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<div class="section" id="the-mathjax-callback-signal-class">
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<span id="api-signal"></span><h1>The MathJax.Callback.Signal Class<a class="headerlink" href="#the-mathjax-callback-signal-class" title="Permalink to this headline">¶</a></h1>
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<p>The <tt class="docutils literal"><span class="pre">MathJax.Callback.Signal</span></tt> object is one of the key mechanisms
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used by MathJax to synchronize its actions with those that occur
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asynchronously, like loading files and stylesheets. A <cite>Signal</cite> object
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is used to pulicise the fact that MathJax has performed certain
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actions, giving other code running the the web page the chance to
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react to those actions. See <a class="reference internal" href="../synchronize.html#synchronization"><em>Synchronizing with MathJax</em></a> for more details, and <a class="reference internal" href="../signals.html#using-signals"><em>Using Signals</em></a> in particular for examples of how to specify and use
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MathJax <cite>Signal</cite> objects.</p>
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<p>The <cite>Callback Signal</cite> object is a subclass of the <a class="reference internal" href="queue.html#api-queue"><em>Callback Queue</em></a> object.</p>
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<div class="section" id="properties">
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<h2>Properties<a class="headerlink" href="#properties" title="Permalink to this headline">¶</a></h2>
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<dl class="describe">
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<dt>
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<tt class="descname">name</tt></dt>
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<dd><p>The name of the signal. Each signal is named so that
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various components can access it. The first one to request a
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particular signal causes it to be created, and other requests for
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the signal return references to the same object.</p>
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</dd></dl>
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<dl class="describe">
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<dt>
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<tt class="descname">posted</tt></dt>
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<dd><p>Array used internally to stored the post history so that when new
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listeners express interests in this signal, they can be informed
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of the signals that have been posted so far. This can be cleared
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using the signal’s <a class="reference internal" href="message.html#Clear" title="Clear"><tt class="xref py py-meth docutils literal"><span class="pre">Clear()</span></tt></a> method.</p>
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</dd></dl>
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<dl class="describe">
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<dt>
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<tt class="descname">listeners</tt></dt>
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<dd><p>Array of callbacks to the listeners who have expressed interest in
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hearing about posts to this signal. When a post occurs, the
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listeners are called in each turn, passing them the message that
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was posted.</p>
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</dd></dl>
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</div>
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<div class="section" id="methods">
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<h2>Methods<a class="headerlink" href="#methods" title="Permalink to this headline">¶</a></h2>
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<dl class="method">
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<dt id="Post">
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<tt class="descname">Post</tt><big>(</big><em>message</em><span class="optional">[</span>, <em>callback</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#Post" title="Permalink to this definition">¶</a></dt>
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<dd><p>Posts a message to all the listeners for the signal. The listener
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callbacks are called in turn (with the message as an argument),
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and if any return a <cite>Callback</cite> object, the posting will be
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suspended until the callback is exectured. In this way, the
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<a class="reference internal" href="#Post" title="Post"><tt class="xref py py-meth docutils literal"><span class="pre">Post()</span></tt></a> call can operate asynchronously, and so the
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<cite>callback</cite> parameter is used to synchronize with its operation;
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the <cite>callback</cite> will be called when all the listeners have responded
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to the post.</p>
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<p>If a <a class="reference internal" href="#Post" title="Post"><tt class="xref py py-meth docutils literal"><span class="pre">Post()</span></tt></a> to this signal occurs while waiting for the
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response from a listener (either because a listener returned a
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<cite>Callback</cite> object and we are waiting for it to complete when the
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<a class="reference internal" href="#Post" title="Post"><tt class="xref py py-meth docutils literal"><span class="pre">Post()</span></tt></a> occurred, or because the listener itself called the
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<tt class="docutils literal"><span class="pre">Post()</span></tt> method), the new message will be queued and will be
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posted after the current message has been sent to all the
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listeners, and they have all responded. This is another way in
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which posting can be asynchronous; the only sure way to know that
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a posting has occurred is through its <cite>callback</cite>. When the posting
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is complete, the callback is called, passing it the signal object
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that has just completed.</p>
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<p>Returns the callback object (or a blank callback object if none
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was provided).</p>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Parameters :</th><td class="field-body"><ul class="first simple">
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<li><strong>message</strong> — the message to send through the signal</li>
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<li><strong>callback</strong> — called after the message is posted</li>
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</ul>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Returns :</th><td class="field-body"><p class="first last">the callback or a blank callback</p>
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</td>
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</tr>
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</tbody>
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</table>
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</dd></dl>
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<dl class="method">
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<dt>
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<tt class="descname">Clear</tt><big>(</big><span class="optional">[</span><em>callback</em><span class="optional">]</span><big>)</big></dt>
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<dd><p>This causes the history of past messages to be cleared so new
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listeners will not receive them. Note that since the signal may
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be operating asynchronously, the <a class="reference internal" href="message.html#Clear" title="Clear"><tt class="xref py py-meth docutils literal"><span class="pre">Clear()</span></tt></a> may be queued for
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later. In this way, the <a class="reference internal" href="#Post" title="Post"><tt class="xref py py-meth docutils literal"><span class="pre">Post()</span></tt></a> and <a class="reference internal" href="message.html#Clear" title="Clear"><tt class="xref py py-meth docutils literal"><span class="pre">Clear()</span></tt></a>
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operations will be performed in the proper order even when they
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are delayed. The <cite>callback</cite> is called when the <a class="reference internal" href="message.html#Clear" title="Clear"><tt class="xref py py-meth docutils literal"><span class="pre">Clear()</span></tt></a>
