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+
+<section id="progress-events">
+<span id="devcycle-progress-events"></span><h1 id="progress-events"><span id="devcycle-progress-events"></span>Progress Events</h1>
+<div class="contents local topic" id="contents">
+<ul class="small-gap">
+<li><a class="reference internal" href="#module-loading-and-progress-events" id="id3">Module loading and progress events</a></li>
+<li><a class="reference internal" href="#handling-progress-events" id="id4">Handling progress events</a></li>
+<li><a class="reference internal" href="#displaying-load-status" id="id5">Displaying load status</a></li>
+<li><a class="reference internal" href="#the-lasterror-attribute" id="id6">The <code>lastError</code> attribute</a></li>
+<li><a class="reference internal" href="#the-readystate-attribute" id="id7">The <code>readyState</code> attribute</a></li>
+<li><a class="reference internal" href="#the-exitstatus-attribute" id="id8">The <code>exitStatus</code> attribute</a></li>
+</ul>
+</div>
+<p>There are five types of events that developers can respond to in Native Client:
+progress, message, view change, focus, and input events (each described in the
+glossary below). This chapter describes how to monitor progress events (events
+that occur during the loading and execution of a Native Client module). This
+chapter assumes you are familiar with the material presented in the
+<a class="reference internal" href="/native-client/overview.html"><em>Technical Overview</em></a>.</p>
+<aside class="note">
+The load_progress example illustrates progress event handling. You can find
+this code in the <code>/examples/tutorial/load_progress/</code> directory in the Native
+Client SDK download.
+</aside>
+<section id="module-loading-and-progress-events">
+<h2 id="module-loading-and-progress-events">Module loading and progress events</h2>
+<p>The Native Client runtime reports a set of state changes during the module
+loading process by means of DOM progress events. This set of events is a direct
+port of the proposed W3C <a class="reference external" href="http://www.w3.org/TR/progress-events/">Progress Events</a> standard (except for the <code>crash</code>
+event which is an extension of the W3C standard). The following table lists the
+events types reported by the Native Client runtime:</p>
+<table border="1" class="docutils">
+<colgroup>
+</colgroup>
+<thead valign="bottom">
+<tr class="row-odd"><th class="head">Event</th>
+<th class="head">Description</th>
+<th class="head">Number of
+times
+triggered</th>
+<th class="head">When event is
+triggered</th>
+<th class="head">How you might
+react to
+event</th>
+</tr>
+</thead>
+<tbody valign="top">
+<tr class="row-even"><td><code>loadstart</code></td>
+<td>Native Client has
+started to load a
+Native Client
+module.</td>
+<td>once</td>
+<td>This is the
+first
+progress
+event
+triggered
+after the
+Native Client
+module is
+instantiated
+and
+initialized.</td>
+<td>Display a
+status
+message, such
+as
+&#8220;Loading...&#8221;</td>
+</tr>
+<tr class="row-odd"><td><code>progress</code></td>
+<td>Part of the module
+has been loaded.</td>
+<td>zero or
+more</td>
+<td>After
+<code>loadstart</code>
+has been
+dispatched.</td>
+<td>Display a
+progress bar.</td>
+</tr>
+<tr class="row-even"><td><code>error</code></td>
+<td>The Native Client
+module failed to
+start execution
+(includes any
+error before or
+during
+initialization of
+the module). The
+<code>lastError</code>
+attribute
+(mentioned later)
+provides details
+on the error
+(initialization
+failed, sel_ldr
+did not start,
+and so on).</td>
+<td>zero or
+once</td>
+<td>After the
+last
+<code>progress</code>
+event has
+been
+dispatched,
+or after
+<code>loadstart</code>
