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<div class="section" id="events">
<h1>Events<a class="headerlink" href="#events" title="Permalink to this headline">¶</a></h1>
<p>The Component Architecture provides a way to dispatch events to event
handlers. Event handlers are registered as <em>subscribers</em>
a.k.a. <em>handlers</em>.</p>
<p>Before we can start we need to import <tt class="docutils literal"><span class="pre">zope.component.event</span></tt> to make
the dispatching effective:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">zope.component.event</span>
</pre></div>
</div>
<p>Consider two event classes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Event1</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">Event2</span><span class="p">(</span><span class="n">Event1</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span>
</pre></div>
</div>
<p>Now consider two handlers for these event classes:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">called</span> <span class="o">=</span> <span class="p">[]</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">zope.component</span>
<span class="gp">>>> </span><span class="nd">@zope.component.adapter</span><span class="p">(</span><span class="n">Event1</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">handler1</span><span class="p">(</span><span class="n">event</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">called</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nd">@zope.component.adapter</span><span class="p">(</span><span class="n">Event2</span><span class="p">)</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">handler2</span><span class="p">(</span><span class="n">event</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">called</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
<p>We can register them with the Component Architecture:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">provideHandler</span><span class="p">(</span><span class="n">handler1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">provideHandler</span><span class="p">(</span><span class="n">handler2</span><span class="p">)</span>
</pre></div>
</div>
<p>Now let’s go through the events. We’ll see that the handlers have been
called accordingly:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">zope.event</span> <span class="kn">import</span> <span class="n">notify</span>
<span class="gp">>>> </span><span class="n">notify</span><span class="p">(</span><span class="n">Event1</span><span class="p">())</span>
<span class="gp">>>> </span><span class="n">called</span>
<span class="go">[1]</span>
<span class="gp">>>> </span><span class="k">del</span> <span class="n">called</span><span class="p">[:]</span>
<span class="gp">>>> </span><span class="n">notify</span><span class="p">(</span><span class="n">Event2</span><span class="p">())</span>
<span class="gp">>>> </span><span class="n">called</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">called</span>
<span class="go">[1, 2]</span>
</pre></div>
</div>
<div class="section" id="object-events">
<h2>Object events<a class="headerlink" href="#object-events" title="Permalink to this headline">¶</a></h2>
<p>The <tt class="docutils literal"><span class="pre">objectEventNotify</span></tt> function is a subscriber to dispatch
ObjectEvents to interested adapters.</p>
<p>First create an object class:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">IUseless</span><span class="p">(</span><span class="n">zope</span><span class="o">.</span><span class="n">interface</span><span class="o">.</span><span class="n">Interface</span><span class="p">):</span>
<span class="gp">... </span> <span class="sd">"""Useless object"""</span>
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">UselessObject</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="sd">"""Useless object"""</span>
<span class="gp">... </span> <span class="n">zope</span><span class="o">.</span><span class="n">interface</span><span class="o">.</span><span class="n">implements</span><span class="p">(</span><span class="n">IUseless</span><span class="p">)</span>
</pre></div>
</div>
<p>Then create an event class:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">IObjectThrownEvent</span><span class="p">(</span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">interfaces</span><span class="o">.</span><span class="n">IObjectEvent</span><span class="p">):</span>
<span class="gp">... </span> <span class="sd">"""An object has been thrown away"""</span>
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">ObjectThrownEvent</span><span class="p">(</span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">interfaces</span><span class="o">.</span><span class="n">ObjectEvent</span><span class="p">):</span>
<span class="gp">... </span> <span class="sd">"""An object has been thrown away"""</span>
<span class="gp">... </span> <span class="n">zope</span><span class="o">.</span><span class="n">interface</span><span class="o">.</span><span class="n">implements</span><span class="p">(</span><span class="n">IObjectThrownEvent</span><span class="p">)</span>
</pre></div>
</div>
<p>Create an object and an event:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">hammer</span> <span class="o">=</span> <span class="n">UselessObject</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">event</span> <span class="o">=</span> <span class="n">ObjectThrownEvent</span><span class="p">(</span><span class="n">hammer</span><span class="p">)</span>
</pre></div>
</div>
<p>Then notify the event to the subscribers.
