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<title>AsyncQueue</title> |
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<h1><img src="http://yuilibrary.com/img/yui-logo.png"></h1> |
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<a href="#toc" class="jump">Jump to Table of Contents</a> |
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<h1>AsyncQueue</h1> |
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<div class="yui3-g"> |
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<div class="yui3-u-3-4"> |
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<div id="main"> |
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<div class="content"><div class="intro" class="component"> |
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<p> |
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AsyncQueue allows you create a chain of function callbacks executed via |
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<code>setTimeout</code> that are guaranteed to run in order. This can |
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enable progressive incremental rendering of your UI so your users can |
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begin to see and interact with your page while the infrastructure is |
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being built. Similarly, process-intensive operations that will lock up |
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the UI while the JavaScript is being executed can be broken up into |
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chunks, helping to keep your interface responsive. |
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</p> |
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</div> |
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<h2 id="getting-started">Getting Started</h2> |
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<p> |
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To include the source files for AsyncQueue and its dependencies, first load |
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the YUI seed file if you haven't already loaded it. |
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</p> |
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<pre class="code prettyprint"><script src="http://yui.yahooapis.com/3.10.3/build/yui/yui-min.js"></script></pre> |
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<p> |
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Next, create a new YUI instance for your application and populate it with the |
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modules you need by specifying them as arguments to the <code>YUI().use()</code> method. |
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YUI will automatically load any dependencies required by the modules you |
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specify. |
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</p> |
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<pre class="code prettyprint"><script> |
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// Create a new YUI instance and populate it with the required modules. |
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YUI().use('async-queue', function (Y) { |
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// AsyncQueue is available and ready for use. Add implementation |
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// code here. |
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}); |
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</script></pre> |
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<p> |
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For more information on creating YUI instances and on the |
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<a href="http://yuilibrary.com/yui/docs/api/classes/YUI.html#method_use"><code>use()</code> method</a>, see the |
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documentation for the <a href="../yui/index.html">YUI Global Object</a>. |
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</p> |
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<h2 id="using">Using AsyncQueue</h2> |
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<h3 id="interacting">Creating and interacting with an AsyncQueue</h3> |
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<p> |
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AsyncQueues manage an array of callbacks that can be either simple function |
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references or <a href="#callbacks">objects with specific keys</a>. The |
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primary methods on AsyncQueue are <code>add</code> and |
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<code>run</code>. |
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</p> |
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<p> |
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When <code>run()</code> is invoked, each callback is executed in turn, |
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either synchronously or via <code>setTimeout</code> (depending on the |
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configuration of the callback or of the AsyncQueue instance). |
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</p> |
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<p> |
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Queued callbacks can also be promoted to the top of the queue or removed |
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from it. |
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</p> |
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<pre class="code prettyprint">var q = new Y.AsyncQueue(callbackB, someTask, callbackA, callbackC); |
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q.add(callbackD, callbackE); // B, someTask, A, C, D, E |
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q.promote(callbackA); // A, B, someTask, C, D, E |
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q.remove(someTask); // A, B, C, D, E |
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q.run(); // execute A, then B, then C, then D, then E</pre> |
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<h4 id="stopping">Pausing and stopping an AsyncQueue</h4> |
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<p> |
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In addition to <code>run()</code>, AsyncQueue instances also have |
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<code>pause()</code> and <code>stop()</code> methods to interrupt the run |
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state. |
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</p> |
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<p> |
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To wait for an external process to complete, such as an XHR request, call |
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<code>pause()</code>, then <code>run()</code> again to resume |
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execution. |
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</p> |
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<p> |
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Call <code>stop()</code> to terminate execution and flush the AsyncQueue. |
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</p> |
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<pre class="code prettyprint">// Seed the instance with callbacks |
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var q = new Y.AsyncQueue( |
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MyApp.doSomething, |
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// The second callback will pause the Queue and send an XHR for data |
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function () { |
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q.pause(); |
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// Send the asynchronous XHR |
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Y.io(MyApp.getDataUri(), { on: { |
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success : function (xid,o) { |
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try { |
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var data = Y.JSON.parse(o.responseText); |
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} |
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catch (e) { |
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MyApp.showErrorStatus(); |
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q.stop(); |
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} |
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MyApp.processData(data); |
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// In the XHR callback, restart the AsyncQueue if successful |
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q.run(); |
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}, |
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failure : function () { |
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MyApp.showErrorStatus(); |
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// Stop the AsyncQueue if anything goes wrong |
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q.stop(); |
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} |
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}}); |
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}, |
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// The third callback will do partial updates until complete |
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{ |
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fn: Y.bind(MyApp.updateUI,MyApp), |
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until: function () { |
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return MyApp.remainingData >= 0; |
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} |
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}, |
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MyApp.doSomethingElse); |
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q.run();</pre> |
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<h4 id="callbacks">About AsyncQueue callbacks</h4> |
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<p> |
