Using the EventManager

This tutorial explores the various features of Zend\EventManager.

Terminology

• An Event is a named action.
• A Listener is any PHP callback that reacts to an event.
• An EventManager aggregates listeners for one or more named events, and triggers events.

Typically, an event will be modeled as an object, containing metadata surrounding when and how it was triggered, including the event name, what object triggered the event (the “target”), and what parameters were provided. Events are named, which allows a single listener to branch logic based on the event.

Getting started

The minimal things necessary to start using events are:

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

The simplest example looks something like this:

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 result in the following:

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

Note

Throughout this tutorial, we use closures as listeners. However, any valid PHP callback can be attached as a listeners: PHP function names, static class methods, object instance methods, functors, or closures. We use closures within this post simply for illustration and simplicity.

If you were paying attention to the example, you will have noted the null argument. Why is it there?

Typically, you will compose an EventManager within a class, to allow triggering actions within methods. The middle argument to trigger() is the “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\EventManager;
use Zend\EventManager\EventManagerAwareInterface;
use Zend\EventManager\EventManagerInterface;

class Example implements EventManagerAwareInterface
{
    protected $events;

    public function setEventManager(EventManagerInterface $events)
    {
        $events->setIdentifiers(array(
            __CLASS__,
            get_class($this)
        ));
        $this->events = $events;
    }

    public function getEventManager()
    {
        if (!$this->events) {
            $this->setEventManager(new EventManager());
        }
        return $this->events;
    }

    public function do($foo, $baz)
    {
        $params = compact('foo', 'baz');
        $this->getEventManager()->trigger(__FUNCTION__, $this, $params);
    }

}

$example = new Example();

$example->getEventManager()->attach('do', function($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 the target, the instance of Example, 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 a new question: What is the call to setIdentifiers() for?

Shared managers

One aspect that the EventManager implementation provides is an ability to compose a SharedEventManagerInterface implementation.

Zend\EventManager\SharedEventManagerInterface describes an object that aggregates listeners for events attached to objects with specific identifiers. It does not trigger events itself. Instead, an EventManager instance that composes a SharedEventManager will query the SharedEventManager for listeners on identifiers it’s interested in, and trigger those listeners as well.

How does this work, exactly?

Consider the following:

use Zend\EventManager\SharedEventManager;

$sharedEvents = new SharedEventManager();
$sharedEvents->attach('Example', 'do', function ($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)
    );
});

This looks almost identical to the previous example; the key difference is that there is an additional argument at the start of the list, 'Example'. This code is basically saying, “Listen to the ‘do’ event of the ‘Example’ target, and, when notified, execute this callback.”

This is where the setIdentifiers() argument of EventManager comes into play. The method allows passing a string, or an array of strings, defining the name or names of the context or targets the given instance will be interested in. If an array is given, then any listener on any of the targets given will be notified.

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

$example = new Example();
$example->getEventManager()->setSharedManager($sharedEvents);
$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 setEventManager() method is that we defined it to listen both on __CLASS__ and get_class($this). This means that calling do() on our SubExample class would also trigger the shared listener! It also means that, if desired, we could attach to specifically SubExample, and listeners attached to only the Example target 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.

Note

We recommend using class names, interface names, and/or abstract class names for identifiers. This makes determining what events are available easier, as well as finding which listeners might be attaching to those events. Interfaces make a particularly good use case, as they allow attaching to a group of related classes a single operation.

At any point, if you do not want to notify shared listeners, pass a null value to setSharedManager():

$events->setSharedManager(null);

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

$events->setSharedManager($sharedEvents);

Wildcards

So far, with both a normal EventManager instance and with the SharedEventManager instance, we’ve seen the usage of singular strings representing the event and target names to which we want to attach. What if you want to attach a listener to multiple events or targets?

The answer is to supply an array of events or targets, or a wildcard, *.

