred are wrapped by this `Error` if `!(result instanceof Error)`.

XMLHttpRequestError:

> Thrown when an `XMLHttpRequest` does not complete successfully for any reason. The `req` property of the error is the failed `XMLHttpRequest` object, and for convenience the `number` property corresponds to `req.status`.

## Constructors

Deferred():

> Encapsulates a sequence of callbacks in response to a value that may not yet be available. This is modeled after the Deferred class from Twisted [3].

> Why do we want this? JavaScript has no threads, and even if it did, threads are hard. Deferreds are a way of abstracting non-blocking events, such as the final response to an `XMLHttpRequest`.

> The sequence of callbacks is internally represented as a list of 2-tuples containing the callback/errback pair. For example, the following call sequence:

>     var d = new Deferred();
>     d.addCallback(myCallback);
>     d.addErrback(myErrback);
>     d.addBoth(myBoth);
>     d.addCallbacks(myCallback, myErrback);

> is translated into a Deferred with the following internal representation:

>     [
>         [myCallback, null],
>         [null, myErrback],
>         [myBoth, myBoth],
>         [myCallback, myErrback]
>     ]

> The Deferred also keeps track of its current status (fired). Its status may be one of the following three values:

> > Value | Condition
> > --- | ---
> > -1 | no value yet (initial condition)
> > 0 | success
> > 1 | error

> A Deferred will be in the error state if one of the following conditions are met:

> 1. The result given to callback or errback is "`instanceof Error`"
> 2. The callback or errback threw while executing. If the thrown object is not `instanceof Error`, it will be wrapped with GenericError.

> Otherwise, the Deferred will be in the success state. The state of the Deferred determines the next element in the callback sequence to run.

> When a callback or errback occurs with the example deferred chain, something equivalent to the following will happen (imagine that exceptions are caught and returned as-is):

>     // d.callback(result) or d.errback(result)
>     if (!(result instanceof Error)) {
>         result = myCallback(result);
>     }
>     if (result instanceof Error) {
>         result = myErrback(result);
>     }
>     result = myBoth(result);
>     if (result instanceof Error) {
>         result = myErrback(result);
>     } else {
>         result = myCallback(result);
>     }

> The result is then stored away in case another step is added to the callback sequence. Since the Deferred already has a value available, any new callbacks added will be called immediately.

> There are two other "advanced" details about this implementation that are useful:

> Callbacks are allowed to return Deferred instances, so you can build complicated sequences of events with (relative) ease.

> The creator of the Deferred may specify a canceller. The canceller is a function that will be called if Deferred.prototype.cancel is called before the Deferred fires. You can use this to allow an `XMLHttpRequest` to be cleanly cancelled, for example. Note that cancel will fire the Deferred with a CancelledError (unless your canceller throws or returns a different `Error`), so errbacks should be prepared to handle that `Error` gracefully for cancellable Deferred instances.

Deferred.prototype.addBoth(func):

> Add the same function as both a callback and an errback as the next element on the callback sequence. This is useful for code that you want to guarantee to run, e.g. a finalizer.

> If additional arguments are given, then `func` will be replaced with MochiKit.Base.partial.apply(null, arguments). This differs from Twisted, because the result of the callback or errback will be the last argument passed to `func`.

> If `func` returns a Deferred, then it will be chained (its value or error will be passed to the next callback). Note that once the returned `Deferred` is chained, it can no longer accept new callbacks.

Deferred.prototype.addCallback(func[, ...]):

> Add a single callback to the end of the callback sequence.

> If additional arguments are given, then `func` will be replaced with MochiKit.Base.partial.apply(null, arguments). This differs from Twisted, because the result of the callback will be the last argument passed to `func`.

> If `func` returns a Deferred, then it will be chained (its value or error will be passed to the next callback). Note that once the returned `Deferred` is chained, it can no longer accept new callbacks.

Deferred.prototype.addCallbacks(callback, errback):

> Add separate callback and errback to the end of the callback sequence. Either callback or errback may be `null`, but not both.

> If `callback` or `errback` returns a Deferred, then it will be chained (its value or error will be passed to the next callback). Note that once the returned `Deferred` is chained, it can no longer accept new callbacks.

Deferred.prototype.addErrback(func):

> Add a single errback to the end of the callback sequence.

> If additional arguments are given, then `func` will be replaced with MochiKit.Base.partial.apply(null, arguments). This differs from Twisted, because the result of the errback will be the last argument passed to `func`.

> If `func` returns a Deferred, then it will be chained (its value or error will be passed to the next callback). Note that once the returned `Deferred` is chained, it can no longer accept new callbacks.

Deferred.prototype.callback([result]):

> Begin the callback sequence with a non-`Error` result. Result may be any value except for a Deferred.

> Either `.callback` or `.errback` should be called exactly once on a Deferred.

Deferred.prototype.cancel():

> Cancels a Deferred that has not yet received a value, or is waiting on another Deferred as its value.

> If a canceller is defined, the canceller is called. If the canceller did not return an `Error`, or there was no canceller, then the errback chain is started with CancelledError.

Deferred.prototype.errback([result]):

> Begin the callback sequence with an error result. Result may be any value except for a Deferred, but if `!(result instanceof Error)`, it will be wrapped with GenericError.

> Either `.callback` or `.errback` should be called exactly once on a Deferred.

DeferredLock():

> A lock for asynchronous systems.

> The `locked` property of a DeferredLock will be `true` if it locked, `false` otherwise. Do not change this property.

DeferredLock.prototype.acquire():

> Attempt to acquire the lock. Returns a Deferred that fires on lock acquisition with the DeferredLock as the value. If the lock is locked, then the Deferred goes into a waiting list.