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AsyncQueue callbacks can be simple function references or object literals |
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with the following keys: |
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</p> |
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<table> |
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<thead> |
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<tr> |
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<th>property</th> |
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<th>description</th> |
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<th>default</th> |
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</tr> |
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</thead> |
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<tbody> |
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<tr> |
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<td><code>fn</code></td> |
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<td><strong>Required</strong>. The callback function to execute.</td> |
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<td>(none)</td> |
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</tr> |
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<tr> |
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<td><code>context</code></td> |
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<td>The context from which to execute the callback function.</td> |
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<td>The AsyncQueue instance</td> |
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</tr> |
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<tr> |
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<td><code>args</code></td> |
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<td>Array of arguments that will be passed as individual args to the callback function.</td> |
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<td>(none)</td> |
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</tr> |
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<tr> |
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<td><code>timeout</code></td> |
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<td>Millisecond delay before each execution of this callback. Set to -1 to trigger synchronous execution.</td> |
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<td>10</td> |
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</tr> |
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<tr> |
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<td><code>iterations</code></td> |
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<td>The number of times to execute this callback before shifting it from the queue.</td> |
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<td>1</td> |
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</tr> |
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<tr> |
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<td><code>until</code></td> |
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<td>A function that will return <code>true</code> when the current callback can be shifted from the queue.</td> |
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<td>a function that tests against <code>iterations</code></td> |
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</tr> |
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<tr> |
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<td><code>id</code></td> |
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<td>Name given to this callback for ease of reference.</td> |
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<td>(none)</td> |
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</tr> |
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<tr> |
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<td><code>autoContinue</code></td> |
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<td>Set to <code>false</code> to automatically <code>pause()</code> after this callback.</td> |
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<td>true</td> |
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</tr> |
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</tbody> |
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</table> |
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<h4 id="defaults">Class- and instance-level callback defaults</h4> |
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<p> |
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AsyncQueue provides three places to configure callbacks (in decreasing |
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precedence order): |
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</p> |
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<ol> |
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<li>The callback object</li> |
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<li>The AsyncQueue instance's <code>defaults</code> collection</li> |
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<li>The class static <code>defaults</code> collection</li> |
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</ol> |
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<pre class="code prettyprint">// All AsyncQueue instances will execute all callbacks synchronously by default |
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Y.AsyncQueue.defaults.timeout = -1; |
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var q = new Y.AsyncQueue(); |
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// run every callback in this instance twice before moving to the next callback |
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q.defaults.iterations = 2; |
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q.add(functionA, |
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{ |
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fn: functionB, |
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timeout: 100 // this callback will be executed asynchronously |
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}); |
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// functionA executes twice immediately, then after 100 milliseconds functionB |
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// is executed, then after another 100ms functionB is executed again. |
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q.run();</pre> |
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<h4 id="sync">Synchronous mode for callback execution</h4> |
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<p> |
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One of the main goals of the AsyncQueue is to provide a mechanism to |
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prevent process-intensive operations from locking up the UI. By default, |
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AsyncQueue callbacks are executed via <code>setTimeout</code> to facilitate |
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this. The <code>timeout</code> configuration accepts -1 as a value to |
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trigger synchronous callback execution. Use this setting with caution. |
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</p> |
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<h4 id="chaining">About timeout chaining</h4> |
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<p> |
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Timeout chaining is a strategy to address the lack of <a |
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href="http://en.wikipedia.org/wiki/Thread_(computer_science)">multithreading</a> |
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in JavaScript. When complex or iterative code executes it can cause the |
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page to stop responding until the running JavaScript process completes; it |
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can also cause "non-responsive script" or "long-running script" dialogs to |
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be presented to the user. Both outcomes are detrimental to user |
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experience. |
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</p> |
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<p> |
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To address this, the operation can be split into chunks, and |
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<code>setTimeout</code> can be used to yield control back to other |
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operations between each chunk. A common use case for this technique is to |
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allow browser reflows to display DOM modifications incrementally while |
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batches of work are being done in JavaScript. For iterative functions, the |
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code can execute a portion of the overall work, then schedule itself to run |
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via <code>setTimeout</code>. |
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</p> |
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<p>The basic form of an iterative timeout chain is:</p> |
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<pre class="code prettyprint">(function () { |
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/* do a chunk of the work */ |
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if (/* process completion check fails */) { |
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// Schedule myself for re-execution, picking up where I left off |
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setTimeout(arguments.callee,0); |
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} |
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})();</pre> |
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<p> |
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When dealing with <code>setTimeout</code>, it's easy to introduce race |
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conditions. Because all timeouts are scheduled against the same timer and |
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only one can run at a time, when two timeouts are separately scheduled, it |
