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<div class="section" id="the-mathjax-callback-class">
<span id="api-callback"></span><h1>The MathJax.Callback Class<a class="headerlink" href="#the-mathjax-callback-class" title="Permalink to this headline">¶</a></h1>
<p>The <code class="docutils literal"><span class="pre">MathJax.Callback</span></code> object is one of the key mechanisms used by
MathJax to synchronize its actions with those that occur
asynchronously, like loading files and stylesheets. A <cite>Callback</cite>
object is used to tie the execution of a function to the completion of
an asynchronous action. See <a class="reference internal" href="../advanced/synchronize.html#synchronization"><span class="std std-ref">Synchronizing with MathJax</span></a> for more details, and <a class="reference internal" href="../advanced/callbacks.html#using-callbacks"><span class="std std-ref">Using Callbacks</span></a> in particular for examples of how to specify and
use MathJax <cite>Callback</cite> objects.</p>
<div class="section" id="specifying-a-callback">
<h2>Specifying a callback<a class="headerlink" href="#specifying-a-callback" title="Permalink to this headline">¶</a></h2>
<p>When a method includes a callback as one of its arguments, that
callback can be specified in a number of different ways, depending on
the functionality that is required of the callback. The easiest case
is to simply provide a function to be called, but it is also possible
to include data to pass to the function when it is executed, and even
the object that will be used as the javascript <cite>this</cite> object when the
function is called.</p>
<p>Most functions that take callbacks as arguments accept a <cite>callback
specification</cite> rather than an actual callback object, though you can
use the <code class="xref py py-meth docutils literal"><span class="pre">MathJax.Callback()</span></code> function to convert a callback
specification into a Callback object if needed.</p>
<p>A callback specification is any one of the following:</p>
<blockquote>
<div><dl class="describe">
<dt>
<code class="descname">fn</code></dt>
<dd><p>A function that is to be called when the callback is executed.
No additional data is passed to it (other than what it is
called with at the time the callback is executed), and <cite>this</cite>
will be the window object.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">[fn]</code></dt>
<dd><p>An array containing a function to be called when the callback
is executed (as above).</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">[fn, data...]</code></dt>
<dd><p>An array containing a function together with data to be passed
to that function when the callback is executed; <cite>this</cite> is still
the window object. For example,</p>
<div class="highlight-javascript"><div class="highlight"><pre><span></span><span class="p">[</span><span class="kd">function</span> <span class="p">(</span><span class="nx">x</span><span class="p">,</span><span class="nx">y</span><span class="p">)</span> <span class="p">{</span><span class="k">return</span> <span class="nx">x</span><span class="o">+</span><span class="nx">y</span><span class="p">},</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
</pre></div>
</div>
<p>would specify a callback that would pass <code class="docutils literal"><span class="pre">2</span></code> and <code class="docutils literal"><span class="pre">3</span></code> to
the given function, and it would return their sum, <code class="docutils literal"><span class="pre">5</span></code>, when
the callback is executed.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">[object, fn]</code></dt>
<dd><p>An array containing an object to use as <cite>this</cite> and a function to
call for the callback. For example,</p>
<div class="highlight-javascript"><div class="highlight"><pre><span></span><span class="p">[{</span><span class="nx">x</span><span class="o">:</span><span class="s1">'foo'</span><span class="p">,</span> <span class="nx">y</span><span class="o">:</span><span class="s1">'bar'</span><span class="p">},</span> <span class="kd">function</span> <span class="p">()</span> <span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">x</span><span class="p">}]</span>
</pre></div>
</div>
<p>would produce a callback that returns the string <code class="docutils literal"><span class="pre">"foo"</span></code>
when it is called.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">[object, fn, data...]</code></dt>
<dd><p>Similar to the previous case, but with data that is passed to
the function as well.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">["method", object]</code></dt>
<dd><p>Here, <cite>object</cite> is an object that has a method called <cite>method</cite>, and
the callback will execute that method (with the object as
<cite>this</cite>) when it is called. For example,</p>
<div class="highlight-javascript"><div class="highlight"><pre><span></span><span class="p">[</span><span class="s2">"toString"</span><span class="p">,[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">]]</span>
</pre></div>
</div>
<p>would call the <cite>toString</cite> method on the array <code class="docutils literal"><span class="pre">[1,2,3,4]</span></code> when
the callback is called, returning <code class="docutils literal"><span class="pre">1,2,3,4</span></code>.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">["method", object, data...]</code></dt>