+if no
+<code>progress</code>
+event was
+dispatched.</td>
+<td>Inform user
+that the
+application
+failed to
+load.</td>
+</tr>
+<tr class="row-odd"><td><code>abort</code></td>
+<td>Loading of the
+Native Client
+module was
+aborted by the
+user.</td>
+<td>zero or
+once</td>
+<td>After the
+last
+<code>progress</code>
+event has
+been
+dispatched,
+or after
+<code>loadstart</code>
+if no
+<code>progress</code>
+event was
+dispatched.</td>
+<td>It&#8217;s not
+likely you
+will want to
+respond to
+this event.</td>
+</tr>
+<tr class="row-even"><td><code>load</code></td>
+<td>The Native Client
+module was
+successfully
+loaded, and
+execution was
+started. (The
+module was
+initialized
+successfully.)</td>
+<td>zero or
+once</td>
+<td>After the
+last
+<code>progress</code>
+event has
+been
+dispatched,
+or after
+<code>loadstart</code>
+if no
+<code>progress</code>
+event was
+dispatched.</td>
+<td>Remove the
+progress bar.</td>
+</tr>
+<tr class="row-odd"><td><code>loadend</code></td>
+<td>Loading of the
+Native Client
+module has
+stopped. Load
+succeeded
+(<code>load</code>),
+failed
+(<code>error</code>), or
+was aborted
+(<code>abort</code>).</td>
+<td>once</td>
+<td>After an
+<code>error</code>,
+<code>abort</code>, or
+<code>load</code>
+event was
+dispatched.</td>
+<td>Indicate
+loading is
+over
+(regardless
+of failure or
+not).</td>
+</tr>
+<tr class="row-even"><td><code>crash</code></td>
+<td>The Native Client
+module is not
+responding (died
+on an
+<code>assert()</code> or
+<code>exit()</code>) after
+a successful
+load. This event
+is unique to
+Native Client and
+is not part of
+the W3C Progress
+Events standard.
+The <code>exitStatus</code>
+attribute provides
+the numeric exit
+status value.</td>
+<td>zero or
+once</td>
+<td>After a
+<code>loadend</code>.</td>
+<td>Notify user
+that the
+module did
+something
+illegal.</td>
+</tr>
+</tbody>
+</table>
+<p>The sequence of events for a successful module load is as follows:</p>
+<table border="1" class="docutils">
+<colgroup>
+</colgroup>
+<thead valign="bottom">
+<tr class="row-odd"><th class="head">Event is dispatched</th>
+<th class="head">... then this task is attempted</th>
+</tr>
+</thead>
+<tbody valign="top">
+<tr class="row-even"><td><code>loadstart</code></td>
+<td>load the manifest file</td>
+</tr>
+<tr class="row-odd"><td><code>progress</code> (first time)</td>
+<td>load the module</td>
+</tr>
+<tr class="row-even"><td><code>progress</code> (subsequent times)</td>
+<td>&nbsp;</td>
+</tr>
+<tr class="row-odd"><td><code>load</code></td>
+<td>start executing the module</td>
+</tr>
+<tr class="row-even"><td><code>loadend</code></td>
+<td>&nbsp;</td>
+</tr>
+</tbody>
+</table>
+<p>Errors that occur during loading are logged to the JavaScript console in Google
+Chrome (select the menu icon <img alt="menu-icon" src="/native-client/images/menu-icon.png" /> &gt; Tools &gt; JavaScript console).</p>
+</section><section id="handling-progress-events">
+<h2 id="handling-progress-events">Handling progress events</h2>
+<p>You should add event listeners in a <code>&lt;script&gt;</code> element to listen for these
+events before the <code>&lt;embed&gt;</code> element is parsed. For example, the following code
+adds a listener for the <code>load</code> event to a parent <code>&lt;div&gt;</code> element that also
+contains the Native Client <code>&lt;embed&gt;</code> element. First, the listener is
+attached. Then, when the listener <code>&lt;div&gt;</code> receives the <code>load</code> event, the
+JavaScript <code>moduleDidLoad()</code> function is called. The following code is
+excerpted from the example in <code>getting_started/part1/</code>:</p>
+<pre class="prettyprint">
+&lt;!--
+Load the published pexe.
