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<section class="tex2jax_ignore mathjax_ignore" id="monitorhandler">
<h1>MonitorHandler<a class="headerlink" href="#monitorhandler" title="Link to this heading"></a></h1>
<p>The <em>MonitorHandler</em> is a system for watching changes in properties or Attributes on objects. A
monitor can be thought of as a sort of trigger that responds to change.</p>
<p>The main use for the MonitorHandler is to report changes to the client; for example the client
Session may ask Evennia to monitor the value of the Character’s <code class="docutils literal notranslate"><span class="pre">health</span></code> attribute and report
whenever it changes. This way the client could for example update its health bar graphic as needed.</p>
<section id="using-the-monitorhandler">
<h2>Using the MonitorHandler<a class="headerlink" href="#using-the-monitorhandler" title="Link to this heading"></a></h2>
<p>The MontorHandler is accessed from the singleton <code class="docutils literal notranslate"><span class="pre">evennia.MONITOR_HANDLER</span></code>. The code for the handler
is in <code class="docutils literal notranslate"><span class="pre">evennia.scripts.monitorhandler</span></code>.</p>
<p>Here’s how to add a new monitor:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">evennia</span><span class="w"> </span><span class="kn">import</span> <span class="n">MONITOR_HANDLER</span>
<span class="n">MONITOR_HANDLER</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">fieldname</span><span class="p">,</span> <span class="n">callback</span><span class="p">,</span>
<span class="n">idstring</span><span class="o">=</span><span class="s2">&quot;&quot;</span><span class="p">,</span> <span class="n">persistent</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">obj</span></code> (<a class="reference internal" href="Typeclasses.html"><span class="std std-doc">Typeclassed</span></a> entity) - the object to monitor. Since this must be
typeclassed, it means you can’t monitor changes on <a class="reference internal" href="Sessions.html"><span class="std std-doc">Sessions</span></a> with the monitorhandler, for
example.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">fieldname</span></code> (str) - the name of a field or <a class="reference internal" href="Attributes.html"><span class="std std-doc">Attribute</span></a> on <code class="docutils literal notranslate"><span class="pre">obj</span></code>. If you want to
monitor a database field you must specify its full name, including the starting <code class="docutils literal notranslate"><span class="pre">db_</span></code> (like
<code class="docutils literal notranslate"><span class="pre">db_key</span></code>, <code class="docutils literal notranslate"><span class="pre">db_location</span></code> etc). Any names not starting with <code class="docutils literal notranslate"><span class="pre">db_</span></code> are instead assumed to be the names
of Attributes. This difference matters, since the MonitorHandler will automatically know to watch
the <code class="docutils literal notranslate"><span class="pre">db_value</span></code> field of the Attribute.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">callback</span></code>(callable) - This will be called as <code class="docutils literal notranslate"><span class="pre">callback(fieldname=fieldname,</span> <span class="pre">obj=obj,</span> <span class="pre">**kwargs)</span></code>
when the field updates.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">idstring</span></code> (str) - this is used to separate multiple monitors on the same object and fieldname.
This is required in order to properly identify and remove the monitor later. It’s also used for
saving it.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">persistent</span></code> (bool) - if True, the monitor will survive a server reboot.</p></li>
</ul>
<p>Example:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span><span class="w"> </span><span class="nn">evennia</span><span class="w"> </span><span class="kn">import</span> <span class="n">MONITOR_HANDLER</span> <span class="k">as</span> <span class="n">monitorhandler</span>
<span class="k">def</span><span class="w"> </span><span class="nf">_monitor_callback</span><span class="p">(</span><span class="n">fieldname</span><span class="o">=</span><span class="s2">&quot;&quot;</span><span class="p">,</span> <span class="n">obj</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="c1"># reporting callback that works both</span>
<span class="c1"># for db-fields and Attributes</span>
<span class="k">if</span> <span class="n">fieldname</span><span class="o">.</span><span class="n">startswith</span><span class="p">(</span><span class="s2">&quot;db_&quot;</span><span class="p">):</span>
<span class="n">new_value</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">fieldname</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span> <span class="c1"># an attribute </span>
<span class="n">new_value</span> <span class="o">=</span> <span class="n">obj</span><span class="o">.</span><span class="n">attributes</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">fieldname</span><span class="p">)</span>
<span class="n">obj</span><span class="o">.</span><span class="n">msg</span><span class="p">(</span><span class="sa">f</span><span class="s2">&quot;</span><span class="si">{</span><span class="n">obj</span><span class="o">.</span><span class="n">key</span><span class="si">}</span><span class="s2">.</span><span class="si">{</span><span class="n">fieldname</span><span class="si">}</span><span class="s2"> changed to &#39;</span><span class="si">{</span><span class="n">new_value</span><span class="si">}</span><span class="s2">&#39;.&quot;</span><span class="p">)</span>
<span class="c1"># (we could add _some_other_monitor_callback here too)</span>
<span class="c1"># monitor Attribute (assume we have obj from before)</span>
<span class="n">monitorhandler</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="s2">&quot;desc&quot;</span><span class="p">,</span> <span class="n">_monitor_callback</span><span class="p">)</span>
<span class="c1"># monitor same db-field with two different callbacks (must separate by id_string)</span>
<span class="n">monitorhandler</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="s2">&quot;db_key&quot;</span><span class="p">,</span> <span class="n">_monitor_callback</span><span class="p">,</span> <span class="n">id_string</span><span class="o">=</span><span class="s2">&quot;foo&quot;</span><span class="p">)</span>
<span class="n">monitorhandler</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="s2">&quot;db_key&quot;</span><span class="p">,</span> <span class="n">_some_other_monitor_callback</span><span class="p">,</span> <span class="n">id_string</span><span class="o">=</span><span class="s2">&quot;bar&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>A monitor is uniquely identified by the combination of the <em>object instance</em> it is monitoring, the
<em>name</em> of the field/attribute to monitor on that object and its <code class="docutils literal notranslate"><span class="pre">idstring</span></code> (<code class="docutils literal notranslate"><span class="pre">obj</span></code> + <code class="docutils literal notranslate"><span class="pre">fieldname</span></code> +
<code class="docutils literal notranslate"><span class="pre">idstring</span></code>). The <code class="docutils literal notranslate"><span class="pre">idstring</span></code> will be the empty string unless given explicitly.</p>
<p>So to “un-monitor” the above you need to supply enough information for the system to uniquely find
the monitor to remove:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">monitorhandler</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="s2">&quot;desc&quot;</span><span class="p">)</span>
<span class="n">monitorhandler</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="s2">&quot;db_key&quot;</span><span class="p">,</span> <span class="n">idstring</span><span class="o">=</span><span class="s2">&quot;foo&quot;</span><span class="p">)</span>
<span class="n">monitorhandler</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="s2">&quot;db_key&quot;</span><span class="p">,</span> <span class="n">idstring</span><span class="o">=</span><span class="s2">&quot;bar&quot;</span><span class="p">)</span>
</pre></div>
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<h3><a href="../index.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">MonitorHandler</a><ul>
<li><a class="reference internal" href="#using-the-monitorhandler">Using the MonitorHandler</a></li>
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