Earlier this year, I wrote about Aspects, Intercepting Filters, Signal Slots, and Events, in order to compare these similar approaches to handling both asychronous programming as well as handling cross-cutting application concerns in a cohesive way.

I took the research I did for that article, and applied it to what was then a "SignalSlot" implementation within Zend Framework 2, and refactored that work into a new "EventManager" component. This article is intended to get you up and running with it.

Table of Contents

• Assumptions
• Terminology
• Getting Started
• EventCollection vs EventManager
• Global Static Listeners
• Listener Aggregates
• Introspecting Results
• Short Circuiting Listener Execution
• Keeping it in Order
• Custom Event Objects
• Putting it Together: A Simple Caching Example
• Fin
• Updates

Assumptions

You must have Zend Framework 2 installed either:

• From a development snapshot (the ZF2 blog has the latest links at the time of writing), or
• From cloning the ZF2 git repo

Terminology

• An Event Manager is an object that aggregates listeners for one or more named events, and which triggers events.
• A Listener is a callback that can react to an event.
• An Event is an action.

Typically, an event will be modeled as an object, containing metadata surrounding when and how it was triggered — what the calling object was, what parameters are available, etc. Events are also typically named, which can allow a single listener to branch logic based on the current event (though purists would argue you should never do this).

Getting Started

The minimal things necessary to get started are:

• An EventManager instance
• One or more listeners on one or more events
• A call to trigger() an event

So, here we go:

use Zend\EventManager\EventManager;

$events = new EventManager();

$events->attach('do', function($e) {
    $event  = $e->getName();
    $params = $e->getParams();
    printf(
        'Handled event "%s", with parameters %s',
        $event,
        json_encode($params)
    );
});

$params = array('foo' => 'bar', 'baz' => 'bat');
$events->trigger('do', null, $params);

The above will output:

Handled event "do", with parameters {"foo":"bar","baz":"bat"}

Pretty simple!

Note: throughout this post, I use closures as listeners. However, any valid PHP callback can be attached as a listeners — PHP function names, static class methods, object instance methods, or closures. I use closures within this post simply for illustration and simplicity.

But what's that null, second argument for?

Typically, you will compose an EventManager within a class, to allow triggering actions within methods. The middle argument to trigger() is a "context" or "target", and in the case described, would be the current object instance. This gives event listeners access to the calling object, which can often be useful.

use Zend\EventManager\EventCollection,
    Zend\EventManager\EventManager;

class Example
{
    protected $events;
    
    public function setEventManager(EventCollection $events)
    {
        $this->events = $events;
    }
    
    public function events()
    {
        if (!$this->events) {
            $this->setEventManager(new EventManager(
                array(__CLASS__, get_called_class())
            );
        }
        return $this->events;
    }
    
    public function do($foo, $baz)
    {
        $params = compact('foo', 'baz');
        $this->events()->trigger(__FUNCTION__, $this, $params);
    }

}

$example = new Example();

$example->events()->attach('do', function($e) {
    $event  = $e->getName();
    $target = get_class($e->getTarget()); // "Example"
    $params = $e->getParams();
    printf(
        'Handled event "%s" on target "%s", with parameters %s',
        $event,
        $target,
        json_encode($params)
    );
});

$example->do('bar', 'bat');

The above is basically the same as the first example. The main difference is that we're now using that middle argument in order to pass a context on to the listeners. Our listener is now retrieving that ($e->getTarget()), and doing something with it.

If you're reading this critically, you should have two questions:

• What is this EventCollection bit?
• What is that argument being passed to the EventManager constructor?

The answer to the first will lead us into the second.

EventCollection vs EventManager

One principle we're trying to follow with ZF2 is the Liskov Substitution Principle. One typical interpretation of this is that strong interfaces should be defined for any class for which there could be a potential substitution, so that consumers may use other implementations without worrying about variances in internal behavior.

As such, we developed an interface, EventCollection that describes an object capable of aggregating listeners for events, and triggering those events. EventManager is the standard implementation we provide.

Global Static Listeners

One aspect that the EventManager implementation provides is an ability to interface with a StaticEventCollection. This interface allows attaching listeners not only on events, but on events emitted by specific contexts or targets. The EventManager, when notifying listeners, will also pull listeners for the event from the StaticEventCollection object it subscribes to, and notify them.

How does this work, exactly?

