native_client_sdk/doc_generated/devguide/coding/progress-events.html

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   3 <section id="progress-events">
   4 <span id="devcycle-progress-events"></span><h1 id="progress-events"><span id="devcycle-progress-events"></span>Progress Events</h1>
   5 <div class="contents local" id="contents" style="display: none">
   6 <ul class="small-gap">
   7 <li><a class="reference internal" href="#module-loading-and-progress-events" id="id3">Module loading and progress events</a></li>
   8 <li><a class="reference internal" href="#handling-progress-events" id="id4">Handling progress events</a></li>
   9 <li><a class="reference internal" href="#displaying-load-status" id="id5">Displaying load status</a></li>
  10 <li><a class="reference internal" href="#the-lasterror-attribute" id="id6">The <code>lastError</code> attribute</a></li>
  11 <li><a class="reference internal" href="#the-readystate-attribute" id="id7">The <code>readyState</code> attribute</a></li>
  12 <li><a class="reference internal" href="#the-exitstatus-attribute" id="id8">The <code>exitStatus</code> attribute</a></li>
  13 </ul>
  14
  15 </div><p>There are five types of events that developers can respond to in Native Client:
  16 progress, message, view change, focus, and input events (each described in the
  17 glossary below). This section describes how to monitor progress events (events
  18 that occur during the loading and execution of a Native Client module). This
  19 section assumes you are familiar with the material presented in the
  20 <a class="reference internal" href="/native-client/overview.html"><em>Technical Overview</em></a>.</p>
  21 <aside class="note">
  22 The load_progress example illustrates progress event handling. You can find
  23 this code in the <code>/pepper_&lt;version&gt;/examples/tutorial/load_progress/</code>
  24 directory in the Native Client SDK download.
  25 </aside>
  26 <h2 id="module-loading-and-progress-events">Module loading and progress events</h2>
  27 <p>The Native Client runtime reports a set of state changes during the module
  28 loading process by means of DOM progress events. This set of events is a direct
  29 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>
  30 event which is an extension of the W3C standard). The following table lists the
  31 events types reported by the Native Client runtime:</p>
  32 <table border="1" class="docutils">
  33 <colgroup>
  34 </colgroup>
  35 <thead valign="bottom">
  36 <tr class="row-odd"><th class="head">Event</th>
  37 <th class="head">Times
  38 triggered</th>
  39 <th class="head">When
  40 triggered</th>
  41 <th class="head">How you might
  42 respond</th>
  43 </tr>
  44 </thead>
  45 <tbody valign="top">
  46 <tr class="row-even"><td><dl class="first last docutils">
  47 <dt><code>loadstart</code></dt>
  48 <dd>Native Client has started to
  49 load a Native Client module.</dd>
  50 </dl>
  51 </td>
  52 <td>once</td>
  53 <td>The
  54 first
  55 progress
  56 event
  57 after the
  58 Native Client
  59 module is
  60 instantiated
  61 and
  62 initialized.</td>
  63 <td>Display a
  64 status
  65 message, such
  66 as
  67 &#8220;Loading...&#8221;</td>
  68 </tr>
  69 <tr class="row-odd"><td><dl class="first last docutils">
  70 <dt><code>progress</code></dt>
  71 <dd>Part of the module has been
  72 loaded.</dd>
  73 </dl>
  74 </td>
  75 <td>zero or
  76 more</td>
  77 <td>After
  78 <code>loadstart</code>
  79 has been
  80 dispatched.</td>
  81 <td>Display a
  82 progress bar.</td>
  83 </tr>
  84 <tr class="row-even"><td><dl class="first last docutils">
  85 <dt><code>error</code></dt>
  86 <dd>The Native Client module failed
  87 to start execution (includes any
  88 error before or during
  89 initialization of the module).