+Note: Since this module does not use any real-estate in the browser, its
+width and height are set to 0.
+
+Note: The &lt;embed&gt; element is wrapped inside a &lt;div&gt;, which has both a 'load'
+and a 'message' event listener attached.  This wrapping method is used
+instead of attaching the event listeners directly to the &lt;embed&gt; element to
+ensure that the listeners are active before the NaCl module 'load' event
+fires.  This also allows you to use PPB_Messaging.PostMessage() (in C) or
+pp::Instance.PostMessage() (in C++) from within the initialization code in
+your module.
+--&gt;
+&lt;div id=&quot;listener&quot;&gt;
+  &lt;script type=&quot;text/javascript&quot;&gt;
+    var listener = document.getElementById('listener');
+    listener.addEventListener('load', moduleDidLoad, true);
+    listener.addEventListener('message', handleMessage, true);
+  &lt;/script&gt;
+
+  &lt;embed id=&quot;hello_tutorial&quot;
+         width=0 height=0
+         src=&quot;hello_tutorial.nmf&quot;
+         type=&quot;application/x-pnacl&quot; /&gt;
+&lt;/div&gt;
+</pre>
+<p>Event listeners can be added to any DOM object. Since listeners set at the
+outermost scope capture events for their contained elements, you can set
+listeners on outer elements (including the <code>&lt;body&gt;</code> element) to handle events
+from inner elements. For more information, see the W3 specifications for <a class="reference external" href="http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-flow-capture">event
+flow capture</a> and
+<a class="reference external" href="http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-registration">event listener registration</a>.</p>
+</section><section id="displaying-load-status">
+<h2 id="displaying-load-status">Displaying load status</h2>
+<p>One common response to progress events is to display the percentage of the
+module that has been loaded. In the load_progress example, when the <code>progress</code>
+event is triggered the <code>moduleLoadProgress</code> function is called. This function
+uses the <code>lengthComputable</code>, <code>loaded</code>, and <code>total</code> attributes (described
+in the proposed W3C <a class="reference external" href="http://www.w3.org/TR/progress-events/">Progress Events</a>
+standard) of the event to calculate the percentage of the module that has
+loaded.</p>
+<pre class="prettyprint">
+function moduleLoadProgress(event) {
+  var loadPercent = 0.0;
+  var loadPercentString;
+  if (event.lengthComputable &amp;&amp; event.total &gt; 0) {
+    loadPercent = event.loaded / event.total * 100.0;
+    loadPercentString = loadPercent + '%';
+    common.logMessage('progress: ' + event.url + ' ' + loadPercentString +
+                     ' (' + event.loaded + ' of ' + event.total + ' bytes)');
+  } else {
+    // The total length is not yet known.
+    common.logMessage('progress: Computing...');
+  }
+}
+</pre>
+</section><section id="the-lasterror-attribute">
+<h2 id="the-lasterror-attribute">The <code>lastError</code> attribute</h2>
+<p>The <code>&lt;embed&gt;</code> element has a <code>lastError</code> attribute that is set to an
+informative string whenever a load failure (an <code>error</code> or <code>abort</code> event)
+occurs.</p>
+<p>The following code adds an event listener before the <code>&lt;embed&gt;</code> element to
+capture and handle an error in loading the Native Client module. The
+<code>handleError()</code> function listens for an <code>error</code> event. When an error occurs,
+this function prints the contents of the <code>lastError</code> attribute
+(<code>embed_element.lastError</code>) as an alert.</p>
+<pre class="prettyprint">
+function domContentLoaded(name, tc, config, width, height) {
+  var listener = document.getElementById('listener');
+  ...
+  listener.addEventListener('error', moduleLoadError, true);
+  ...