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operation is completed.</p>
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<p>Returns the callback (or a blank callback if none is provided).</p>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Parameters :</th><td class="field-body"><ul class="first simple">
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<li><strong>callback</strong> — called after the signal history is cleared</li>
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</ul>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Returns :</th><td class="field-body"><p class="first last">the callback or a blank callback</p>
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</td>
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</tr>
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</tbody>
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</table>
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</dd></dl>
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<dl class="method">
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<dt id="Interest">
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<tt class="descname">Interest</tt><big>(</big><em>callback</em><span class="optional">[</span>, <em>ignorePast</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#Interest" title="Permalink to this definition">¶</a></dt>
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<dd><p>This method registers a new listener on the signal. It creates a
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<cite>Callback</cite> object from the callback specification, attaches it to
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the signal, and returns that <cite>Callback</cite> object. When new messages
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are posted to the signal, it runs the callback, passing it the
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message that was posted. If the callback itself returns a
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<cite>Callback</cite> object, that indicates that the listener has started an
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asynchronous operation and the poster should wait for that
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callback to complete before allowing new posts on the signal.</p>
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<p>If <cite>ignorePast</cite> is <tt class="docutils literal"><span class="pre">false</span></tt> or not present, then before
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<a class="reference internal" href="#Interest" title="Interest"><tt class="xref py py-meth docutils literal"><span class="pre">Interest()</span></tt></a> returns, the callback will be called with all
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the past messages that have been sent to the signal.</p>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Parameters :</th><td class="field-body"><ul class="first simple">
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<li><strong>callback</strong> — called whenever a message is posted (past or present)</li>
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<li><strong>ignorePast</strong> — <tt class="docutils literal"><span class="pre">true</span></tt> means ignore previous messages</li>
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</ul>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Returns :</th><td class="field-body"><p class="first last">the callback object</p>
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</td>
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</tr>
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</tbody>
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</table>
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</dd></dl>
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<dl class="method">
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<dt id="NoInterest">
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<tt class="descname">NoInterest</tt><big>(</big><em>callback</em><big>)</big><a class="headerlink" href="#NoInterest" title="Permalink to this definition">¶</a></dt>
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<dd><p>This removes a listener from the signal so that no new messages
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will be sent to it. The callback should be the one returned by
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the original <a class="reference internal" href="#Interest" title="Interest"><tt class="xref py py-meth docutils literal"><span class="pre">Interest()</span></tt></a> call that attached the listener to
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the signal in the first place. Once removed, the listener will no
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longer receive messages from the signal.</p>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Parameters :</th><td class="field-body"><ul class="first simple">
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<li><strong>callback</strong> — the listener to be removed from signal</li>
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</ul>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Returns :</th><td class="field-body"><p class="first last"><tt class="docutils literal"><span class="pre">null</span></tt></p>
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</td>
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</tr>
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</tbody>
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</table>
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</dd></dl>
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<dl class="method">
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<dt id="MessageHook">
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<tt class="descname">MessageHook</tt><big>(</big><em>message</em>, <em>callback</em><big>)</big><a class="headerlink" href="#MessageHook" title="Permalink to this definition">¶</a></dt>
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<dd><p>This creates a callback that is called whenever the signal posts
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the given message. This is a little easier than having to write a
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function that must check the message each time it is called.
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Although the <cite>message</cite> here is a string, if a message posted to the
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signal is an array, then only the first element of that array is
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used to match against message. That way, if a message contains an
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identifier plus arguments, the hook will match the identifier and
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still get called with the complete set of arguments.</p>
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<p>Returns the <cite>Callback</cite> object that was produced.</p>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Parameters :</th><td class="field-body"><ul class="first simple">
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<li><strong>message</strong> — the message to look for from the signal</li>
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<li><strong>callback</strong> — called when the message is posted</li>
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</ul>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Returns :</th><td class="field-body"><p class="first last">the callback object</p>
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</td>
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</tr>
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</tbody>
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</table>
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</dd></dl>
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<dl class="method">
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<dt id="ExecuteHook">
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<tt class="descname">ExecuteHook</tt><big>(</big><em>message</em><big>)</big><a class="headerlink" href="#ExecuteHook" title="Permalink to this definition">¶</a></dt>
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<dd><p>Used internally to call the listeners when a particular
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message is posted to the signal.</p>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Parameters :</th><td class="field-body"><ul class="first simple">
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<li><strong>message</strong> — the posted message</li>
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</ul>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Returns :</th><td class="field-body"><p class="first last"><tt class="docutils literal"><span class="pre">null</span></tt></p>
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</td>
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</tr>
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</tbody>
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</table>
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</dd></dl>
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<h3><a href="../index.html">Table Of Contents</a></h3>
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<ul>
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<li><a class="reference internal" href="#">The MathJax.Callback.Signal Class</a><ul>
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<li><a class="reference internal" href="#properties">Properties</a></li>
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<li><a class="reference internal" href="#methods">Methods</a></li>
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</ul>
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title="previous chapter">The MathJax.Callback.Queue Class</a></p>
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<h4>Next topic</h4>
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