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copied!<h2>First of all, there are no events in PHP</h2> <p>An event is an abstraction for callback functions with their name. Typically, we'd define them as <code>$eventName => $listener</code>, where <code>$listener</code> is a callback function for the <code>$eventName</code></p> <p></p> <h2>What is the difference between events and regular callback functions?</h2> <p>Again - the core point to remember, is that events are callback functions. Nothing more.</p> <p>The only difference between them, is how we do invoke them. </p> <blockquote> <blockquote> <p><sub>An event is defined on bootstrap step with its required arguments, but invoked on demand without arguments. While the callback function is invoked with arguments and only</p> </blockquote> </blockquote> <p>Consider this example,</p> <pre><code><?php $eventManager = new EventManager(); $eventManager->attach('my_event', function(){ print_r(func_get_args()); }, array('foo', 'bar')); </code></pre> <p>As we have just defined an event, we'd invoke like,</p> <p><code>$eventManager->trigger('my_event');</code></p> <p>This will output: <code>Array([0] => [foo], [1] => [bar]</code></p> <h2>That's related to JavaScript!</h2> <p>Since most of us are familiar with JavaScript even-driven architecture, its worth nothing to mention an example of its common usage:</p> <pre><code>var a = document.getElementsByTagName('a')[0]; a.onclick = function(event) { // <-- We define an event with the event argument event.preventDefault(); alert('A element was clicked'); } a.click(); // <-- but we invoke it without arguments // or If you want a Jquery $("a").click(function(event){ event.preventDefault(); alert('A element was clicked'); }); $("a").click(); </code></pre> <p>Since in PHP we don't have such event-driven nature, we can replace it with our own class that manage events and takes a full advantage of it.</p> <h2>Why use them?</h2> <p>While events confuse so many people, they are extremely useful.</p> <p>Imagine you have a <strong>Content Management System (CMS)</strong>, where your users can decide how to handle <code>404</code> errors. Say, they can handle with <br /></p> <p>1) Show a blank page <br /> 2) Redirect to <code>/</code> <br /> 3) Show a custom message <br /></p> <p>Without events you would have to do it, like</p> <pre><code>if ($router->isMatched($request)){ //do dispatch etc } else { // Here you start handling 404 errors switch($config->read('404_way_handle')){ case 'show_blank': die(); break; case 'show_msg': echo 'Some custom message'; break; case 'redirect': // do redirect break; } } </code></pre> <p>With an event you can simplify the readability and keep the code more maintainable:</p> <pre><code>if ($router->isMatched($request)){ // do dispatch } else { $eventManager->trigger('404_handler'); } </code></pre> <p>while <code>404_handler</code> itself looks like</p> <pre><code> $eventManager->attach('404_handler', function(){ switch($config->read('404_way_handle')){ case 'show_blank': die(); break; case 'show_msg': echo 'Some custom message'; break; case 'redirect': // do redirect break; } }, $config); </code></pre> <p><br /></p> <h2>So let's break it down</h2> <p>1) Events improve readability, which is great for future<br /></p> <p>2) Events do adhere to the <strong>Single-Responsibility Principle</strong>, because you can simply inject <code>$eventManager</code> to your classes that need it, while callback functions could break it or could introduce a <strong>global state</strong> too (Which is bad for unit-testings).</p> <p>3) There are distinct types of logic - template logic, business logic, error handler logic, data access logic etc etc. Events <strong>do simplify</strong> your application logic by decoupling business (or another kind) logic from its <strong>configuration</strong> logic, so that you end up with clear application logic.</p> <blockquote> <blockquote> <p>You can watch this <a href="http://www.youtube.com/watch?v=O6dwYklDVp0" rel="nofollow">lecture</a> if you want to know how they do work in Zend Framework 2 (watch it even if you're not familiar with Zend Framework 2)</sub></p> </blockquote> </blockquote> <p><strong>Events in MVC-related architectures</strong></p> <p>Since you've been talking about frameworks, its worth nothing to mention, that there could be events in <code>MVC-related</code> architectures too. And since events are callback functions you can abstract common <code>boostrap</code> events in your MVC-like architecture, like this.</p> <pre><code>$mvcEvent->on(MVC_EVENT::ROUTE_MATCH, function(){ $mvcEvent->on(MVC_EVENT::DISTPATCH, function($content){ echo $mvcEvent->trigger(MVC_EVENT::RENDER, $content); }); }); </code></pre> <p>Note : <strong>In pure MVC theory, there are no events at all</strong>. They do acts as helpers, but again - in frameworks you can abstract them and call them "events".</p>

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