+  common.createNaClModule(name, tc, config, width, height);
+}
+
+function moduleLoadError() {
+  common.logMessage('error: ' + common.naclModule.lastError);
+}
+</pre>
+</section><section id="the-readystate-attribute">
+<h2 id="the-readystate-attribute">The <code>readyState</code> attribute</h2>
+<p>You can use the <code>readyState</code> attribute to monitor the loading process. This
+attribute is particularly useful if you don&#8217;t care about the details of
+individual progress events or when you want to poll for current load state
+without registering listeners. The value of <code>readyState</code> progresses as follows
+for a successful load:</p>
+<table border="1" class="docutils">
+<colgroup>
+</colgroup>
+<thead valign="bottom">
+<tr class="row-odd"><th class="head">Event</th>
+<th class="head"><code>readyState</code> value</th>
+</tr>
+</thead>
+<tbody valign="top">
+<tr class="row-even"><td>(before any events)</td>
+<td><code>undefined</code></td>
+</tr>
+<tr class="row-odd"><td><code>loadstart</code></td>
+<td>1</td>
+</tr>
+<tr class="row-even"><td><code>progress</code></td>
+<td>3</td>
+</tr>
+<tr class="row-odd"><td><code>load</code></td>
+<td>4</td>
+</tr>
+<tr class="row-even"><td><code>loadend</code></td>
+<td>4</td>
+</tr>
+</tbody>
+</table>
+<p>The following code demonstrates how to monitor the loading process using the
+<code>readyState</code> attribute. As before, the script that adds the event listeners
+precedes the <code>&lt;embed&gt;</code> element so that the event listeners are in place before
+the progress events are generated.</p>
+<pre class="prettyprint">
+&lt;html&gt;
+...
+  &lt;body id=&quot;body&quot;&gt;
+    &lt;div id=&quot;status_div&quot;&gt;
+    &lt;/div&gt;
+    &lt;div id=&quot;listener_div&quot;&gt;
+      &lt;script type=&quot;text/javascript&quot;&gt;
+         var stat = document.getElementById('status_div');
+         function handleEvent(e) {
+           var embed_element = document.getElementById('my_embed');
+           stat.innerHTML +=
+           '&lt;br&gt;' + e.type + ': readyState = ' + embed_element.readyState;
+         }
+         var listener_element = document.getElementById('listener_div');
+         listener_element.addEventListener('loadstart', handleEvent, true);
+         listener_element.addEventListener('progress', handleEvent, true);
+         listener_element.addEventListener('load', handleEvent, true);
+         listener_element.addEventListener('loadend', handleEvent, true);
+      &lt;/script&gt;
+      &lt;embed
+        name=&quot;naclModule&quot;
+        id=&quot;my_embed&quot;
+        width=0 height=0
+        src=&quot;my_example.nmf&quot;
+        type=&quot;application/x-pnacl&quot; /&gt;
+    &lt;/div&gt;
+  &lt;/body&gt;
+&lt;/html&gt;
+</pre>
+</section><section id="the-exitstatus-attribute">
+<h2 id="the-exitstatus-attribute">The <code>exitStatus</code> attribute</h2>
+<p>This read-only attribute is set if the application calls <code>exit(n)</code>,
+<code>abort()</code>, or crashes. Since NaCl modules are event handlers, there is no
+need to call <code>exit(n)</code> in normal execution. If the module does exit or
+crash, the <code>crash</code> progress event is issued and the <code>exitStatus</code> attribute
+will contain the numeric value of the exit status:</p>
+<ul class="small-gap">
+<li>In the case of explicit calls to <code>exit(n)</code>, the numeric value will be
+<code>n</code> (between 0 and 255).</li>
+<li>In the case of crashes and calls to <code>abort()</code>, the numeric value will
+be non-zero, but the exact value will depend on the chosen libc and the
+target architecture, and may change in the future. Applications should not
+rely on the <code>exitStatus</code> value being stable in these cases, but the value
+may nevertheless be useful for temporary debugging.</li>
+</ul>
+</section></section>
+
+{{/partials.standard_nacl_article}}