At the application level, you grab an instance of StaticEventManager, and start attaching events to it.

use Zend\EventManager\StaticEventManager;

$events = StaticEventManager::getInstance();
$events->attach('Example', 'do', function($e) {
    $event  = $e->getName();
    $target = get_class($e->getTarget()); // "Example"
    $params = $e->getParams();
    printf(
        'Handled event "%s" on target "%s", with parameters %s',
        $event,
        $target,
        json_encode($params)
    );
});

You'll notice it looks almost the same as the original example. The only difference is there is a new argument at the beginning of the list, to which we attached the name 'Example'. This code is basically saying, "Listen to the 'do' event of the 'Example' target, and, when notified, execute this callback."

This is finally where the constructor argument of EventManager comes into play. The constructor allows passing a string, or an array of strings, defining the name of the context or target the given instance will be interested in. If an array is given, then any listener on any of the targets given will be notified. Listeners attached directly to the EventManager will be executed before any attached statically.

So, getting back to our example, let's assume that the above static listener is registered, and also that the Example class is defined as above. We can then execute the following:

$example = new Example();
$example->do('bar', 'bat');

and expect the following to be echo'd:

Handled event "do" on target "Example", with parameters {"foo":"bar","baz":"bat"}

Now, let's say we extended Example as follows:

    class SubExample extends Example
    {
    }

One interesting aspect of our EventManager construction is that we defined it to listen both on __CLASS__ and get_called_class(). This means that calling do() on our SubExample class would also trigger the event we attached statically! It also means that, if desired, we could attach to specifically SubExample, and listeners on simply Example would not be triggered.

Finally, the names used as contexts or targets need not be class names; they can be some name that only has meaning in your application if desired. As an example, you could have a set of classes that respond to "log" or "cache" — and listeners on these would be notified by any of them.

At any point, if you do not want the EventManager attached to a class to notify statically attached listeners, you can simply pass a null value to the setStaticConnections() method:

$events->setStaticConnections(null);

and they will be ignored. If at any point, you want to enable them again, pass the StaticEventManager instance:

$events->setStaticConnections(StaticEventManager::getInstance());

Listener Aggregates

Oftentimes, you may want a single class to listen to multiple events, attaching one or more instance methods as listeners. To make this paradigm easy, you can simply implement the HandlerAggregate interface. This interface defines two methods, attach(EventCollection $events) and detach(EventCollection $events). Basically, you pass an EventManager instance to one and/or the other, and then it's up to the implementing class to determine what to do.

As an example:

use Zend\EventManager\Event,
    Zend\EventManager\EventCollection,
    Zend\EventManager\HandlerAggregate,
    Zend\Log\Logger;

class LogEvents implements HandlerAggregate
{
    protected $handlers = array();
    protected $log;

    public function __construct(Logger $log)
    {
        $this->log = $log;
    }

    public function attach(EventCollection $events)
    {
        $this->handlers[] = $events->attach('do', array($this, 'log'));
        $this->handlers[] = $events->attach('doSomethingElse', array($this, 'log'));
    }
    
    public function detach(EventCollection $events)
    {
        foreach ($this->handlers as $key => $handler) {
            $events->detach($handler);
            unset($this->handlers[$key];
        }
        $this->handlers = array();
    }

    public function log(Event $e)
    {
        $event  = $e->getName();
        $params = $e->getParams();
        $log->info(sprintf('%s: %s', $event, json_encode($params)));
    }
}

You would then attach it as follows:

$doLog = new LogEvents($logger);
$events->attachAggregate($doLog);

and any events it handles would then be notified when they are triggered. This allows you to have stateful event listeners.

You'll notice the detach() method implementation. Just like attach(), it accepts an EventManager, and then calls detach for each handler it has aggregated. This is possible because EventManager::attach() returns an object representing the listener — which we've aggregated within our aggregate's attach() method previously.

Introspecting Results

Sometimes you'll want to know what your listeners returned. One thing to remember is that you may have multiple listeners on the same event; the interface for results must be consistent regardless of the number of listeners.

The EventManager implementation by default returns a ResponseCollection object. This class extends PHP's SplStack, allowing you to loop through responses in reverse order (since the last one executed is likely the one you're most interested in). It also implements the following methods:

• first() will retrieve the first result received
• last() will retrieve the last result received
• contains($value) allows you to test all values to see if a given one was received, and returns simply a boolean true if found, and false if not.

