Aura Signal

The Aura Signal package is a SignalSlots/EventHandler implementation for PHP. With it, we can invoke handlers ("slots" or "hooks") whenever an object sends a signal ("notification" or "event") to the signal manager.

This package is compliant with PSR-0, PSR-1, and PSR-2. If you notice compliance oversights, please send a patch via pull request.

Basic Usage

Instantiating the Signal Manager

First, instantiate the signal Manager class. The easiest way to do this is to call the Aura.Signal/scripts/instance.php script.

<?php
$signal = require '/path/to/Aura.Signal/scripts/instance.php';

Adding Signal Handlers

Before we can send a signal to the Manager, we will need to add a handler for it. To add a handler, specify:

1. The class expected to be sending the signal. This can be '*' for "any class", or a fully-qualified class name.

2. The name of the signal.

3. A closure or callback to handle the signal.

For example, to add a closure that will be executed every time an object of the class Vendor\Package\Example sends a signal called 'example_signal':

<?php
$signal->handler(
    'Vendor\Package\Example',
    'example_signal',
    function ($arg) { echo $arg; }
);

Signals By Class

To send a signal, the sending class must have an instance of the Manager. The class should call the send() method with the originating object (itself), the signal being sent, and arguments to pass to the signal handler.

For example, we will define the Vendor\Package\Example class, and have it send a signal to the Manager.

<?php
namespace Vendor\Package;
use Aura\Signal\Manager as SignalManager;

class Example
{
    protected $signal;
    
    public function __construct(SignalManager $signal)
    {
        $this->signal = $signal;
    }
    
    public function doSomething($text)
    {
        echo $text;
        $this->signal->send($this, 'example_signal', $text);
    }
}

Now whenever we call the doSomething() method, it will send the 'example_signal' to the Manager, and the Manager will invoke the handler for that signal.

Signal Inheritance

If a class sends a signal, and no handler has been set for it, then the Manager will do nothing. However, if a handler has been set for a parent class, and one of its child classes sends a signal handled for the parent, the Manager will handle that signal for the child as well.

For example, if we have these two classes, and call doSomethingElse() on each of them ...

<?php
namespace Vendor\Package;
use Aura\Signal\Manager as SignalManager;

class ExampleChild extends Example
{
    public function doSomethingElse($text)
    {
        echo $text . $text . $text;
        $this->signal->send($this, 'example_signal', $text);
    }
}

class ExampleOther
{
    protected $signal;
    
    public function __construct(SignalManager $signal)
    {
        $this->signal = $signal;
    }
    
    public function doSomethingElse($text)
    {
        echo $text . $text . $text;
        $this->signal->send($this, 'example_signal', $text)
    }
}

... then the Manager will handle the signal from ExampleChild because its parent has a handler for it. The Manager will not handle the signal for ExampleOther because no handlers for it or its parents have been added to the Manager.

Signals By Object

It is possible to tie a handler to an object instance, so that only signals sent from that specific object will be handled. To do so, pass the object instance as the $sender for the handler.

<?php
/**
 * @var Aura\Signal\Manager $signal
 */
$object = new Vendor\Package\ExampleChild($signal);

$signal->handler(
    $object,
    'example_signal',
    function ($arg) { echo "$arg!!!";}
);

If that specific object instance sends the example_signal then the handler will be triggered, but no other instance of ExampleChild will trigger the handler when it sends the same signal. This is useful for setting signal handlers from within an object that contains its own callback; for example:

<?php
namespace Vendor\Package;
use Aura\Signal\Manager as SignalManager;

class ExampleAnotherChild extends Example
{
    public function __construct(SignalManager $signal)
    {
        parent::__construct();
        $this->signal = $signal;
        $this->signal->handler($this, 'preAction', [$this, 'preAction']);
        $this->signal->handler($this, 'postAction', [$this, 'postAction']);
    }
    
    public function action()
    {
        $this->signal->send($this, 'preAction');
        $this->doSomething( __METHOD__ );
        $this->signal->send($this, 'postAction');
    }
    
    public function preAction()
    {
        // happens before the main action() logic
    }
    
    public function postAction()
    {
        // happens after the main action() logic
    }
}

When ExampleAnotherChild::action() is called, the code:

1. Sends a 'preAction' signal to the Manager, which in turn calls the preAction() method on the object

2. Calls the doSomething() method on the object (n.b., remember that the doSomething() method sends an 'example_signal' of its own to the Manager)

3. Sends a 'postAction' signal to the Manager, which in turn calls the postAction() method on the object.

If there are class-based handlers for ExampleAnotherChild class or its parents, those will also be executed. This means we can set up combinations of handlers to be applied to classes overall, along with handlers that are tied to specific objects.