<dd><p>Similar to the previous case, but with data that is passed to
the method. E.g.,</p>
<div class="highlight-javascript"><div class="highlight"><pre><span></span><span class="p">[</span><span class="s2">"slice"</span><span class="p">,[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">],</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">]</span>
</pre></div>
</div>
<p>would perform the equivalent of <code class="docutils literal"><span class="pre">[1,2,3,4].slice(1,3)</span></code>,
which returns the array <code class="docutils literal"><span class="pre">[2,3]</span></code> as a result.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">{hook: fn, data: [...], object: this}</code></dt>
<dd><p>Here the data for the callback are given in an associative
array of <cite>key:value</cite> pairs. The value of <cite>hook</cite> is the
function to call, the value of <cite>data</cite> is an array of the
arguments to pass to the function, and the value of <cite>object</cite>
is the object to use as <cite>this</cite> in the function call. The
specification need not include all three <cite>key:value</cite> pairs; any
that are missing get default values (a function that does
nothing, an empty array, and the window object, respectively).</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">"string"</code></dt>
<dd><p>This specifies a callback where the string is executed via an
<code class="docutils literal"><span class="pre">eval()</span></code> statement. The code is run in the global context,
so any variables or functions created by the string become
part of the global namespace. The return value is the value of
the last statement executed in the string.</p>
</dd></dl>
</div></blockquote>
<div class="section" id="executing-a-callback-object">
<h3>Executing a Callback Object<a class="headerlink" href="#executing-a-callback-object" title="Permalink to this headline">¶</a></h3>
<p>The <cite>Callback</cite> object is itself a function, and calling that function
executes the callback. You can pass the callback additional
parameters, just as you can any function, and these will be added to
the callback function’s argument list following any data that was
supplied at the time the callback was created. For example</p>
<div class="highlight-javascript"><div class="highlight"><pre><span></span><span class="kd">var</span> <span class="nx">f</span> <span class="o">=</span> <span class="kd">function</span> <span class="p">(</span><span class="nx">x</span><span class="p">,</span><span class="nx">y</span><span class="p">)</span> <span class="p">{</span><span class="k">return</span> <span class="nx">x</span> <span class="o">+</span> <span class="s2">" and "</span> <span class="o">+</span><span class="nx">y</span><span class="p">}</span>
<span class="kd">var</span> <span class="nx">cb</span> <span class="o">=</span> <span class="nx">MathJax</span><span class="p">.</span><span class="nx">Callback</span><span class="p">([</span><span class="nx">f</span><span class="p">,</span> <span class="s2">"foo"</span><span class="p">]);</span>
<span class="kd">var</span> <span class="nx">result</span> <span class="o">=</span> <span class="nx">cb</span><span class="p">(</span><span class="s2">"bar"</span><span class="p">);</span> <span class="c1">// sets result to "foo and bar"</span>
</pre></div>
</div>
<p>Usually, the callback is not executed by the code that creates it (as
it is in the example above), but by some other code that runs at a
later time at the completion of some other activity (say the loading
of a file), or in response to a user action. For example:</p>
<div class="highlight-javascript"><div class="highlight"><pre><span></span><span class="kd">function</span> <span class="nx">f</span><span class="p">(</span><span class="nx">x</span><span class="p">)</span> <span class="p">{</span><span class="nx">alert</span><span class="p">(</span><span class="s2">"x contains "</span><span class="o">+</span><span class="nx">x</span><span class="p">)};</span>
<span class="kd">function</span> <span class="nx">DelayedX</span><span class="p">(</span><span class="nx">time</span><span class="p">)</span> <span class="p">{</span>
<span class="kd">var</span> <span class="nx">x</span> <span class="o">=</span> <span class="s2">"hi"</span><span class="p">;</span>
<span class="nx">setTimeout</span><span class="p">(</span><span class="nx">MathJax</span><span class="p">.</span><span class="nx">Callback</span><span class="p">([</span><span class="nx">f</span><span class="p">,</span> <span class="nx">x</span><span class="p">],</span> <span class="nx">time</span><span class="p">));</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">DelayedX</span></code> function arranges for the function <code class="docutils literal"><span class="pre">f</span></code> to be called at
a later time, passing it the value of a local variable, <code class="docutils literal"><span class="pre">x</span></code>. Normally,
this would require the use of a closure, but that is not needed when a
<cite>MathJax.Callback</cite> object is used.</p>
</div>
</div>
<div class="section" id="callback-object-properties">
<h2>Callback Object Properties<a class="headerlink" href="#callback-object-properties" title="Permalink to this headline">¶</a></h2>