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is possible for them to execute out of intended order. |
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</p> |
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<p> |
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AsyncQueue supports both "chunked operations" (by specifying callback |
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timeouts) and "iterative operations" (by specifying callback |
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<code>iterations</code> or <code>until</code> functions). Furthermore, |
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AsyncQueue manages the callback sequence and can therefore guarantee the |
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execution order, so you avoid race conditions. |
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</p> |
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<h4 id="events">Exposed events</h4> |
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<p> |
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AsyncQueue is based on EventTarget and instances emit the following events |
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throughout their lifecycle: |
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</p> |
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<table> |
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<thead> |
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<tr> |
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<th>Event</th> |
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<th>When</th> |
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<th>Event payload</th> |
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</tr> |
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</thead> |
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<tbody> |
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<tr> |
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<td><code>add</code></td> |
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<td>Callbacks are added to the AsyncQueue.</td> |
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<td><code>{ callbacks: (Array of callbacks added) }</code></td> |
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</tr> |
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<tr> |
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<td><code>promote</code></td> |
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<td>A callback is promoted.</td> |
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<td><code>{ callback : (callback) }</code></td> |
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</tr> |
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<tr> |
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<td><code>remove</code></td> |
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<td>A callback is removed.</td> |
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<td><code>{ callback : (callback) }</code></td> |
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</tr> |
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<tr> |
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<td><code>execute</code></td> |
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<td>A callback is executed.</td> |
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<td><code>{ callback : (callback) }</code></td> |
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</tr> |
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<tr> |
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<td><code>shift</code></td> |
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<td>A callback is shifted from the AsyncQueue.</td> |
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<td><code>{ callback : (callback) }</code></td> |
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</tr> |
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<tr> |
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<td><code>complete</code></td> |
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<td>After the last callback is finished executing. <em>NOT</em> fired after <code>stop()</code>.</td> |
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<td>(none)</td> |
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</tr> |
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</tbody> |
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</table> |
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</div> |
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</div> |
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</div> |
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<div class="yui3-u-1-4"> |
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<div class="sidebar"> |
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<div id="toc" class="sidebox"> |
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<div class="hd"> |
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<h2 class="no-toc">Table of Contents</h2> |
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</div> |
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<div class="bd"> |
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<ul class="toc"> |
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<li> |
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<a href="#getting-started">Getting Started</a> |
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</li> |
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<li> |
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<a href="#using">Using AsyncQueue</a> |
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<ul class="toc"> |
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<li> |
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<a href="#interacting">Creating and interacting with an AsyncQueue</a> |
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<ul class="toc"> |
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<li> |
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<a href="#stopping">Pausing and stopping an AsyncQueue</a> |
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</li> |
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<li> |
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<a href="#callbacks">About AsyncQueue callbacks</a> |
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</li> |
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<li> |
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<a href="#defaults">Class- and instance-level callback defaults</a> |
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</li> |
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<li> |
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<a href="#sync">Synchronous mode for callback execution</a> |
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</li> |
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<li> |
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<a href="#chaining">About timeout chaining</a> |
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</li> |
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<li> |
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<a href="#events">Exposed events</a> |
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</li> |
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</ul> |
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</li> |
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</ul> |
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</li> |
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</ul> |
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</div> |
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</div> |
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<div class="sidebox"> |
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<div class="hd"> |
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<h2 class="no-toc">Examples</h2> |
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</div> |
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<div class="bd"> |
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<ul class="examples"> |
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<li data-description="This example employs AsyncQueue to incrementally construct an application interface; this illustrates the approach you'd take to allow chunked rendering of the UI in a process-intensive application."> |
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<a href="queue-app.html">Building a UI with AsyncQueue</a> |
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</li> |
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</ul> |
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</div> |
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</div> |
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</div> |
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</div> |
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</div> |
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</div> |
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<script src="../assets/vendor/prettify/prettify-min.js"></script> |
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<script>prettyPrint();</script> |
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<script> |
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YUI.Env.Tests = { |
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examples: [], |
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project: '../assets', |
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assets: '../assets/async-queue', |
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name: 'async-queue', |
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title: 'AsyncQueue', |
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newWindow: '', |
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auto: false |
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}; |
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YUI.Env.Tests.examples.push('queue-app'); |
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</script> |
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|
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<script src="../assets/yui/test-runner.js"></script> |
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</body> |
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</html> |