Since the subscribers list is empty, nothing happens.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">event</span><span class="o">.</span><span class="n">objectEventNotify</span><span class="p">(</span><span class="n">event</span><span class="p">)</span>
</pre></div>
</div>
<p>Now create an handler for the event:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">events</span> <span class="o">=</span> <span class="p">[]</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">record</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="c">#*</span>
<span class="gp">... </span> <span class="n">events</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">args</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">provideHandler</span><span class="p">(</span><span class="n">record</span><span class="p">,</span> <span class="p">[</span><span class="n">IUseless</span><span class="p">,</span> <span class="n">IObjectThrownEvent</span><span class="p">])</span>
</pre></div>
</div>
<p>The event is notified to the subscriber:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">event</span><span class="o">.</span><span class="n">objectEventNotify</span><span class="p">(</span><span class="n">event</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">events</span> <span class="o">==</span> <span class="p">[(</span><span class="n">hammer</span><span class="p">,</span> <span class="n">event</span><span class="p">)]</span>
<span class="go">True</span>
</pre></div>
</div>
<p>Following test demonstrates how a subscriber can raise an exception
to prevent an action.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">provideHandler</span><span class="p">(</span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">event</span><span class="o">.</span><span class="n">objectEventNotify</span><span class="p">)</span>
</pre></div>
</div>
<p>Let’s create a container:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">ToolBox</span><span class="p">(</span><span class="nb">dict</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__delitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">notify</span><span class="p">(</span><span class="n">ObjectThrownEvent</span><span class="p">(</span><span class="bp">self</span><span class="p">[</span><span class="n">key</span><span class="p">]))</span>
<span class="gp">... </span> <span class="k">return</span> <span class="nb">super</span><span class="p">(</span><span class="n">ToolBox</span><span class="p">,</span><span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__delitem__</span><span class="p">(</span><span class="n">key</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">container</span> <span class="o">=</span> <span class="n">ToolBox</span><span class="p">()</span>
</pre></div>
</div>
<p>And put the object into the container:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">container</span><span class="p">[</span><span class="s">'Red Hammer'</span><span class="p">]</span> <span class="o">=</span> <span class="n">hammer</span>
</pre></div>
</div>
<p>Create an handler function that will raise an error when called:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Veto</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">>>> </span><span class="k">def</span> <span class="nf">callback</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="n">event</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">assert</span><span class="p">(</span><span class="n">item</span> <span class="o">==</span> <span class="n">event</span><span class="o">.</span><span class="n">object</span><span class="p">)</span>
<span class="gp">... </span> <span class="k">raise</span> <span class="n">Veto</span>
</pre></div>
</div>
<p>Register the handler:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">zope</span><span class="o">.</span><span class="n">component</span><span class="o">.</span><span class="n">provideHandler</span><span class="p">(</span><span class="n">callback</span><span class="p">,</span> <span class="p">[</span><span class="n">IUseless</span><span class="p">,</span> <span class="n">IObjectThrownEvent</span><span class="p">])</span>
</pre></div>
</div>
<p>Then if we try to remove the object, an ObjectThrownEvent is fired:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">del</span> <span class="n">container</span><span class="p">[</span><span class="s">'Red Hammer'</span><span class="p">]</span>
<span class="gp">... </span>
<span class="gt">Traceback (most recent call last):</span>
<span class="c">...</span>
<span class="k">raise</span> <span class="n">Veto</span>
<span class="gr">Veto</span>
</pre></div>
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<li><a class="reference internal" href="#">Events</a><ul>
<li><a class="reference internal" href="#object-events">Object events</a></li>
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