<dl class="describe">
<dt>
<code class="descname">hook</code></dt>
<dd><p>The function to be called when the callback is executed.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">data</code></dt>
<dd><p>An array containing the arguments to pass to the callback
function when it is executed.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">object</code></dt>
<dd><p>The object to use as <cite>this</cite> during the call to the callback
function.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">called</code></dt>
<dd><p>Set to <code class="docutils literal"><span class="pre">true</span></code> after the callback has been called, and undefined
otherwise. A callback will not be executed a second time unless
the callback’s <a class="reference internal" href="#reset" title="reset"><code class="xref py py-meth docutils literal"><span class="pre">reset()</span></code></a> method is called first, or its
<code class="docutils literal"><span class="pre">autoReset</span></code> property is set to <code class="docutils literal"><span class="pre">true</span></code>.</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">autoReset</code></dt>
<dd><p>Set this to <code class="docutils literal"><span class="pre">true</span></code> if you want to be able to call the callback
more than once. (This is the case for signal listeners, for example).</p>
</dd></dl>
<dl class="describe">
<dt>
<code class="descname">isCallback</code></dt>
<dd><p>Always set to <code class="docutils literal"><span class="pre">true</span></code> (used to detect if an object is a callback
or not).</p>
</dd></dl>
</div>
<div class="section" id="callback-object-methods">
<h2>Callback Object Methods<a class="headerlink" href="#callback-object-methods" title="Permalink to this headline">¶</a></h2>
<dl class="method">
<dt id="reset">
<code class="descname">reset</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#reset" title="Permalink to this definition">¶</a></dt>
<dd><p>Clears the callback’s <cite>called</cite> property.</p>
</dd></dl>
</div>
<div class="section" id="mathjax-callback-methods">
<h2>MathJax.Callback Methods<a class="headerlink" href="#mathjax-callback-methods" title="Permalink to this headline">¶</a></h2>
<dl class="method">
<dt id="Delay">
<code class="descname">Delay</code><span class="sig-paren">(</span><em>time</em><span class="optional">[</span>, <em>callback</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#Delay" title="Permalink to this definition">¶</a></dt>
<dd><p>Waits for the specified time (given in milliseconds) and then
performs the callback. It returns the Callback object (or a blank
one if none was supplied). The returned callback structure has a
<cite>timeout</cite> property set to the result of the <code class="docutils literal"><span class="pre">setTimeout()</span></code> call
that was used to perform the wait so that you can cancel the wait,
if needed. Thus <code class="xref py py-meth docutils literal"><span class="pre">MathJax.Callback.Delay()</span></code> can be used to
start a timeout delay that executes the callback if an action
doesn’t occur within the given time (and if the action does occur,
the timeout can be canceled). Since
<code class="xref py py-meth docutils literal"><span class="pre">MathJax.Callback.Delay()</span></code> returns a callback structure, it
can be used in a callback queue to insert a delay between queued
commands.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>time</strong> — the amount of time to wait</li>
<li><strong>callback</strong> — the callback specification</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Returns:</th><td class="field-body"><p class="first last">the callback object</p>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="method">
<dt id="Queue">
<code class="descname">Queue</code><span class="sig-paren">(</span><span class="optional">[</span><em>callback</em>, <em>...</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#Queue" title="Permalink to this definition">¶</a></dt>
<dd><p>Creates a <cite>MathJax.CallBack.Queue</cite> object and pushes the given
callbacks into the queue. See <a class="reference internal" href="../advanced/queues.html#using-queues"><span class="std std-ref">Using Queues</span></a>
for more details about MathJax queues.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>callback</strong> — one or more callback specifications</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Returns:</th><td class="field-body"><p class="first last">the <cite>Queue</cite> object</p>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="method">
<dt id="Signal">
<code class="descname">Signal</code><span class="sig-paren">(</span><em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#Signal" title="Permalink to this definition">¶</a></dt>
<dd><p>Looks for a named signal, creates it if it doesn’t already exist,
and returns the signal object. See
<a class="reference internal" href="../advanced/signals.html#using-signals"><span class="std std-ref">Using Signals</span></a> for more details.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>name</strong> — name of the signal to get or create</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Returns:</th><td class="field-body"><p class="first last">the <cite>Signal</cite> object</p>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="method">