  90 The <code>lastError</code> attribute
  91 (mentioned later) provides
  92 details on the error
  93 (initialization failed, sel_ldr
  94 did not start, and so on).</dd>
  95 </dl>
  96 </td>
  97 <td>zero or
  98 once</td>
  99 <td>After the last
 100 <code>progress</code>
 101 event has been
 102 dispatched,
 103 or after
 104 <code>loadstart</code>
 105 if no
 106 <code>progress</code>
 107 event was
 108 dispatched.</td>
 109 <td>Inform user
 110 that the
 111 application
 112 failed to
 113 load.</td>
 114 </tr>
 115 <tr class="row-odd"><td><dl class="first last docutils">
 116 <dt><code>abort</code></dt>
 117 <dd>Loading of the NativeClient
 118 module was aborted by the user.</dd>
 119 </dl>
 120 </td>
 121 <td>zero or
 122 once</td>
 123 <td>After the last
 124 <code>progress</code>
 125 event has been
 126 dispatched, or
 127 after
 128 <code>loadstart</code>
 129 if no
 130 <code>progress</code>
 131 event was
 132 dispatched.</td>
 133 <td>It&#8217;s not
 134 likely you
 135 will want to
 136 respond to
 137 this event.</td>
 138 </tr>
 139 <tr class="row-even"><td><dl class="first last docutils">
 140 <dt><code>load</code></dt>
 141 <dd>The Native Client module was
 142 successfully loaded, and
 143 execution was started.
 144 (The module was initialized
 145 successfully.)</dd>
 146 </dl>
 147 </td>
 148 <td>zero or
 149 once</td>
 150 <td>After the
 151 last
 152 <code>progress</code>
 153 event has been
 154 dispatched, or
 155 after
 156 <code>loadstart</code>
 157 if no
 158 <code>progress</code>
 159 event was
 160 dispatched.</td>
 161 <td>Remove the
 162 progress bar.</td>
 163 </tr>
 164 <tr class="row-odd"><td><dl class="first last docutils">
 165 <dt><code>loadend</code></dt>
 166 <dd>Loading of the Native Client
 167 module has stopped. Load
 168 succeeded (<code>load</code>), failed
 169 (<code>error</code>), or was aborted
 170 (<code>abort</code>).</dd>
 171 </dl>
 172 </td>
 173 <td>once</td>
 174 <td>After an
 175 <code>error</code>,
 176 <code>abort</code>, or
 177 <code>load</code>
 178 event was
 179 dispatched.</td>
 180 <td>Indicate
 181 loading is
 182 over
 183 (regardless of
 184 failure or
 185 not).</td>
 186 </tr>
 187 <tr class="row-even"><td><dl class="first last docutils">
 188 <dt><code>crash</code></dt>
 189 <dd>The Native Client module is not
 190 responding (died on an
 191 <code>assert()</code> or <code>exit()</code>)
 192 after a successful load. This
 193 event is unique to Native Client
 194 and is not part of the W3C
 195 Progress Events standard. The
 196 <code>exitStatus</code> attribute
 197 provides the numeric exit
 198 status.</dd>
 199 </dl>
 200 </td>
 201 <td>zero or
 202 once</td>
 203 <td>After a
 204 <code>loadend</code>.</td>
 205 <td>Notify user
 206 that the
 207 module did
 208 something
 209 illegal.</td>
 210 </tr>
 211 </tbody>
 212 </table>
 213 <p>The sequence of events for a successful module load is as follows:</p>
 214 <table border="1" class="docutils">
 215 <colgroup>
 216 </colgroup>
 217 <thead valign="bottom">
 218 <tr class="row-odd"><th class="head">Event is dispatched</th>
 219 <th class="head">... then this task is attempted</th>
 220 </tr>
 221 </thead>
 222 <tbody valign="top">
 223 <tr class="row-even"><td><code>loadstart</code></td>
 224 <td>load the manifest file</td>
 225 </tr>
 226 <tr class="row-odd"><td><code>progress</code> (first time)</td>
 227 <td>load the module</td>
 228 </tr>
 229 <tr class="row-even"><td><code>progress</code> (subsequent times)</td>
 230 <td>&nbsp;</td>
 231 </tr>
 232 <tr class="row-odd"><td><code>load</code></td>
 233 <td>start executing the module</td>
 234 </tr>
 235 <tr class="row-even"><td><code>loadend</code></td>
 236 <td>&nbsp;</td>
 237 </tr>
 238 </tbody>
 239 </table>
 240 <p>Errors that occur during loading are logged to the JavaScript console in Google
 241 Chrome (select the menu icon <img alt="menu-icon" src="/native-client/images/menu-icon.png" /> &gt; Tools &gt; JavaScript console).</p>
 242 <h2 id="handling-progress-events">Handling progress events</h2>
 243 <p>You should add event listeners in a <code>&lt;script&gt;</code> element to listen for these
 244 events before the <code>&lt;embed&gt;</code> element is parsed. For example, the following code
 245 adds a listener for the <code>load</code> event to a parent <code>&lt;div&gt;</code> element that also
 246 contains the Native Client <code>&lt;embed&gt;</code> element. First, the listener is
 247 attached. Then, when the listener <code>&lt;div&gt;</code> receives the <code>load</code> event, the
