<div class="intro" class="component">

<p>

AsyncQueue allows you create a chain of function callbacks executed via

<code>setTimeout</code> that are guaranteed to run in order. This can

enable progressive incremental rendering of your UI so your users can

begin to see and interact with your page while the infrastructure is

being built. Similarly, process-intensive operations that will lock up

the UI while the JavaScript is being executed can be broken up into

chunks, helping to keep your interface responsive.

</p>

</div>

{{>getting-started}}

<h2 id="using">Using AsyncQueue</h2>

<h3 id="interacting">Creating and interacting with an AsyncQueue</h3>

<p>

AsyncQueues manage an array of callbacks that can be either simple function

references or <a href="#callbacks">objects with specific keys</a>. The

primary methods on AsyncQueue are <code>add</code> and

<code>run</code>.

</p>

<p>

When <code>run()</code> is invoked, each callback is executed in turn,

either synchronously or via <code>setTimeout</code> (depending on the

configuration of the callback or of the AsyncQueue instance).

</p>

<p>

Queued callbacks can also be promoted to the top of the queue or removed

from it.

</p>

```

var q = new Y.AsyncQueue(callbackB, someTask, callbackA, callbackC);

q.add(callbackD, callbackE); // B, someTask, A, C, D, E

q.promote(callbackA); // A, B, someTask, C, D, E

q.remove(someTask); // A, B, C, D, E

q.run(); // execute A, then B, then C, then D, then E

```

<h4 id="stopping">Pausing and stopping an AsyncQueue</h4>

<p>

In addition to <code>run()</code>, AsyncQueue instances also have

<code>pause()</code> and <code>stop()</code> methods to interrupt the run

state.

</p>

<p>

To wait for an external process to complete, such as an XHR request, call

<code>pause()</code>, then <code>run()</code> again to resume

execution.

</p>

<p>

Call <code>stop()</code> to terminate execution and flush the AsyncQueue.

</p>

```

// Seed the instance with callbacks

var q = new Y.AsyncQueue(

MyApp.doSomething,

// The second callback will pause the Queue and send an XHR for data

function () {

q.pause();

// Send the asynchronous XHR

Y.io(MyApp.getDataUri(), { on: {

success : function (xid,o) {

try {

var data = Y.JSON.parse(o.responseText);

}

catch (e) {

MyApp.showErrorStatus();

q.stop();

}

MyApp.processData(data);

// In the XHR callback, restart the AsyncQueue if successful

q.run();

},

failure : function () {

MyApp.showErrorStatus();

// Stop the AsyncQueue if anything goes wrong

q.stop();

}

}});

},

// The third callback will do partial updates until complete

{

fn: Y.bind(MyApp.updateUI,MyApp),

until: function () {

return MyApp.remainingData >= 0;

}

},

MyApp.doSomethingElse);

q.run();

```

<h4 id="callbacks">About AsyncQueue callbacks</h4>

<p>

AsyncQueue callbacks can be simple function references or object literals

with the following keys:

</p>

<table>

<thead>

<tr>

<th>property</th>

<th>description</th>

<th>default</th>

</tr>

</thead>

<tbody>

<tr>

<td><code>fn</code></td>

<td><strong>Required</strong>. The callback function to execute.</td>

<td>(none)</td>

</tr>

<tr>

<td><code>context</code></td>

<td>The context from which to execute the callback function.</td>

<td>The AsyncQueue instance</td>

</tr>

<tr>

<td><code>args</code></td>

<td>Array of arguments that will be passed as individual args to the callback function.</td>

<td>(none)</td>

</tr>

<tr>

<td><code>timeout</code></td>

<td>Millisecond delay before each execution of this callback. Set to -1 to trigger synchronous execution.</td>

<td>10</td>

</tr>

<tr>

<td><code>iterations</code></td>

<td>The number of times to execute this callback before shifting it from the queue.</td>

<td>1</td>

</tr>

<tr>

<td><code>until</code></td>

<td>A function that will return <code>true</code> when the current callback can be shifted from the queue.</td>

<td>a function that tests against <code>iterations</code></td>

</tr>

<tr>

<td><code>id</code></td>

<td>Name given to this callback for ease of reference.</td>

<td>(none)</td>

</tr>

<tr>

<td><code>autoContinue</code></td>

<td>Set to <code>false</code> to automatically <code>pause()</code> after this callback.</td>