DeferredLock.prototype.release():

> Release the lock. If there is a waiting list, then the first Deferred in that waiting list will be called back.

DeferredList(list, [fireOnOneCallback, fireOnOneErrback, consumeErrors, canceller]):

> Combine a list of Deferred into one. Track the callbacks and return a list of (success, result) tuples, 'success' being a boolean indicating whether result is a normal result or an error.

> Once created, you have access to all Deferred methods, like addCallback, addErrback, addBoth. The behaviour can be changed by the following options:

> `fireOnOneCallback`:
>   Flag for launching the callback once the first Deferred of the list has returned.
> `fireOnOneErrback`:
>   Flag for calling the errback at the first error of a Deferred.
> `consumeErrors`:
>   Flag indicating that any errors raised in the Deferreds should be consumed by the DeferredList.

> Example:

>     // We need to fetch data from 2 different urls
>     var d1 = loadJSONDoc(url1);
>     var d2 = loadJSONDoc(url2);
>     var l1 = new DeferredList([d1, d2], false, false, true);
>     l1.addCallback(function (resultList) {
>         MochiKit.Base.map(function (result) {
>             if (result[0]) {
>                 alert("Data is here: " + result[1]);
>             } else {
>                 alert("Got an error: " + result[1]);
>             }
>         }, resultList);
>     });

## Functions

callLater(seconds, func[, args...]):

> Call `func(args...)` after at least `seconds` seconds have elapsed. This is a convenience method for:

>     func = partial.apply(extend(null, arguments, 1));
>     return wait(seconds).addCallback(function (res) { return func() });

> Returns a cancellable Deferred.

doSimpleXMLHttpRequest(url[, queryArguments...]):

> Perform a simple `XMLHttpRequest` and wrap it with a Deferred that may be cancelled.

> Note that currently, only `200` (OK) and `304` (NOT_MODIFIED) are considered success codes at this time, other status codes will result in an errback with an `XMLHttpRequestError`.

> `url`:
>   The URL to GET
> `queryArguments`:

>   If this function is called with more than one argument, a `"?"` and the result of MochiKit.Base.queryString with the rest of the arguments are appended to the URL.

>   For example, this will do a GET request to the URL `http://example.com?bar=baz`:

>       doSimpleXMLHttpRequest("http://example.com", {bar: "baz"});

> returns:
>   Deferred that will callback with the `XMLHttpRequest` instance on success

evalJSONRequest(req):

> Evaluate a JSON [4] `XMLHttpRequest`

> `req`:
>   The request whose `.responseText` property is to be evaluated
> returns:
>   A JavaScript object

fail([result]):

> Return a Deferred that has already had `.errback(result)` called.

> See `succeed` documentation for rationale.

> `result`:
>   The result to give to Deferred.prototype.errback(result).
> returns:
>   A `new` Deferred()

gatherResults(deferreds):

> A convenience function that returns a DeferredList from the given `Array` of Deferred instances that will callback with an `Array` of just results when they're available, or errback on the first array.

getXMLHttpRequest():

> Return an `XMLHttpRequest` compliant object for the current platform.

> In order of preference:

> - `new XMLHttpRequest()`
> - `new ActiveXObject('Msxml2.XMLHTTP')`
> - `new ActiveXObject('Microsoft.XMLHTTP')`
> - `new ActiveXObject('Msxml2.XMLHTTP.4.0')`

maybeDeferred(func[, argument...]):

> Call a `func` with the given arguments and ensure the result is a Deferred.

> `func`:
>   The function to call.
> returns:
>   A new Deferred based on the call to `func`. If `func` does not naturally return a Deferred, its result or error value will be wrapped by one.

loadJSONDoc(url):

> Do a simple `XMLHttpRequest` to a URL and get the response as a JSON [4] document.

> `url`:
>   The URL to GET
> returns:
>   Deferred that will callback with the evaluated JSON [4] response upon successful `XMLHttpRequest`

sendXMLHttpRequest(req[, sendContent]):

> Set an `onreadystatechange` handler on an `XMLHttpRequest` object and send it off. Will return a cancellable Deferred that will callback on success.

> Note that currently, only `200` (OK) and `304` (NOT_MODIFIED) are considered success codes at this time, other status codes will result in an errback with an `XMLHttpRequestError`.

> `req`:
>   An preconfigured `XMLHttpRequest` object (open has been called).
> `sendContent`:
>   Optional string or DOM content to send over the `XMLHttpRequest`.
> returns:
>   Deferred that will callback with the `XMLHttpRequest` instance on success.

succeed([result]):

> Return a Deferred that has already had `.callback(result)` called.

> This is useful when you're writing synchronous code to an asynchronous interface: i.e., some code is calling you expecting a Deferred result, but you don't actually need to do anything asynchronous. Just return `succeed(theResult)`.

> See `fail` for a version of this function that uses a failing Deferred rather than a successful one.

> `result`:
>   The result to give to Deferred.prototype.callback(result)
> returns:
>   a `new` Deferred

wait(seconds[, res]):

> Return a new cancellable Deferred that will `.callback(res)` after at least `seconds` seconds have elapsed.

# See Also

[1] | AJAX, Asynchronous JavaScript and XML: http://en.wikipedia.org/wiki/AJAX

[2] | Twisted, an event-driven networking framework written in Python: http://twistedmatrix.com/

[3] | Twisted Deferred Reference: http://twistedmatrix.com/projects/core/documentation/howto/defer.html

[4] | (1, 2, 3) JSON, JavaScript Object Notation: http://json.org/

# Authors

- Bob Ippolito <bob@redivi.com>

# Copyright

Copyright 2005 Bob Ippolito <bob@redivi.com>. This program is dual-licensed free software; you can redistribute it and/or modify it under the terms of the MIT License or the Academic Free License v2.1.