 248 JavaScript <code>moduleDidLoad()</code> function is called. The following code is
 249 excerpted from the example in <code>getting_started/part1/</code>:</p>
 250 <pre class="prettyprint">
 251 &lt;!--
 252 Load the published pexe.
 253 Note: Since this module does not use any real-estate in the browser, its
 254 width and height are set to 0.
 255
 256 Note: The &lt;embed&gt; element is wrapped inside a &lt;div&gt;, which has both a 'load'
 257 and a 'message' event listener attached.  This wrapping method is used
 258 instead of attaching the event listeners directly to the &lt;embed&gt; element to
 259 ensure that the listeners are active before the NaCl module 'load' event
 260 fires.  This also allows you to use PPB_Messaging.PostMessage() (in C) or
 261 pp::Instance.PostMessage() (in C++) from within the initialization code in
 262 your module.
 263 --&gt;
 264 &lt;div id=&quot;listener&quot;&gt;
 265   &lt;script type=&quot;text/javascript&quot;&gt;
 266     var listener = document.getElementById('listener');
 267     listener.addEventListener('load', moduleDidLoad, true);
 268     listener.addEventListener('message', handleMessage, true);
 269   &lt;/script&gt;
 270
 271   &lt;embed id=&quot;hello_tutorial&quot;
 272          width=0 height=0
 273          src=&quot;hello_tutorial.nmf&quot;
 274          type=&quot;application/x-pnacl&quot; /&gt;
 275 &lt;/div&gt;
 276 </pre>
 277 <p>Event listeners can be added to any DOM object. Since listeners set at the
 278 outermost scope capture events for their contained elements, you can set
 279 listeners on outer elements (including the <code>&lt;body&gt;</code> element) to handle events
 280 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
 281 flow capture</a> and
 282 <a class="reference external" href="http://www.w3.org/TR/DOM-Level-2-Events/events.html#Events-registration">event listener registration</a>.</p>
 283 <h2 id="displaying-load-status">Displaying load status</h2>
 284 <p>One common response to progress events is to display the percentage of the
 285 module that has been loaded. In the load_progress example, when the <code>progress</code>
 286 event is triggered the <code>moduleLoadProgress</code> function is called. This function
 287 uses the <code>lengthComputable</code>, <code>loaded</code>, and <code>total</code> attributes (described
 288 in the proposed W3C <a class="reference external" href="http://www.w3.org/TR/progress-events/">Progress Events</a>
 289 standard) of the event to calculate the percentage of the module that has
 290 loaded.</p>
 291 <pre class="prettyprint">
 292 function moduleLoadProgress(event) {
 293   var loadPercent = 0.0;
 294   var loadPercentString;
 295   if (event.lengthComputable &amp;&amp; event.total &gt; 0) {
 296     loadPercent = event.loaded / event.total * 100.0;
 297     loadPercentString = loadPercent + '%';
 298     common.logMessage('progress: ' + event.url + ' ' + loadPercentString +
 299                      ' (' + event.loaded + ' of ' + event.total + ' bytes)');
 300   } else {
 301     // The total length is not yet known.
 302     common.logMessage('progress: Computing...');
 303   }
 304 }
 305 </pre>
 306 <h2 id="the-lasterror-attribute">The <code>lastError</code> attribute</h2>
 307 <p>The <code>&lt;embed&gt;</code> element has a <code>lastError</code> attribute that is set to an
 308 informative string whenever a load failure (an <code>error</code> or <code>abort</code> event)
 309 occurs.</p>
 310 <p>The following code adds an event listener before the <code>&lt;embed&gt;</code> element to
 311 capture and handle an error in loading the Native Client module. The
 312 <code>handleError()</code> function listens for an <code>error</code> event. When an error occurs,
 313 this function prints the contents of the <code>lastError</code> attribute
 314 (<code>embed_element.lastError</code>) as an alert.</p>
 315 <pre class="prettyprint">
 316 function domContentLoaded(name, tc, config, width, height) {
 317   var listener = document.getElementById('listener');
 318   ...