Advanced Usage

Handler Position Groups

By default, all Handler objects will be appended to the Manager stack, and will be processed the order they were added. Sometimes you will need a Handler to be processed in a different order; for example, before or after all others. If so, you can pass a $position value when adding a Handler to the Manager. (The default $position for Handler objects is 5000.)

<?php
// add a closure at position 1000, which means it will be processed
// before all handlers at the default position 5000.
$closure = function() { 
    echo "Before all others."; 
    return "First closure";
};
$signal->handler('Vendor\Package\ExampleChild', 'example_signal', $closure, 1000);

// add a closure at position 9000, which means it will be processed
// after all handlers at the default position 5000.
$closure = function() { 
    echo "After all others."; 
    return "Second closure";
};
$signal->handler('Vendor\Package\ExampleChild', 'example_signal', $closure, 1000);

Handler objects added at a position will still be appended within that position group.

Result Inspection

After a signal has been sent, we can review the results returned by every handler for that signal.

<?php
// send a signal
$this->signal->send($this, 'example_signal');

// get the result collection
$results =  $this->signal->getResults();

// go through each result ...
foreach ($results as $result) {
    
    // ... and echo the value returned by the Handler callback
    echo $result->value;
}

The getResults() method returns a ResultCollection of Result objects, each of which has these properties:

• $origin: The object that sent the signal.

• $sender: The sender expected by the Handler.

• $signal: The signal that was sent by the origin.

• $value: The value returned by the Handler callback.

If you need only the last result, you can call getLast() on the ResultCollection object.

<?php
// send a signal and retain the results from each Handler
$results = $this->signal->send($this, 'example_signal');

// get the last result
$result = $results->getLast();

// and echo the value returned by the last Handler callback
echo $result->value;

Stopping Signal Processing

Sometimes it will be necessary to stop processing signal handlers. If a handler callback returns the Aura\Signal\Manager::STOP constant, then no more handlers for that signal will be processed.

First we define the handlers; note that the second one returns the STOP constant:

<?php
// add signal handlers
$signal->handler(
    'Vendor\Package\Example',
    'mock_signal',
    function() { return 'first'; }
);

$signal->handler(
    'Vendor\Package\Example',
    'mock_signal',
    function() { return \Aura\Signal\Manager::STOP; }
);

$signal->handler(
    'Vendor\Package\Example',
    'mock_signal',
    function() { return 'third'; }
);

Then, from inside an object, we send a signal:

<?php
$results = $this->signal->send($this, 'mock_signal');
// Or you can get via 
// $results = $this->signal->getResults();

Normally, $results would have three entries. In this case it has only two, because the second handler returned \aura\signal\Manager::STOP. As such, the third handler was never executed. You can call ResultCollection::isStopped() to see if the Manager stopped processing handlers in this way.

<?php
if ($results->isStopped()) {
    $result = $results->getLast();
    echo "Processing for signal 'mock_signal' stopped "
       . "by handler for " . $result->sender;
}

Setting Handlers at Construction

It is possible to set the Handler definitions for a Manager at construction time. This allows us to use one or more config files to define the Handler stack for a Manager.

Given this file at /path/to/signal_handlers.php ...

<?php
return [
    // first handler, with a closure
    [
        'Vendor\Package\Example',
        'mock_signal',
        function() { return 'foo'; },
    ],
    // second handler, with a static callback
    [
        'Vendor\Package\Example',
        'mock_signal',
        ['Vendor\Package\SomeClass', 'someMethod'],
    ],
    // third handler, with a closure and position
    [
        'Vendor\Package\Example',
        'mock_signal',
        function() { return 'baz'; },
        1000,
    ],
];

... we can configure a Manager like so:

<?php
namespace Aura\Signal;
$handlers = require '/path/to/signal_handlers.php';
$signal = new Manager(
    new HandlerFactory,
    new ResultFactory,
    new ResultCollection,
    $handlers
);

That is the equivalent of calling $signal->handler() three times to add each handler.

Thanks

Thanks to Richard "Cyberlot" Thomas for the original suggestion, Galactic Void for bringing it back up, and Matthew Weier O'Phinney.