<td>true</td>

</tr>

</tbody>

</table>

<h4 id="defaults">Class- and instance-level callback defaults</h4>

<p>

AsyncQueue provides three places to configure callbacks (in decreasing

precedence order):

</p>

<ol>

<li>The callback object</li>

<li>The AsyncQueue instance's <code>defaults</code> collection</li>

<li>The class static <code>defaults</code> collection</li>

</ol>

```

// All AsyncQueue instances will execute all callbacks synchronously by default

Y.AsyncQueue.defaults.timeout = -1;

var q = new Y.AsyncQueue();

// run every callback in this instance twice before moving to the next callback

q.defaults.iterations = 2;

q.add(functionA,

{

fn: functionB,

timeout: 100 // this callback will be executed asynchronously

});

// functionA executes twice immediately, then after 100 milliseconds functionB

// is executed, then after another 100ms functionB is executed again.

q.run();

```

<h4 id="sync">Synchronous mode for callback execution</h4>

<p>

One of the main goals of the AsyncQueue is to provide a mechanism to

prevent process-intensive operations from locking up the UI. By default,

AsyncQueue callbacks are executed via <code>setTimeout</code> to facilitate

this. The <code>timeout</code> configuration accepts -1 as a value to

trigger synchronous callback execution. Use this setting with caution.

</p>

<h4 id="chaining">About timeout chaining</h4>

<p>

Timeout chaining is a strategy to address the lack of <a

href="http://en.wikipedia.org/wiki/Thread_(computer_science)">multithreading</a>

in JavaScript. When complex or iterative code executes it can cause the

page to stop responding until the running JavaScript process completes; it

can also cause "non-responsive script" or "long-running script" dialogs to

be presented to the user. Both outcomes are detrimental to user

experience.

</p>

<p>

To address this, the operation can be split into chunks, and

<code>setTimeout</code> can be used to yield control back to other

operations between each chunk. A common use case for this technique is to

allow browser reflows to display DOM modifications incrementally while

batches of work are being done in JavaScript. For iterative functions, the

code can execute a portion of the overall work, then schedule itself to run

via <code>setTimeout</code>.

</p>

<p>The basic form of an iterative timeout chain is:</p>

```

(function () {

/* do a chunk of the work */

if (/* process completion check fails */) {

// Schedule myself for re-execution, picking up where I left off

setTimeout(arguments.callee,0);

}

})();

```

<p>

When dealing with <code>setTimeout</code>, it's easy to introduce race

conditions. Because all timeouts are scheduled against the same timer and

only one can run at a time, when two timeouts are separately scheduled, it

is possible for them to execute out of intended order.

</p>

<p>

AsyncQueue supports both "chunked operations" (by specifying callback

timeouts) and "iterative operations" (by specifying callback

<code>iterations</code> or <code>until</code> functions). Furthermore,

AsyncQueue manages the callback sequence and can therefore guarantee the

execution order, so you avoid race conditions.

</p>

<h4 id="events">Exposed events</h4>

<p>

AsyncQueue is based on EventTarget and instances emit the following events

throughout their lifecycle:

</p>

<table>

<thead>

<tr>

<th>Event</th>

<th>When</th>

<th>Event payload</th>

</tr>

</thead>

<tbody>

<tr>

<td><code>add</code></td>

<td>Callbacks are added to the AsyncQueue.</td>

<td><code>{ callbacks: (Array of callbacks added) }</code></td>

</tr>

<tr>

<td><code>promote</code></td>

<td>A callback is promoted.</td>

<td><code>{ callback : (callback) }</code></td>

</tr>

<tr>

<td><code>remove</code></td>

<td>A callback is removed.</td>

<td><code>{ callback : (callback) }</code></td>

</tr>

<tr>

<td><code>execute</code></td>

<td>A callback is executed.</td>

<td><code>{ callback : (callback) }</code></td>

</tr>

<tr>

<td><code>shift</code></td>

<td>A callback is shifted from the AsyncQueue.</td>

<td><code>{ callback : (callback) }</code></td>

</tr>

<tr>

<td><code>complete</code></td>

<td>After the last callback is finished executing. <em>NOT</em> fired after <code>stop()</code>.</td>

<td>(none)</td>

</tr>

</tbody>

</table>