A generic callback wrapper which enables lazy DI instantiation

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2.1.0 2019-11-26 15:57 UTC

This package is auto-updated.

Last update: 2020-07-26 17:46:09 UTC


Signal is a generic callback wrapper / proxy which enables custom resolution, including lazy instantiation and dependency injection in places where it might not otherwise be supported.

If you think you know what the above means and want to skip the explanation you can simply:


composer require hiraeth/signal

Othewise, read on to understand...

What's the Problem?

I was looking at using igorw/evenement and saw this:

$emitter->on('user.created', function (User $user) use ($logger) {
    $logger->log(sprintf("User '%s' was created.", $user->getLogin()));

Lots of libraries support generic callbacks which are usually shown as anonymous functions. This is great if you write a lot of custom wiring, not so great if you separate wiring/configuration data from wiring logic, and use classes, e.g.:

FILE: config.php

return [
	'events' => [
		'user.created' => 'UserCreatedHandler'

Now when you read the config, you could do:

foreach ($config['events'] as $event => $handler) {
	$emitter->on($event, $container->get($handler))

But then you're resolving all your handlers and their dependencies up front. So maybe instead you do:

foreach ($config['events'] as $event => $handler) {
	$emitter->on($event, function(...$params) use ($container, $handler) {
		$handler = $container->get($handler);
		return $handler(...$params);

OK, that's not so bad, but now you're kind of assuming your handlers will always implement __invoke. And what if you need to handle different callback styles elsewhere? Now you have custom proxy callbacks all over.

Is There a Better Way?


$signal = new Hiraeth\Utils\Signal(function($signal) use ($container) {
	if (is_string($signal)) {
		if (function_exists($signal)) {
			return $signal;

		if (strpos($signal, '::') !== FALSE) {
			list($class, $method) = explode('::', $signal);

			return [$container->get($class), $method];

		if (class_exists($signal)) {
			return [$container->get($signal), '__invoke'];

	return NULL;


foreach ($config['events'] as $event => $handler) {
	$emitter->on($event, $signal->create($handler))

Isn't that nice?

If you want to make it even nicer, you can move your resolver functionality into a separate class that implements __invoke($signal):

$resolver = new Resolver($container);
$signal   = new Hiraeth\Utils\Signal($resolver);

Is that All?

No... because whatever $handler is gets passed to your custom resolver, you can do whatever you want when you resolve the handler. For example, maybe you want to handle "artisan" callbacks (not sure why, but whatever):

$signal = str_replace('@', '::', $signal);

Maybe you want to create URL callbacks:

if (preg_match('#^https?://#', $signal)) {
	$client = $container->get('APIClient');

	return function() use ($client) {

Who the hell knows! The world is your oyster.

OK, Aren't You Still Instantiating Everything Up Front?

No... $signal->create($handler) does not return the resolved handler. Rather, it merely tracks the $handler and returns a proxy callback in its place, so the handler isn't resolved until it actually needs to be. See the one test it has:

class SignalTest extends PHPUnit\Framework\TestCase
	public function testProxy()
		// Create a new instance of signal with a totally useless
		// resolver that always returns the same callback.

		$signal = new Hiraeth\Utils\Signal(function($signal) use (&$target) {
			$this->assertSame($signal, 'fake_signal');

			return $target = new class {
				public function __invoke($foo, $bar)
					return $foo . ' ' . $bar;

		// Create a new proxy for the signal as $foobar
		// this does not yet call the resolver, rather $foobar
		// contains a callback that will call the resolver when
		// it gets called.

		$foobar = $signal->create('fake_signal');

		$this->assertNotSame($target, $foobar);

		// Calling foobar resolves the signal to a target handler
		// our anonymous class above, and calls it to get the result.

		$result = $foobar('foo', 'bar');

		$this->assertSame($result, 'foo bar');

		// Once resolved, calling $signal->create() will return the
		// resolved handler directly without a proxy.

		$newbar = $signal->create('fake_signal');

		$this->assertSame($target, $newbar);

Run The Test Yourself

php vendor/bin/phpunit --bootstrap vendor/autoload.php test/routines