Typically, you should not worry about the return values from events, as the object triggering the event shouldn't really have much insight into what listeners are attached. However, sometimes you may want to short-circuit execution if interesting results are obtained.

Short Circuiting Listener Execution

You may want to short-ciruit execution if a particular result is obtained, or if a listener determines that something is wrong, or that it can return something quicker than the target.

As examples, one rationale for adding an EventManager is as a caching mechanism. You can trigger one event early in the method, returning if a cache is found, and trigger another event late in the method, seeding the cache.

The EventManager component offers two ways to handle this. The first is to pass a callback as the last argument to trigger(); callback; if that callback returns a boolean true, execution is halted.

Here's an example:

public function someExpensiveCall($criteria1, $criteria2)
{
    $params  = compact('criteria1', 'criteria2');
    $results = $this->events()->trigger(__FUNCTION__, $this, $params, function ($r) {
        return ($r instanceof SomeResultClass);
    });
    if ($results->stopped()) {
        return $results->last();
    }
    
    // ... do some work ...
}

With this paradigm, we know that the likely reason of execution halting is due to the last result meeting the test callback criteria; as such, we simply return that last result.

The other way to halt execution is within a listener, acting on the Event object it receives. In this case, the listener calls stopPropagation(true), and the EventManager will then return without notifying any additional listeners.

$events->attach('do', function ($e) {
    $e->stopPropagation();
    return new SomeResultClass();
});

This, of course, raises some ambiguity when using the trigger paradigm, as you can no longer be certain that the last result meets the criteria it's searching on. As such, my recommendation is you use one approach or the other.

Keeping it in Order

On occasion, you may be concerned about the order in which listeners execute. As an example, you may want to do any logging early, to ensure that if short-circuiting occurs, you've logged; or if implementing a cache, you may want to return early if a cache hit is found, and execute late when saving to a cache.

Each of EventManager::attach() and StaticEventManager::attach() accept one additional argument, a priority. By default, if this is omitted, listeners get a priority of 1, and are executed in the order in which they are attached. If you provide a priority value, you can influence order of execution. Higher priority values execute earlier, while lower (negative) values execute later.

To borrow an example from earlier:

$priority = 100;
$events->attach('Example', 'do', function($e) {
    $event  = $e->getName();
    $target = get_class($e->getTarget()); // "Example"
    $params = $e->getParams();
    printf(
        'Handled event "%s" on target "%s", with parameters %s',
        $event,
        $target,
        json_encode($params)
    );
}, $priority);

This would execute with high priority, meaning it would execute early. If we changed $priority to -100, it would execute with low priority, executing late.

While you can't necessarily know all the listeners attached, chances are you can make adequate guesses when necessary in order to set appropriate priority values. My advice is to avoid setting a priority value unless absolutely necessary.

Custom Event Objects

Hopefully some of you have been wondering, "where and when is the Event object created"? In all of the examples above, it's created based on the arguments passed to trigger() — the event name, target, and parameters. Sometimes, however, you may want greater control over the object, however.

As an example, as we've been developing the ZF2 MVC layer, we've been adding event awareness to several of the core MVC components. One thing that looks like a code smell is when you have code like this:

$routeMatch = $e->getParam('route-match', false);
if (!$routeMatch) {
    // Oh noes! we cannot do our work! whatever shall we do?!?!?!
}

The problems with this are several. First, relying on string keys is going to very quickly run into problems — typos when setting or retrieving the argument can lead to hard to debug situations. Second, we now have a documentation issue; how do we document expected arguments? how do we document what we're shoving into the event. Third, as a side effect, we can't use IDE or editor hinting support — string keys give these tools nothing to work with.

Similarly, we found ourselves writing some wierd hacks around how we represent a computational result of a method when triggering an event. As an example:

// in the method:
$params['__RESULT'] = $computedResult;
$events->trigger(__FUNCTION__ . '.post', $this, $params);

// in the listener:
$result = $e->getParam('__RESULT__');
if (!$result) {
    // Oh noes! we cannot do our work! whatever shall we do?!?!?!
}

Sure, that key may be unique, but it suffers from a lot of the same issues.

So, the solution is to create custom events. As an example, we have a custom "MvcEvent" in the ZF2 MVC layer. This event composes a router, route match object, request and response objects, and also a result. We end up with code like this in our listeners:

$response = $e->getResponse();
$result   = $e->getResult();
if (is_string($result)) {
    $content = $view->render('layout.phtml', array('content' => $result));
    $response->setContent($content);
}

But how do we use this custom event? Simple: trigger() can accept an event object instead of any of the event name, target, or params arguments.