<dt id="ExecuteHooks">
<code class="descname">ExecuteHooks</code><span class="sig-paren">(</span><em>hooks</em><span class="optional">[</span>, <em>data</em><span class="optional">[</span>, <em>reset</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#ExecuteHooks" title="Permalink to this definition">¶</a></dt>
<dd><p>Calls each callback in the <cite>hooks</cite> array (or the single hook if it
is not an array), passing it the arguments stored in the data
array. If <cite>reset</cite> is <code class="docutils literal"><span class="pre">true</span></code>, then the callback’s
<a class="reference internal" href="#reset" title="reset"><code class="xref py py-meth docutils literal"><span class="pre">reset()</span></code></a> method will be called before each hook is
executed. If any of the hooks returns a <cite>Callback</cite> object, then
it collects those callbacks and returns a new callback that will
execute when all the ones returned by the hooks have been
completed. Otherwise, <code class="xref py py-meth docutils literal"><span class="pre">MathJax.Callback.ExecuteHooks()</span></code>
returns <code class="docutils literal"><span class="pre">null</span></code>.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>hooks</strong> — array of hooks to be called, or a hook</li>
<li><strong>data</strong> — array of arguments to pass to each hook in turn</li>
<li><strong>reset</strong> — <code class="docutils literal"><span class="pre">true</span></code> if the <a class="reference internal" href="#reset" title="reset"><code class="xref py py-meth docutils literal"><span class="pre">reset()</span></code></a> method should be called</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Returns:</th><td class="field-body"><p class="first last">callback that waits for all the hooks to complete, or <code class="docutils literal"><span class="pre">null</span></code></p>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="method">
<dt id="Hooks">
<code class="descname">Hooks</code><span class="sig-paren">(</span><em>reset</em><span class="sig-paren">)</span><a class="headerlink" href="#Hooks" title="Permalink to this definition">¶</a></dt>
<dd><p>Creates a prioritized list of hooks that are called in order based
on their priority (low priority numbers are handled first). This
is meant to replace <code class="xref py py-meth docutils literal"><span class="pre">MathJax.Callback.ExecuteHooks()</span></code> and is
used internally for signal callbacks, pre- and post-filters, and
other lists of callbacks.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>reset</strong> — <code class="docutils literal"><span class="pre">true</span></code> if callbacks can be called more than once</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Returns:</th><td class="field-body"><p class="first last">the <cite>Hooks</cite> object</p>
</td>
</tr>
</tbody>
</table>
<p>The list has the following methods:</p>
<dl class="method">
<dt id="Add">
<code class="descname">Add</code><span class="sig-paren">(</span><em>hook</em><span class="optional">[</span>, <em>priority</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#Add" title="Permalink to this definition">¶</a></dt>
<dd><p>Add a callback to the prioritized list. If <code class="docutils literal"><span class="pre">priority</span></code> is
not provided, the default is 10. The <code class="docutils literal"><span class="pre">hook</span></code> is a <cite>Callback</cite>
specification as described above.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>hook</strong> — callback specification to add to the list</li>
<li><strong>priority</strong> — priority of the hook in the list (default: 10)</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Returns:</th><td class="field-body"><p class="first last">the callback object being added</p>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="method">
<dt>
<code class="descname">Remove</code><span class="sig-paren">(</span><em>hook</em><span class="sig-paren">)</span></dt>
<dd><p>Remove a given hook (as returned from <a class="reference internal" href="#Add" title="Add"><code class="xref py py-meth docutils literal"><span class="pre">Add()</span></code></a> above)
from the prioritized list.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first simple">
<li><strong>hook</strong> — the callback to be removed</li>
</ul>
</td>
</tr>
<tr class="field-even field"><th class="field-name">Returns:</th><td class="field-body"><p class="first last"><code class="docutils literal"><span class="pre">null</span></code></p>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="method">
<dt id="Execute">
<code class="descname">Execute</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#Execute" title="Permalink to this definition">¶</a></dt>
<dd><p>Execute the list of callbacks, resetting them if requested.
If any of the hooks return callbacks, then <code class="docutils literal"><span class="pre">Execute()</span></code>
returns a callback that will be executed when they all have
completed.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Returns:</th><td class="field-body">a callback object or <code class="docutils literal"><span class="pre">null</span></code></td>
</tr>
</tbody>
</table>
</dd></dl>
</dd></dl>
</div>
</div>
</div>
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