Consider the following examples:

// Multiple named events:
$events->attach(
    array('foo', 'bar', 'baz'), // events
    $listener
);

// All events via wildcard:
$events->attach(
    '*', // all events
    $listener
);

// Multiple named targets:
$sharedEvents->attach(
    array('Foo', 'Bar', 'Baz'), // targets
    'doSomething', // named event
    $listener
);

// All targets via wildcard
$sharedEvents->attach(
    '*', // all targets
    'doSomething', // named event
    $listener
);

// Mix and match: multiple named events on multiple named targets:
$sharedEvents->attach(
    array('Foo', 'Bar', 'Baz'), // targets
    array('foo', 'bar', 'baz'), // events
    $listener
);

// Mix and match: all events on multiple named targets:
$sharedEvents->attach(
    array('Foo', 'Bar', 'Baz'), // targets
    '*', // events
    $listener
);

// Mix and match: multiple named events on all targets:
$sharedEvents->attach(
    '*', // targets
    array('foo', 'bar', 'baz'), // events
    $listener
);

// Mix and match: all events on all targets:
$sharedEvents->attach(
    '*', // targets
    '*', // events
    $listener
);

The ability to specify multiple targets and/or events when attaching can slim down your code immensely.

Listener aggregates

Another approach to listening to multiple events is via a concept of listener aggregates, represented by Zend\EventManager\ListenerAggregateInterface. Via this approach, a single class can listen to multiple events, attaching one or more instance methods as listeners.

This interface defines two methods, attach(EventManagerInterface $events) and detach(EventManagerInterface $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\EventInterface;
use Zend\EventManager\EventManagerInterface;
use Zend\EventManager\ListenerAggregateInterface;
use Zend\Log\Logger;

class LogEvents implements ListenerAggregateInterface
{
    protected $listeners = array();
    protected $log;

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

    public function attach(EventManagerInterface $events)
    {
        $this->listeners[] = $events->attach('do', array($this, 'log'));
        $this->listeners[] = $events->attach('doSomethingElse', array($this, 'log'));
    }

    public function detach(EventCollection $events)
    {
        foreach ($this->listeners as $index => $listener) {
            if ($events->detach($listener)) {
                unset($this->listeners[$index];
            }
        }
    }

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

You can attach this using either attach() or attachAggregate():

$logListener = new LogEvents($logger);

$events->attachAggregate($logListener); // OR
$events->attach($logListener);

Any events the aggregate attaches to will then be notified when triggered.

Why bother? For a couple of reasons:

• Aggregates allow you to have stateful listeners. The above example demonstrates this via the composition of the logger; another example would be tracking configuration options.
• Aggregates make detaching listeners easier. When you call attach() normally, you receive a Zend\Stdlib\CallbackHandler instance; the only way to detach() a listener is to pass that instance back – which means if you want to detach later, you need to keep that instance somewhare. Aggregates typically do this for you – as you can see in the example above.

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 Zend\EventManager\ResponseCollection instance. 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-ciruiting 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(); 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->getEventManager()->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, we recommend that you standardize on 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 SharedEentManager::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. However, if you provide a priority value, you can influence order of execution.

• Higher priority values execute earlier.
• Lower (negative) priority 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. We advise avoiding 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.

As an example, 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, consider how you might 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 the application instance, the router, the route match object, request and response objects, the view model, 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: Implementing a simple caching system

In previous sections, 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? One standard that has emerged is the use of __RESULT__, as double-underscored variables are typically reserved for the sytem.

Here’s what the method will look like:

public function someExpensiveCall($criteria1, $criteria2)
{
    $params  = compact('criteria1', 'criteria2');
    $results = $this->getEventManager()->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 ListenerAggregate, which would have allowed keeping the $cache instance as a stateful property, instead of importing it into closures.

Another approach would be to move the body of the method to a listener as well, which would allow using the priority system in order to implement caching. That would look like this:

public function setEventManager(EventManagerInterface $events)
{
    $this->events = $events;
    $events->setIdentifiers(array(__CLASS__, get_class($this)));
    $events->attach('someExpensiveCall', array($this, 'doSomeExpensiveCall'));
}

public function someExpensiveCall($criteria1, $criteria2)
{
    $params  = compact('criteria1', 'criteria2');
    $results = $this->getEventManager()->trigger(
        __FUNCTION__,
        $this,
        $params,
        function ($r) {
            return ($r instanceof SomeResultClass);
        }
    );
    return $results->last();
}

public function doSomeExpensiveCall($e)
{
    // ... do some work ...
    $e->setParam('__RESULT__', $calculatedResult);
    return $calculatedResult;
}

The listeners would then attach to the “someExpensiveCall” event, with the cache lookup listener listening at high priority, and the cache storage listener listening at low (negative) priority.

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.

Conclusion

The EventManager is a powerful component. It drives the workflow of the MVC layer, and is used in countless components to provide hook points for developers to manipulate the workflow. It can be put to any number of uses inside your own code, and is an important part of your Zend Framework toolbox.