<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 Characer’s <codeclass="docutils literal notranslate"><spanclass="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>
<p>The MontorHandler is accessed from the singleton <codeclass="docutils literal notranslate"><spanclass="pre">evennia.MONITOR_HANDLER</span></code>. The code for the handler
is in <codeclass="docutils literal notranslate"><spanclass="pre">evennia.scripts.monitorhandler</span></code>.</p>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">obj</span></code> (<aclass="reference internal"href="Typeclasses.html"><spanclass="doc std std-doc">Typeclassed</span></a> entity) - the object to monitor. Since this must be
typeclassed, it means you can’t monitor changes on <aclass="reference internal"href="Sessions.html"><spanclass="doc std std-doc">Sessions</span></a> with the monitorhandler, for
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">fieldname</span></code> (str) - the name of a field or <aclass="reference internal"href="Attributes.html"><spanclass="doc std std-doc">Attribute</span></a> on <codeclass="docutils literal notranslate"><spanclass="pre">obj</span></code>. If you want to
monitor a database field you must specify its full name, including the starting <codeclass="docutils literal notranslate"><spanclass="pre">db_</span></code> (like
<codeclass="docutils literal notranslate"><spanclass="pre">db_key</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">db_location</span></code> etc). Any names not starting with <codeclass="docutils literal notranslate"><spanclass="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 <codeclass="docutils literal notranslate"><spanclass="pre">db_value</span></code> field of the Attribute.</p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">callback</span></code>(callable) - This will be called as <codeclass="docutils literal notranslate"><spanclass="pre">callback(fieldname=fieldname,</span><spanclass="pre">obj=obj,</span><spanclass="pre">**kwargs)</span></code>
when the field updates.</p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="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
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">persistent</span></code> (bool) - if True, the monitor will survive a server reboot.</p></li>
<spanclass="n">obj</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="sa">f</span><spanclass="s2">"</span><spanclass="si">{</span><spanclass="n">obj</span><spanclass="o">.</span><spanclass="n">key</span><spanclass="si">}</span><spanclass="s2">.</span><spanclass="si">{</span><spanclass="n">fieldname</span><spanclass="si">}</span><spanclass="s2"> changed to '</span><spanclass="si">{</span><spanclass="n">new_value</span><spanclass="si">}</span><spanclass="s2">'."</span><spanclass="p">)</span>
<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 <codeclass="docutils literal notranslate"><spanclass="pre">idstring</span></code> (<codeclass="docutils literal notranslate"><spanclass="pre">obj</span></code> + <codeclass="docutils literal notranslate"><spanclass="pre">fieldname</span></code> +
<codeclass="docutils literal notranslate"><spanclass="pre">idstring</span></code>). The <codeclass="docutils literal notranslate"><spanclass="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