 319   listener.addEventListener('error', moduleLoadError, true);
 320   ...
 321   common.createNaClModule(name, tc, config, width, height);
 322 }
 323
 324 function moduleLoadError() {
 325   common.logMessage('error: ' + common.naclModule.lastError);
 326 }
 327 </pre>
 328 <h2 id="the-readystate-attribute">The <code>readyState</code> attribute</h2>
 329 <p>You can use the <code>readyState</code> attribute to monitor the loading process. This
 330 attribute is particularly useful if you don&#8217;t care about the details of
 331 individual progress events or when you want to poll for current load state
 332 without registering listeners. The value of <code>readyState</code> progresses as follows
 333 for a successful load:</p>
 334 <table border="1" class="docutils">
 335 <colgroup>
 336 </colgroup>
 337 <thead valign="bottom">
 338 <tr class="row-odd"><th class="head">Event</th>
 339 <th class="head"><code>readyState</code> value</th>
 340 </tr>
 341 </thead>
 342 <tbody valign="top">
 343 <tr class="row-even"><td>(before any events)</td>
 344 <td><code>undefined</code></td>
 345 </tr>
 346 <tr class="row-odd"><td><code>loadstart</code></td>
 347 <td>1</td>
 348 </tr>
 349 <tr class="row-even"><td><code>progress</code></td>
 350 <td>3</td>
 351 </tr>
 352 <tr class="row-odd"><td><code>load</code></td>
 353 <td>4</td>
 354 </tr>
 355 <tr class="row-even"><td><code>loadend</code></td>
 356 <td>4</td>
 357 </tr>
 358 </tbody>
 359 </table>
 360 <p>The following code demonstrates how to monitor the loading process using the
 361 <code>readyState</code> attribute. As before, the script that adds the event listeners
 362 precedes the <code>&lt;embed&gt;</code> element so that the event listeners are in place before
 363 the progress events are generated.</p>
 364 <pre class="prettyprint">
 365 &lt;html&gt;
 366 ...
 367   &lt;body id=&quot;body&quot;&gt;
 368     &lt;div id=&quot;status_div&quot;&gt;
 369     &lt;/div&gt;
 370     &lt;div id=&quot;listener_div&quot;&gt;
 371       &lt;script type=&quot;text/javascript&quot;&gt;
 372          var stat = document.getElementById('status_div');
 373          function handleEvent(e) {
 374            var embed_element = document.getElementById('my_embed');
 375            stat.innerHTML +=
 376            '&lt;br&gt;' + e.type + ': readyState = ' + embed_element.readyState;
 377          }
 378          var listener_element = document.getElementById('listener_div');
 379          listener_element.addEventListener('loadstart', handleEvent, true);
 380          listener_element.addEventListener('progress', handleEvent, true);
 381          listener_element.addEventListener('load', handleEvent, true);
 382          listener_element.addEventListener('loadend', handleEvent, true);
 383       &lt;/script&gt;
 384       &lt;embed
 385         name=&quot;naclModule&quot;
 386         id=&quot;my_embed&quot;
 387         width=0 height=0
 388         src=&quot;my_example.nmf&quot;
 389         type=&quot;application/x-pnacl&quot; /&gt;
 390     &lt;/div&gt;
 391   &lt;/body&gt;
 392 &lt;/html&gt;
 393 </pre>
 394 <h2 id="the-exitstatus-attribute">The <code>exitStatus</code> attribute</h2>
 395 <p>This read-only attribute is set if the application calls <code>exit(n)</code>,
 396 <code>abort()</code>, or crashes. Since NaCl modules are event handlers, there is no
 397 need to call <code>exit(n)</code> in normal execution. If the module does exit or
 398 crash, the <code>crash</code> progress event is issued and the <code>exitStatus</code> attribute
 399 will contain the numeric value of the exit status:</p>
 400 <ul class="small-gap">
 401 <li>In the case of explicit calls to <code>exit(n)</code>, the numeric value will be
 402 <code>n</code> (between 0 and 255).</li>
 403 <li>In the case of crashes and calls to <code>abort()</code>, the numeric value will
 404 be non-zero, but the exact value will depend on the chosen libc and the
 405 target architecture, and may change in the future. Applications should not
 406 rely on the <code>exitStatus</code> value being stable in these cases, but the value
 407 may nevertheless be useful for temporary debugging.</li>
 408 </ul>
 409 </section>
 410
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