$event = new CustomEvent();
$event->setSomeKey($value);

// Injected with event name and target:
$events->trigger('foo', $this, $event);

// Injected with event name:
$event->setTarget($this);
$events->trigger('foo', $event);

// Fully encapsulates all necessary properties:
$event->setName('foo');
$event->setTarget($this);
$events->trigger($event);

// Passing a callback following the event object works for 
// short-circuiting, too.
$results = $events->trigger('foo', $this, $event, $callback);

This is a really powerful technique for domain-specific event systems, and definitely worth experimenting with.

Putting it Together: A Simple Caching Example

In the previous section, I indicated that short-circuiting is a way to potentially implement a caching solution. Let's create a full example.

First, let's define a method that could use caching. You'll note that in most of the examples, I've used __FUNCTION__ as the event name; this is a good practice, as it makes it simple to create a macro for triggering events, as well as helps to keep event names unique (as they're usually within the context of the triggering class). However, in the case of a caching example, this would lead to identical events being triggered. As such, I recommend postfixing the event name with semantic names: "do.pre", "do.post", "do.error", etc. I'll use that convention in this example.

Additionally, you'll notice that the $params I pass to the event is usually the list of parameters passed to the method. This is because those are often not stored in the object, and also to ensure the listeners have the exact same context as the calling method. But it raises an interesting problem in this example: what name do we give the result of the method? I've standardized on __RESULT__, as double-underscored variables are typically reserved for the sytem. If you have better suggestions, I'd love to hear them!

Here's what the method will look like:

public function someExpensiveCall($criteria1, $criteria2)
{
    $params  = compact('criteria1', 'criteria2');
    $results = $this->events()->trigger(__FUNCTION__ . '.pre', $this, $params, function ($r) {
        return ($r instanceof SomeResultClass);
    });
    if ($results->stopped()) {
        return $results->last();
    }
    
    // ... do some work ...
    
    $params['__RESULT__'] = $calculatedResult;
    $this->events()->trigger(__FUNCTION__ . '.post', $this, $params);
    return $calculatedResult;
}

Now, to provide some caching listeners. We'll need to attach to each of the 'someExpensiveCall.pre' and 'someExpensiveCall.post' methods. In the former case, if a cache hit is detected, we return it, and move on. In the latter, we store the value in the cache.

We'll assume $cache is defined, and follows the paradigms of Zend_Cache. We'll want to return early if a hit is detected, and execute late when saving a cache (in case the result is modified by another listener). As such, we'll set the 'someExpensiveCall.pre' listener to execute with priority 100, and the 'someExpensiveCall.post' listener to execute with priority -100.

$events->attach('someExpensiveCall.pre', function($e) use ($cache) {
    $params = $e->getParams();
    $key    = md5(json_encode($params));
    $hit    = $cache->load($key);
    return $hit;
}, 100);

$events->attach('someExpensiveCall.post', function($e) use ($cache) {
    $params = $e->getParams();
    $result = $params['__RESULT__'];
    unset($params['__RESULT__']);
    $key    = md5(json_encode($params));
    $cache->save($result, $key);
}, -100);

Note: the above could have been done within a HandlerAggregate, which would have allowed keeping the $cache instance as a stateful property, instead of importing it into closures.

Sure, we could probably simply add caching to the object itself - but this approach allows the same handlers to be attached to multiple events, or to attach multiple listeners to the same events (e.g. an argument validator, a logger and a cache manager). The point is that if you design your object with events in mind, you can easily make it more flexible and extensible, without requiring developers to actually extend it — they can simply attach listeners.

Fin

The EventManager is a powerful new addition to Zend Framework. Already, it's being used with the new MVC prototype to empower some constructs that were difficult to accomplish well in the version 1.X series — as an example, I was able to prototype a ViewRenderer replacement in a handful of lines of code, in a way that properly accomplishes the separation of concerns one expects from MVC. I anticipate we'll be using it much, much more often as version 2 matures.

There are certainly some rough edges — the boiler-plate code for short-circuiting is verbose, and we will likely want to add capabilities such as event globbing — but the foundation is solid and mature at this point in time. Experiment with it, and see what you can accomplish!

Updates

• 2011-10-06: Removed references to triggerUntil(), as that functionality is now incorporated into trigger(). Added section on Custom Event Objects.